[syzbot] [kernel?] WARNING: suspicious RCU usage in __do_softirq

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: c0b832517f62 Add linux-next specific files for 20240402
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=15f64776180000
kernel config: https://syzkaller.appspot.com/x/.config?x=afcaf46d374cec8c
dashboard link: https://syzkaller.appspot.com/bug?extid=dce04ed6d1438ad69656
compiler: Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=10f00471180000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/0d36ec76edc7/disk-c0b83251.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/6f9bb4e37dd0/vmlinux-c0b83251.xz
kernel image: https://storage.googleapis.com/syzbot-assets/2349287b14b7/bzImage-c0b83251.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+dce04e...@syzkaller.appspotmail.com

=============================
WARNING: suspicious RCU usage
6.9.0-rc2-next-20240402-syzkaller #0 Not tainted
-----------------------------
kernel/rcu/tree.c:276 Illegal rcu_softirq_qs() in RCU read-side critical section!

other info that might help us debug this:


rcu_scheduler_active = 2, debug_locks = 1
1 lock held by ksoftirqd/0/16:
#0: ffffffff8e334d20 (rcu_read_lock_sched){....}-{1:2}, at: rcu_lock_acquire include/linux/rcupdate.h:329 [inline]
#0: ffffffff8e334d20 (rcu_read_lock_sched){....}-{1:2}, at: rcu_read_lock_sched include/linux/rcupdate.h:933 [inline]
#0: ffffffff8e334d20 (rcu_read_lock_sched){....}-{1:2}, at: pfn_valid include/linux/mmzone.h:2019 [inline]
#0: ffffffff8e334d20 (rcu_read_lock_sched){....}-{1:2}, at: __virt_addr_valid+0x183/0x520 arch/x86/mm/physaddr.c:65

stack backtrace:
CPU: 0 PID: 16 Comm: ksoftirqd/0 Not tainted 6.9.0-rc2-next-20240402-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/27/2024
Call Trace:
<IRQ>
__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0x241/0x360 lib/dump_stack.c:114
lockdep_rcu_suspicious+0x221/0x340 kernel/locking/lockdep.c:6712
rcu_softirq_qs+0xd9/0x370 kernel/rcu/tree.c:273
__do_softirq+0x5fd/0x980 kernel/softirq.c:568
invoke_softirq kernel/softirq.c:428 [inline]
__irq_exit_rcu+0xf2/0x1c0 kernel/softirq.c:633
irq_exit_rcu+0x9/0x30 kernel/softirq.c:645
instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1043 [inline]
sysvec_apic_timer_interrupt+0xa6/0xc0 arch/x86/kernel/apic/apic.c:1043
</IRQ>
<TASK>
asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:702
RIP: 0010:debug_lockdep_rcu_enabled+0xd/0x40 kernel/rcu/update.c:320
Code: f5 90 0f 0b 90 90 90 eb c6 0f 1f 40 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa 31 c0 83 3d c7 0f 28 04 00 <74> 1e 83 3d 26 42 28 04 00 74 15 65 48 8b 0c 25 c0 d3 03 00 31 c0
RSP: 0018:ffffc90000157a50 EFLAGS: 00000202
RAX: 0000000000000000 RBX: 00000000000000a0 RCX: 0000000000000001
RDX: dffffc0000000000 RSI: ffffffff8bcae740 RDI: ffffffff8c1f7ec0
RBP: dffffc0000000000 R08: ffffffff92f3a527 R09: 1ffffffff25e74a4
R10: dffffc0000000000 R11: fffffbfff25e74a5 R12: 0000000029373578
R13: 1ffff9200002af64 R14: ffffffff814220f3 R15: ffff88813fff90a0
rcu_read_lock_sched include/linux/rcupdate.h:934 [inline]
pfn_valid include/linux/mmzone.h:2019 [inline]
__virt_addr_valid+0x1a9/0x520 arch/x86/mm/physaddr.c:65
kasan_addr_to_slab+0xd/0x80 mm/kasan/common.c:37
__kasan_record_aux_stack+0x11/0xc0 mm/kasan/generic.c:526
__call_rcu_common kernel/rcu/tree.c:3096 [inