[PATCH] kcov, usb: Fix invalid context sleep in softirq path on PREEMPT_RT

[Yunseong Kim]

When fuzzing USB with syzkaller on a PREEMPT_RT enabled kernel, following
bug is triggered in the ksoftirqd context.

| BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48
| in_atomic(): 0, irqs_disabled(): 1, non_block: 0, pid: 30, name: ksoftirqd/1
| preempt_count: 0, expected: 0
| RCU nest depth: 2, expected: 2
| CPU: 1 UID: 0 PID: 30 Comm: ksoftirqd/1 Tainted: G W 6.16.0-rc1-rt1 #11 PREEMPT_RT
| Tainted: [W]=WARN
| Hardware name: QEMU KVM Virtual Machine, BIOS 2025.02-8 05/13/2025
| Call trace:
| show_stack+0x2c/0x3c (C)
| __dump_stack+0x30/0x40
| dump_stack_lvl+0x148/0x1d8
| dump_stack+0x1c/0x3c
| __might_resched+0x2e4/0x52c
| rt_spin_lock+0xa8/0x1bc
| kcov_remote_start+0xb0/0x490
| __usb_hcd_giveback_urb+0x2d0/0x5e8
| usb_giveback_urb_bh+0x234/0x3c4
| process_scheduled_works+0x678/0xd18
| bh_worker+0x2f0/0x59c
| workqueue_softirq_action+0x104/0x14c
| tasklet_action+0x18/0x8c
| handle_softirqs+0x208/0x63c
| run_ksoftirqd+0x64/0x264
| smpboot_thread_fn+0x4ac/0x908
| kthread+0x5e8/0x734
| ret_from_fork+0x10/0x20

To reproduce on PREEMPT_RT kernel:

$ git remote add rt-devel git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git
$ git fetch rt-devel
$ git checkout -b v6.16-rc1-rt1 v6.16-rc1-rt1

I have attached the syzlang and the C source code converted by syz-prog2c:

Link: https://gist.github.com/kzall0c/9455aaa246f4aa1135353a51753adbbe

Then, run with a PREEMPT_RT config.

This issue was introduced by commit
f85d39dd7ed8 ("kcov, usb: disable interrupts in kcov_remote_start_usb_softirq").

However, this creates a conflict on PREEMPT_RT kernels. The local_irq_save()
call establishes an atomic context where sleeping is forbidden. Inside this
context, kcov_remote_start() is called, which on PREEMPT_RT uses sleeping
locks (spinlock_t and local_lock_t are mapped to rt_mutex). This results in
a sleeping function called from invalid context.

On PREEMPT_RT, interrupt handlers are threaded, so the re-entrancy scenario
is already safely handled by the existing local_lock_t and the global
kcov_remote_lock within kcov_remote_start(). Therefore, the outer
local_irq_save() is not necessary.

This preserves the intended re-entrancy protection for non-RT kernels while
resolving the locking violation on PREEMPT_RT kernels.

After making this modification and testing it, syzkaller fuzzing the
PREEMPT_RT kernel is now running without stopping on latest announced
Real-time Linux.

Link: https://lore.kernel.org/linux-rt-devel/20250610080...@linutronix.de/
Fixes: f85d39dd7ed8 ("kcov, usb: disable interrupts in kcov_remote_start_usb_softirq")
Cc: Andrey Konovalov <andre...@gmail.com>
Cc: Tetsuo Handa <penguin...@i-love.sakura.ne.jp>
Cc: Alan Stern <st...@rowland.harvard.edu>
Cc: Dmitry Vyukov <dvy...@google.com>
Cc: Greg Kroah-Hartman <gre...@linuxfoundation.org>
Cc: Thomas Gleixner <tg...@linutronix.de>
Cc: Sebastian Andrzej Siewior <big...@linutronix.de>
Cc: Byungchul Park <byun...@sk.com>
Cc: sta...@vger.kernel.org
Cc: kasa...@googlegroups.com
Cc: syzk...@googlegroups.com
Cc: linu...@vger.kernel.org
Cc: linux-r...@lists.linux.dev
Signed-off-by: Yunseong Kim <y...@kzalloc.com>
---
include/linux/kcov.h | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/include/linux/kcov.h b/include/linux/kcov.h
index 75a2fb8b16c3..c5e1b2dd0bb7 100644
--- a/include/linux/kcov.h
+++ b/include/linux/kcov.h
@@ -85,7 +85,9 @@ static inline unsigned long kcov_remote_start_usb_softirq(u64 id)
unsigned long flags = 0;

if (in_serving_softirq()) {
+#ifndef CONFIG_PREEMPT_RT
local_irq_save(flags);
+#endif
kcov_remote_start_usb(id);
}

@@ -96,7 +98,9 @@ static inline void kcov_remote_stop_softirq(unsigned long flags)
{
if (in_serving_softirq()) {
kcov_remote_stop();
+#ifndef CONFIG_PREEMPT_RT
local_irq_restore(flags);
+#endif
}
}

--
2.50.0

[Greg Kroah-Hartman]

Why is this only a USB thing? What is unique about it to trigger this
issue?

thanks,

greg k-h

[Tetsuo Handa]

On 2025/07/26 15:36, Greg Kroah-Hartman wrote:
> Why is this only a USB thing? What is unique about it to trigger this
> issue?

I couldn't catch your question. But the answer could be that

__usb_hcd_giveback_urb() is a function which is a USB thing

and

kcov_remote_start_usb_softirq() is calling local_irq_save() despite CONFIG_PREEMPT_RT=y

as shown below.



static void __usb_hcd_giveback_urb(struct urb *urb)
{
(...snipped...)
kcov_remote_start_usb_softirq((u64)urb->dev->bus->busnum) {
if (in_serving_softirq()) {
local_irq_save(flags); // calling local_irq_save() is wrong if CONFIG_PREEMPT_RT=y
kcov_remote_start_usb(id) {
kcov_remote_start(id) {
kcov_remote_start(kcov_remote_handle(KCOV_SUBSYSTEM_USB, id)) {
(...snipped...)
local_lock_irqsave(&kcov_percpu_data.lock, flags) {
__local_lock_irqsave(lock, flags) {
#ifndef CONFIG_PREEMPT_RT
https://elixir.bootlin.com/linux/v6.16-rc7/source/include/linux/local_lock_internal.h#L125
#else
https://elixir.bootlin.com/linux/v6.16-rc7/source/include/linux/local_lock_internal.h#L235 // not calling local_irq_save(flags)
#endif
}
}
(...snipped...)
spin_lock(&kcov_remote_lock) {
#ifndef CONFIG_PREEMPT_RT
https://elixir.bootlin.com/linux/v6.16-rc7/source/include/linux/spinlock.h#L351
#else
https://elixir.bootlin.com/linux/v6.16-rc7/source/include/linux/spinlock_rt.h#L42 // mapped to rt_mutex which might sleep
#endif
}
(...snipped...)
}
}
}
}
}
(...snipped...)
}

[Greg Kroah-Hartman]

Ok, but then how does the big comment section for
kcov_remote_start_usb_softirq() work, where it explicitly states:

* 2. Disables interrupts for the duration of the coverage collection section.
* This allows avoiding nested remote coverage collection sections in the
* softirq context (a softirq might occur during the execution of a work in
* the BH workqueue, which runs with in_serving_softirq() > 0).
* For example, usb_giveback_urb_bh() runs in the BH workqueue with
* interrupts enabled, so __usb_hcd_giveback_urb() might be interrupted in
* the middle of its remote coverage collection section, and the interrupt
* handler might invoke __usb_hcd_giveback_urb() again.


You are removing half of this function entirely, which feels very wrong
to me as any sort of solution, as you have just said that all of that
documentation entry is now not needed.

Are you sure this is ok?

thanks,

greg k-h

[Thomas Gleixner]

On Sat, Jul 26 2025 at 09:59, Greg Kroah-Hartman wrote:
> On Sat, Jul 26, 2025 at 04:44:42PM +0900, Tetsuo Handa wrote:
>> static void __usb_hcd_giveback_urb(struct urb *urb)
>> {
>> (...snipped...)
>> kcov_remote_start_usb_softirq((u64)urb->dev->bus->busnum) {
>> if (in_serving_softirq()) {
>> local_irq_save(flags); // calling local_irq_save() is wrong if CONFIG_PREEMPT_RT=y
>> kcov_remote_start_usb(id) {
>> kcov_remote_start(id) {
>> kcov_remote_start(kcov_remote_handle(KCOV_SUBSYSTEM_USB, id)) {
>> (...snipped...)
>> local_lock_irqsave(&kcov_percpu_data.lock, flags) {
>> __local_lock_irqsave(lock, flags) {
>> #ifndef CONFIG_PREEMPT_RT
>> https://elixir.bootlin.com/linux/v6.16-rc7/source/include/linux/local_lock_internal.h#L125
>> #else
>> https://elixir.bootlin.com/linux/v6.16-rc7/source/include/linux/local_lock_internal.h#L235 // not calling local_irq_save(flags)
>> #endif

Right, it does not invoke local_irq_save(flags), but it takes the
underlying lock, which means it prevents reentrance.

> Ok, but then how does the big comment section for
> kcov_remote_start_usb_softirq() work, where it explicitly states:
>
> * 2. Disables interrupts for the duration of the coverage collection section.
> * This allows avoiding nested remote coverage collection sections in the
> * softirq context (a softirq might occur during the execution of a work in
> * the BH workqueue, which runs with in_serving_softirq() > 0).
> * For example, usb_giveback_urb_bh() runs in the BH workqueue with
> * interrupts enabled, so __usb_hcd_giveback_urb() might be interrupted in
> * the middle of its remote coverage collection section, and the interrupt
> * handler might invoke __usb_hcd_giveback_urb() again.
>
>
> You are removing half of this function entirely, which feels very wrong
> to me as any sort of solution, as you have just said that all of that
> documentation entry is now not needed.

I'm not so sure because kcov_percpu_data.lock is only held within
kcov_remote_start() and kcov_remote_stop(), but the above comment
suggests that the whole section needs to be serialized.

Though I'm not a KCOV wizard and might be completely wrong here.

If the whole section is required to be serialized, then this need
another local lock in kcov_percpu_data to work correctly on RT.

Thanks,

tglx

[Yunseong Kim]

Huge thanks to everyone for the feedback!

While working on earlier patches, running syzkaller on PREEMPT_RT uncovered
numerous sleep-in-atomic-context bugs and other synchronization issues unique to
that environment. This highlighted the need to address these problems.

After receiving comments from maintainers, I realized that my initial patch set
wasn't heading in the right direction.


It seems that the following two patches conflict on PREEMPT_RT kernels:

1. kcov: replace local_irq_save() with a local_lock_t
Link: https://github.com/torvalds/linux/commit/d5d2c51f1e5f
2. kcov, usb: disable interrupts in kcov_remote_start_usb_softirq
Link: https://github.com/torvalds/linux/commit/f85d39dd7ed8


My current approach involves:

* Removing the existing 'kcov_percpu_data.lock'
* Converting 'kcov->lock' and 'kcov_remote_lock' to raw spinlocks
* Relocating the kmalloc call for kcov_remote_add() outside kcov_ioctl_locked(),
as GFP_ATOMIC allocations can potentially sleep under PREEMPT_RT.
: As expected from further testing, keeping the GFP_ATOMIC allocation inside
kcov_remote_add() still leads to sleep in atomic context.

This approach allows us to keep Andrey’s patch d5d2c51f1e5f while making
modifications as Sebastian suggested in his commit f85d39dd7ed8 message,
which I found particularly insightful and full of helpful hints.

The work I'm doing on PATCH v2 involves a number of changes, and I would truly
appreciate any critical feedback. I'm always happy to hear insights!


Best regards,
Yunseong Kim

[Greg Kroah-Hartman]

On Sat, Aug 02, 2025 at 02:26:49PM +0000, Yunseong Kim wrote:
> The KCOV subsystem currently utilizes standard spinlock_t and local_lock_t
> for synchronization. In PREEMPT_RT configurations, these locks can be
> implemented via rtmutexes and may therefore sleep. This behavior is
> problematic as kcov locks are sometimes used in atomic contexts or protect
> data accessed during critical instrumentation paths where sleeping is not
> permissible.
>
> Address these issues to make kcov PREEMPT_RT friendly:
>
> 1. Convert kcov->lock and kcov_remote_lock from spinlock_t to
> raw_spinlock_t. This ensures they remain true, non-sleeping
> spinlocks even on PREEMPT_RT kernels.
>
> 2. Refactor the KCOV_REMOTE_ENABLE path to move memory allocations
> out of the critical section. All necessary struct kcov_remote
> structures are now pre-allocated individually in kcov_ioctl()
> using GFP_KERNEL (allowing sleep) before acquiring the raw
> spinlocks.
>
> 3. Modify the ioctl handling logic to utilize these pre-allocated
> structures within the critical section. kcov_remote_add() is
> modified to accept a pre-allocated structure instead of allocating
> one internally.
>
> 4. Remove the local_lock_t protection for kcov_percpu_data in
> kcov_remote_start/stop(). Since local_lock_t can also sleep under
> RT, and the required protection is against local interrupts when
> accessing per-CPU data, it is replaced with explicit
> local_irq_save/restore().
>
> Link: https://lore.kernel.org/all/20250725201400...@kzalloc.com/t/#u

> Fixes: f85d39dd7ed8 ("kcov, usb: disable interrupts in kcov_remote_start_usb_softirq")
> Cc: Andrey Konovalov <andre...@gmail.com>
> Cc: Tetsuo Handa <penguin...@i-love.sakura.ne.jp>
> Cc: Alan Stern <st...@rowland.harvard.edu>
> Cc: Dmitry Vyukov <dvy...@google.com>
> Cc: Greg Kroah-Hartman <gre...@linuxfoundation.org>
> Cc: Thomas Gleixner <tg...@linutronix.de>
> Cc: Sebastian Andrzej Siewior <big...@linutronix.de>
> Cc: Byungchul Park <byun...@sk.com>
> Cc: sta...@vger.kernel.org
> Cc: kasa...@googlegroups.com
> Cc: syzk...@googlegroups.com
> Cc: linu...@vger.kernel.org
> Cc: linux-r...@lists.linux.dev
> Signed-off-by: Yunseong Kim <y...@kzalloc.com>
> ---

> kernel/kcov.c | 243 +++++++++++++++++++++++++++-----------------------
> 1 file changed, 130 insertions(+), 113 deletions(-)
>
> diff --git a/kernel/kcov.c b/kernel/kcov.c
> index 187ba1b80bda..9c8e4325cff8 100644
> --- a/kernel/kcov.c
> +++ b/kernel/kcov.c
> @@ -54,7 +54,7 @@ struct kcov {
> */
> refcount_t refcount;
> /* The lock protects mode, size, area and t. */
> - spinlock_t lock;
> + raw_spinlock_t lock;
> enum kcov_mode mode;
> /* Size of arena (in long's). */
> unsigned int size;
> @@ -84,13 +84,12 @@ struct kcov_remote {
> struct hlist_node hnode;
> };
>
> -static DEFINE_SPINLOCK(kcov_remote_lock);
> +static DEFINE_RAW_SPINLOCK(kcov_remote_lock);
> static DEFINE_HASHTABLE(kcov_remote_map, 4);
> static struct list_head kcov_remote_areas = LIST_HEAD_INIT(kcov_remote_areas);
>
> struct kcov_percpu_data {
> void *irq_area;
> - local_lock_t lock;
>
> unsigned int saved_mode;
> unsigned int saved_size;
> @@ -99,9 +98,7 @@ struct kcov_percpu_data {
> int saved_sequence;
> };
>
> -static DEFINE_PER_CPU(struct kcov_percpu_data, kcov_percpu_data) = {
> - .lock = INIT_LOCAL_LOCK(lock),
> -};
> +static DEFINE_PER_CPU(struct kcov_percpu_data, kcov_percpu_data);
>
> /* Must be called with kcov_remote_lock locked. */
> static struct kcov_remote *kcov_remote_find(u64 handle)
> @@ -116,15 +113,9 @@ static struct kcov_remote *kcov_remote_find(u64 handle)
> }
>
> /* Must be called with kcov_remote_lock locked. */
> -static struct kcov_remote *kcov_remote_add(struct kcov *kcov, u64 handle)
> +static struct kcov_remote *kcov_remote_add(struct kcov *kcov, u64 handle,
> + struct kcov_remote *remote)
> {
> - struct kcov_remote *remote;
> -
> - if (kcov_remote_find(handle))
> - return ERR_PTR(-EEXIST);
> - remote = kmalloc(sizeof(*remote), GFP_ATOMIC);
> - if (!remote)
> - return ERR_PTR(-ENOMEM);
> remote->handle = handle;
> remote->kcov = kcov;
> hash_add(kcov_remote_map, &remote->hnode, handle);
> @@ -404,9 +395,8 @@ static void kcov_remote_reset(struct kcov *kcov)
> int bkt;
> struct kcov_remote *remote;
> struct hlist_node *tmp;
> - unsigned long flags;
>
> - spin_lock_irqsave(&kcov_remote_lock, flags);
> + raw_spin_lock(&kcov_remote_lock);
> hash_for_each_safe(kcov_remote_map, bkt, tmp, remote, hnode) {
> if (remote->kcov != kcov)
> continue;
> @@ -415,7 +405,7 @@ static void kcov_remote_reset(struct kcov *kcov)
> }
> /* Do reset before unlock to prevent races with kcov_remote_start(). */
> kcov_reset(kcov);
> - spin_unlock_irqrestore(&kcov_remote_lock, flags);
> + raw_spin_unlock(&kcov_remote_lock);
> }
>
> static void kcov_disable(struct task_struct *t, struct kcov *kcov)
> @@ -450,7 +440,7 @@ void kcov_task_exit(struct task_struct *t)
> if (kcov == NULL)
> return;
>
> - spin_lock_irqsave(&kcov->lock, flags);
> + raw_spin_lock_irqsave(&kcov->lock, flags);
> kcov_debug("t = %px, kcov->t = %px\n", t, kcov->t);
> /*
> * For KCOV_ENABLE devices we want to make sure that t->kcov->t == t,
> @@ -475,12 +465,12 @@ void kcov_task_exit(struct task_struct *t)
> * By combining all three checks into one we get:
> */
> if (WARN_ON(kcov->t != t)) {
> - spin_unlock_irqrestore(&kcov->lock, flags);
> + raw_spin_unlock_irqrestore(&kcov->lock, flags);
> return;
> }
> /* Just to not leave dangling references behind. */
> kcov_disable(t, kcov);
> - spin_unlock_irqrestore(&kcov->lock, flags);
> + raw_spin_unlock_irqrestore(&kcov->lock, flags);
> kcov_put(kcov);
> }
>
> @@ -492,14 +482,14 @@ static int kcov_mmap(struct file *filep, struct vm_area_struct *vma)
> struct page *page;
> unsigned long flags;
>
> - spin_lock_irqsave(&kcov->lock, flags);
> + raw_spin_lock_irqsave(&kcov->lock, flags);
> size = kcov->size * sizeof(unsigned long);
> if (kcov->area == NULL || vma->vm_pgoff != 0 ||
> vma->vm_end - vma->vm_start != size) {
> res = -EINVAL;
> goto exit;
> }
> - spin_unlock_irqrestore(&kcov->lock, flags);
> + raw_spin_unlock_irqrestore(&kcov->lock, flags);
> vm_flags_set(vma, VM_DONTEXPAND);
> for (off = 0; off < size; off += PAGE_SIZE) {
> page = vmalloc_to_page(kcov->area + off);
> @@ -511,7 +501,7 @@ static int kcov_mmap(struct file *filep, struct vm_area_struct *vma)
> }
> return 0;
> exit:
> - spin_unlock_irqrestore(&kcov->lock, flags);
> + raw_spin_unlock_irqrestore(&kcov->lock, flags);
> return res;
> }
>
> @@ -525,7 +515,7 @@ static int kcov_open(struct inode *inode, struct file *filep)
> kcov->mode = KCOV_MODE_DISABLED;
> kcov->sequence = 1;
> refcount_set(&kcov->refcount, 1);
> - spin_lock_init(&kcov->lock);
> + raw_spin_lock_init(&kcov->lock);
> filep->private_data = kcov;
> return nonseekable_open(inode, filep);
> }
> @@ -586,10 +576,8 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
> unsigned long arg)
> {
> struct task_struct *t;
> - unsigned long flags, unused;
> - int mode, i;
> - struct kcov_remote_arg *remote_arg;
> - struct kcov_remote *remote;
> + unsigned long unused;
> + int mode;
>
> switch (cmd) {
> case KCOV_ENABLE:
> @@ -627,69 +615,80 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
> kcov_disable(t, kcov);
> kcov_put(kcov);
> return 0;
> - case KCOV_REMOTE_ENABLE:
> - if (kcov->mode != KCOV_MODE_INIT || !kcov->area)
> - return -EINVAL;
> - t = current;
> - if (kcov->t != NULL || t->kcov != NULL)
> - return -EBUSY;
> - remote_arg = (struct kcov_remote_arg *)arg;
> - mode = kcov_get_mode(remote_arg->trace_mode);
> - if (mode < 0)
> - return mode;
> - if ((unsigned long)remote_arg->area_size >
> - LONG_MAX / sizeof(unsigned long))
> - return -EINVAL;
> - kcov->mode = mode;
> - t->kcov = kcov;
> - t->kcov_mode = KCOV_MODE_REMOTE;
> - kcov->t = t;
> - kcov->remote = true;
> - kcov->remote_size = remote_arg->area_size;
> - spin_lock_irqsave(&kcov_remote_lock, flags);
> - for (i = 0; i < remote_arg->num_handles; i++) {
> - if (!kcov_check_handle(remote_arg->handles[i],
> - false, true, false)) {
> - spin_unlock_irqrestore(&kcov_remote_lock,
> - flags);
> - kcov_disable(t, kcov);
> - return -EINVAL;
> - }
> - remote = kcov_remote_add(kcov, remote_arg->handles[i]);
> - if (IS_ERR(remote)) {
> - spin_unlock_irqrestore(&kcov_remote_lock,
> - flags);
> - kcov_disable(t, kcov);
> - return PTR_ERR(remote);
> - }
> - }
> - if (remote_arg->common_handle) {
> - if (!kcov_check_handle(remote_arg->common_handle,
> - true, false, false)) {
> - spin_unlock_irqrestore(&kcov_remote_lock,
> - flags);
> - kcov_disable(t, kcov);
> - return -EINVAL;
> - }
> - remote = kcov_remote_add(kcov,
> - remote_arg->common_handle);
> - if (IS_ERR(remote)) {
> - spin_unlock_irqrestore(&kcov_remote_lock,
> - flags);
> - kcov_disable(t, kcov);
> - return PTR_ERR(remote);
> - }
> - t->kcov_handle = remote_arg->common_handle;
> - }
> - spin_unlock_irqrestore(&kcov_remote_lock, flags);
> - /* Put either in kcov_task_exit() or in KCOV_DISABLE. */
> - kcov_get(kcov);
> - return 0;
> default:
> return -ENOTTY;
> }
> }
>
> +static int kcov_ioctl_locked_remote_enabled(struct kcov *kcov,
> + unsigned int cmd, unsigned long arg,
> + struct kcov_remote *remote_handles,
> + struct kcov_remote *remote_common_handle)
> +{
> + struct task_struct *t;
> + int mode, i, ret;
> + struct kcov_remote_arg *remote_arg;
> +
> + if (kcov->mode != KCOV_MODE_INIT || !kcov->area)
> + return -EINVAL;
> + t = current;
> + if (kcov->t != NULL || t->kcov != NULL)
> + return -EBUSY;
> + remote_arg = (struct kcov_remote_arg *)arg;
> + mode = kcov_get_mode(remote_arg->trace_mode);
> + if (mode < 0)
> + return mode;
> + if ((unsigned long)remote_arg->area_size >
> + LONG_MAX / sizeof(unsigned long))
> + return -EINVAL;
> + kcov->mode = mode;
> + t->kcov = kcov;
> + t->kcov_mode = KCOV_MODE_REMOTE;
> + kcov->t = t;
> + kcov->remote = true;
> + kcov->remote_size = remote_arg->area_size;
> + raw_spin_lock(&kcov_remote_lock);
> + for (i = 0; i < remote_arg->num_handles; i++) {
> + if (!kcov_check_handle(remote_arg->handles[i],
> + false, true, false)) {
> + ret = -EINVAL;
> + goto err;
> + }
> + if (kcov_remote_find(remote_arg->handles[i])) {
> + ret = -EEXIST;
> + goto err;
> + }
> + kcov_remote_add(kcov, remote_arg->handles[i],
> + &remote_handles[i]);
> + }
> + if (remote_arg->common_handle) {
> + if (!kcov_check_handle(remote_arg->common_handle,
> + true, false, false)) {
> + ret = -EINVAL;
> + goto err;
> + }
> + if (kcov_remote_find(remote_arg->common_handle)) {
> + ret = -EEXIST;
> + goto err;
> + }
> + kcov_remote_add(kcov,
> + remote_arg->common_handle, remote_common_handle);
> + t->kcov_handle = remote_arg->common_handle;
> + }
> + raw_spin_unlock(&kcov_remote_lock);
> + /* Put either in kcov_task_exit() or in KCOV_DISABLE. */
> + kcov_get(kcov);
> + return 0;
> +
> +err:
> + raw_spin_unlock(&kcov_remote_lock);
> + kcov_disable(t, kcov);
> + kfree(remote_common_handle);
> + kfree(remote_handles);
> +
> + return ret;
> +}
> +
> static long kcov_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
> {
> struct kcov *kcov;
> @@ -697,6 +696,7 @@ static long kcov_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
> struct kcov_remote_arg *remote_arg = NULL;
> unsigned int remote_num_handles;
> unsigned long remote_arg_size;
> + struct kcov_remote *remote_handles, *remote_common_handle;
> unsigned long size, flags;
> void *area;
>
> @@ -716,16 +716,16 @@ static long kcov_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
> area = vmalloc_user(size * sizeof(unsigned long));
> if (area == NULL)
> return -ENOMEM;
> - spin_lock_irqsave(&kcov->lock, flags);
> + raw_spin_lock_irqsave(&kcov->lock, flags);
> if (kcov->mode != KCOV_MODE_DISABLED) {
> - spin_unlock_irqrestore(&kcov->lock, flags);
> + raw_spin_unlock_irqrestore(&kcov->lock, flags);
> vfree(area);
> return -EBUSY;
> }
> kcov->area = area;
> kcov->size = size;
> kcov->mode = KCOV_MODE_INIT;
> - spin_unlock_irqrestore(&kcov->lock, flags);
> + raw_spin_unlock_irqrestore(&kcov->lock, flags);
> return 0;
> case KCOV_REMOTE_ENABLE:
> if (get_user(remote_num_handles, (unsigned __user *)(arg +
> @@ -743,18 +743,35 @@ static long kcov_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
> return -EINVAL;
> }
> arg = (unsigned long)remote_arg;
> - fallthrough;
> + remote_handles = kmalloc_array(remote_arg->num_handles,
> + sizeof(struct kcov_remote), GFP_KERNEL);
> + if (!remote_handles)
> + return -ENOMEM;
> + remote_common_handle = kmalloc(sizeof(struct kcov_remote), GFP_KERNEL);
> + if (!remote_common_handle) {
> + kfree(remote_handles);
> + return -ENOMEM;
> + }
> +
> + raw_spin_lock_irqsave(&kcov->lock, flags);
> + res = kcov_ioctl_locked_remote_enabled(kcov, cmd, arg,
> + remote_handles, remote_common_handle);
> + raw_spin_unlock_irqrestore(&kcov->lock, flags);
> + kfree(remote_arg);
> + break;
> default:
> /*
> + * KCOV_ENABLE, KCOV_DISABLE:
> * All other commands can be normally executed under a spin lock, so we
> * obtain and release it here in order to simplify kcov_ioctl_locked().
> */
> - spin_lock_irqsave(&kcov->lock, flags);
> + raw_spin_lock_irqsave(&kcov->lock, flags);
> res = kcov_ioctl_locked(kcov, cmd, arg);
> - spin_unlock_irqrestore(&kcov->lock, flags);
> - kfree(remote_arg);
> - return res;
> + raw_spin_unlock_irqrestore(&kcov->lock, flags);
> + break;
> }
> +
> + return res;
> }
>
> static const struct file_operations kcov_fops = {
> @@ -862,7 +879,7 @@ void kcov_remote_start(u64 handle)
> if (!in_task() && !in_softirq_really())
> return;
>
> - local_lock_irqsave(&kcov_percpu_data.lock, flags);
> + local_irq_save(flags);
>
> /*
> * Check that kcov_remote_start() is not called twice in background
> @@ -870,7 +887,7 @@ void kcov_remote_start(u64 handle)
> */
> mode = READ_ONCE(t->kcov_mode);
> if (WARN_ON(in_task() && kcov_mode_enabled(mode))) {
> - local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
> + local_irq_restore(flags);
> return;
> }
> /*
> @@ -879,15 +896,15 @@ void kcov_remote_start(u64 handle)
> * happened while collecting coverage from a background thread.
> */
> if (WARN_ON(in_serving_softirq() && t->kcov_softirq)) {
> - local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
> + local_irq_restore(flags);
> return;
> }
>
> - spin_lock(&kcov_remote_lock);
> + raw_spin_lock(&kcov_remote_lock);
> remote = kcov_remote_find(handle);
> if (!remote) {
> - spin_unlock(&kcov_remote_lock);
> - local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
> + raw_spin_unlock(&kcov_remote_lock);
> + local_irq_restore(flags);
> return;
> }
> kcov_debug("handle = %llx, context: %s\n", handle,
> @@ -908,17 +925,17 @@ void kcov_remote_start(u64 handle)
> size = CONFIG_KCOV_IRQ_AREA_SIZE;
> area = this_cpu_ptr(&kcov_percpu_data)->irq_area;
> }
> - spin_unlock(&kcov_remote_lock);
> + raw_spin_unlock(&kcov_remote_lock);
>
> /* Can only happen when in_task(). */
> if (!area) {
> - local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
> + local_irq_restore(flags);
> area = vmalloc(size * sizeof(unsigned long));
> if (!area) {
> kcov_put(kcov);
> return;
> }
> - local_lock_irqsave(&kcov_percpu_data.lock, flags);
> + local_irq_save(flags);
> }
>
> /* Reset coverage size. */
> @@ -930,7 +947,7 @@ void kcov_remote_start(u64 handle)
> }
> kcov_start(t, kcov, size, area, mode, sequence);
>
> - local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
> + local_irq_restore(flags);
>
> }
> EXPORT_SYMBOL(kcov_remote_start);
> @@ -1004,12 +1021,12 @@ void kcov_remote_stop(void)
> if (!in_task() && !in_softirq_really())
> return;
>
> - local_lock_irqsave(&kcov_percpu_data.lock, flags);
> + local_irq_save(flags);
>
> mode = READ_ONCE(t->kcov_mode);
> barrier();
> if (!kcov_mode_enabled(mode)) {
> - local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
> + local_irq_restore(flags);
> return;
> }
> /*
> @@ -1017,12 +1034,12 @@ void kcov_remote_stop(void)
> * actually found the remote handle and started collecting coverage.
> */
> if (in_serving_softirq() && !t->kcov_softirq) {
> - local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
> + local_irq_restore(flags);
> return;
> }
> /* Make sure that kcov_softirq is only set when in softirq. */
> if (WARN_ON(!in_serving_softirq() && t->kcov_softirq)) {
> - local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
> + local_irq_restore(flags);
> return;
> }
>
> @@ -1037,22 +1054,22 @@ void kcov_remote_stop(void)
> kcov_remote_softirq_stop(t);
> }
>
> - spin_lock(&kcov->lock);
> + raw_spin_lock(&kcov->lock);
> /*
> * KCOV_DISABLE could have been called between kcov_remote_start()
> * and kcov_remote_stop(), hence the sequence check.
> */
> if (sequence == kcov->sequence && kcov->remote)
> kcov_move_area(kcov->mode, kcov->area, kcov->size, area);
> - spin_unlock(&kcov->lock);
> + raw_spin_unlock(&kcov->lock);
>
> if (in_task()) {
> - spin_lock(&kcov_remote_lock);
> + raw_spin_lock(&kcov_remote_lock);
> kcov_remote_area_put(area, size);
> - spin_unlock(&kcov_remote_lock);
> + raw_spin_unlock(&kcov_remote_lock);
> }
>
> - local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
> + local_irq_restore(flags);
>
> /* Get in kcov_remote_start(). */
> kcov_put(kcov);
> --
> 2.50.0
>

Hi,

This is the friendly patch-bot of Greg Kroah-Hartman. You have sent him
a patch that has triggered this response. He used to manually respond
to these common problems, but in order to save his sanity (he kept
writing the same thing over and over, yet to different people), I was
created. Hopefully you will not take offence and will fix the problem
in your patch and resubmit it so that it can be accepted into the Linux
kernel tree.

You are receiving this message because of the following common error(s)
as indicated below:

- This looks like a new version of a previously submitted patch, but you
did not list below the --- line any changes from the previous version.
Please read the section entitled "The canonical patch format" in the
kernel file, Documentation/process/submitting-patches.rst for what
needs to be done here to properly describe this.

If you wish to discuss this problem further, or you have questions about
how to resolve this issue, please feel free to respond to this email and
Greg will reply once he has dug out from the pending patches received
from other developers.

thanks,

greg k-h's patch email bot

[Greg Kroah-Hartman]

why isn't this 4 different patches?

[Yunseong Kim]

Hi Greg,

Thank you for your feedback on the patch. I’ll split it into four separate
patches for v3 to improve clarity.

Best regards,
Yunseong Kim

[Sebastian Andrzej Siewior]

On 2025-07-25 20:14:01 [+0000], Yunseong Kim wrote:
> When fuzzing USB with syzkaller on a PREEMPT_RT enabled kernel, following
> bug is triggered in the ksoftirqd context.
>

…

> This issue was introduced by commit
> f85d39dd7ed8 ("kcov, usb: disable interrupts in kcov_remote_start_usb_softirq").
>
> However, this creates a conflict on PREEMPT_RT kernels. The local_irq_save()
> call establishes an atomic context where sleeping is forbidden. Inside this
> context, kcov_remote_start() is called, which on PREEMPT_RT uses sleeping
> locks (spinlock_t and local_lock_t are mapped to rt_mutex). This results in
> a sleeping function called from invalid context.
>
> On PREEMPT_RT, interrupt handlers are threaded, so the re-entrancy scenario
> is already safely handled by the existing local_lock_t and the global
> kcov_remote_lock within kcov_remote_start(). Therefore, the outer
> local_irq_save() is not necessary.
>
> This preserves the intended re-entrancy protection for non-RT kernels while
> resolving the locking violation on PREEMPT_RT kernels.
>
> After making this modification and testing it, syzkaller fuzzing the
> PREEMPT_RT kernel is now running without stopping on latest announced
> Real-time Linux.

This looks oddly familiar because I removed the irq-disable bits while
adding local-locks.

Commit f85d39dd7ed8 looks wrong not that it shouldn't disable
interrupts. The statement in the added comment

| + * 2. Disables interrupts for the duration of the coverage collection section.
| + * This allows avoiding nested remote coverage collection sections in the
| + * softirq context (a softirq might occur during the execution of a work in
| + * the BH workqueue, which runs with in_serving_softirq() > 0).

is wrong. Softirqs are never nesting. While the BH workqueue is
running another softirq does not occur. The softirq is raised (again)
and will be handled _after_ BH workqueue is done. So this is already
serialised.

The issue is __usb_hcd_giveback_urb() always invokes
kcov_remote_start_usb_softirq(). __usb_hcd_giveback_urb() itself is
invoked from BH context (for the majority of HCDs) and from hardirq
context for the root-HUB. This gets us to the scenario that that we are
in the give-back path in softirq context and then invoke the function
once again in hardirq context.

I have no idea how kcov works but reverting the original commit and
avoiding the false nesting due to hardirq context should do the trick,
an untested patch follows.

This isn't any different than the tasklet handling that was used before
so I am not sure why it is now a problem.

Could someone maybe test this?

--- a/drivers/usb/core/hcd.c
+++ b/drivers/usb/core/hcd.c
@@ -1636,7 +1636,6 @@ static void __usb_hcd_giveback_urb(struct urb *urb)
struct usb_hcd *hcd = bus_to_hcd(urb->dev->bus);
struct usb_anchor *anchor = urb->anchor;
int status = urb->unlinked;
- unsigned long flags;

urb->hcpriv = NULL;
if (unlikely((urb->transfer_flags & URB_SHORT_NOT_OK) &&
@@ -1654,14 +1653,13 @@ static void __usb_hcd_giveback_urb(struct urb *urb)
/* pass ownership to the completion handler */
urb->status = status;
/*
- * Only collect coverage in the softirq context and disable interrupts
- * to avoid scenarios with nested remote coverage collection sections
- * that KCOV does not support.
- * See the comment next to kcov_remote_start_usb_softirq() for details.
+ * This function can be called in task context inside another remote
+ * coverage collection section, but kcov doesn't support that kind of
+ * recursion yet. Only collect coverage in softirq context for now.
*/
- flags = kcov_remote_start_usb_softirq((u64)urb->dev->bus->busnum);
+ kcov_remote_start_usb_softirq((u64)urb->dev->bus->busnum);
urb->complete(urb);
- kcov_remote_stop_softirq(flags);
+ kcov_remote_stop_softirq();

usb_anchor_resume_wakeups(anchor);
atomic_dec(&urb->use_count);
diff --git a/include/linux/kcov.h b/include/linux/kcov.h
index 75a2fb8b16c32..0143358874b07 100644
--- a/include/linux/kcov.h
+++ b/include/linux/kcov.h
@@ -57,47 +57,21 @@ static inline void kcov_remote_start_usb(u64 id)

/*
* The softirq flavor of kcov_remote_*() functions is introduced as a temporary
- * workaround for KCOV's lack of nested remote coverage sections support.
- *
- * Adding support is tracked in https://bugzilla.kernel.org/show_bug.cgi?id=210337.
- *
- * kcov_remote_start_usb_softirq():
- *
- * 1. Only collects coverage when called in the softirq context. This allows
- * avoiding nested remote coverage collection sections in the task context.
- * For example, USB/IP calls usb_hcd_giveback_urb() in the task context
- * within an existing remote coverage collection section. Thus, KCOV should
- * not attempt to start collecting coverage within the coverage collection
- * section in __usb_hcd_giveback_urb() in this case.
- *
- * 2. Disables interrupts for the duration of the coverage collection section.
- * This allows avoiding nested remote coverage collection sections in the
- * softirq context (a softirq might occur during the execution of a work in
- * the BH workqueue, which runs with in_serving_softirq() > 0).
- * For example, usb_giveback_urb_bh() runs in the BH workqueue with
- * interrupts enabled, so __usb_hcd_giveback_urb() might be interrupted in
- * the middle of its remote coverage collection section, and the interrupt
- * handler might invoke __usb_hcd_giveback_urb() again.
+ * work around for kcov's lack of nested remote coverage sections support in
+ * task context. Adding support for nested sections is tracked in:
+ * https://bugzilla.kernel.org/show_bug.cgi?id=210337
*/

-static inline unsigned long kcov_remote_start_usb_softirq(u64 id)
+static inline void kcov_remote_start_usb_softirq(u64 id)
{
- unsigned long flags = 0;
-
- if (in_serving_softirq()) {
- local_irq_save(flags);
+ if (in_serving_softirq() && !in_hardirq())
kcov_remote_start_usb(id);
- }
-
- return flags;
}

-static inline void kcov_remote_stop_softirq(unsigned long flags)
+static inline void kcov_remote_stop_softirq(void)
{
- if (in_serving_softirq()) {
+ if (in_serving_softirq() && !in_hardirq())
kcov_remote_stop();
- local_irq_restore(flags);
- }
}

#ifdef CONFIG_64BIT
@@ -131,11 +105,8 @@ static inline u64 kcov_common_handle(void)
}
static inline void kcov_remote_start_common(u64 id) {}
static inline void kcov_remote_start_usb(u64 id) {}
-static inline unsigned long kcov_remote_start_usb_softirq(u64 id)
-{
- return 0;
-}
-static inline void kcov_remote_stop_softirq(unsigned long flags) {}
+static inline void kcov_remote_start_usb_softirq(u64 id) {}
+static inline void kcov_remote_stop_softirq(void) {}

#endif /* CONFIG_KCOV */
#endif /* _LINUX_KCOV_H */
--
2.50.1

Sebastian

[Yunseong Kim]

Hi Sebastian,

I was waiting for your review — thanks!

Thank you for the detailed analysis and the patch. Your explanation about
the real re-entrancy issue being "softirq vs. hardirq" and the faulty
premise in the original commit makes perfect sense.

> Could someone maybe test this?

As you requested, I have tested your patch on my setup.

I can check that your patch resolves the issue. I have been running
the syzkaller for several hours, and the "sleeping function called
from invalid context" bug is no longer triggered.

I really impressed your "How to Not Break PREEMPT_RT" talk at LPC 22.


Tested-by: Yunseong Kim <y...@kzalloc.com>


Thanks,

Yunseong Kim

[Sebastian Andrzej Siewior]

On 2025-08-09 02:35:48 [+0900], Yunseong Kim wrote:
> Hi Sebastian,
Hi Yunseong,

> > Could someone maybe test this?
>
> As you requested, I have tested your patch on my setup.
>
> I can check that your patch resolves the issue. I have been running
> the syzkaller for several hours, and the "sleeping function called
> from invalid context" bug is no longer triggered.

Thank you. I just sent this as a proper patch assuming kcov still does
what it should. I just don't understand why this triggers after moving
to workqueues and did not with the tasklet setup. Other that than
workqueue code has a bit more overhead, it is the same thing.

> I really impressed your "How to Not Break PREEMPT_RT" talk at LPC 22.

Thank you.

>
> Tested-by: Yunseong Kim <y...@kzalloc.com>
>
>
> Thanks,
>
> Yunseong Kim

Sebastian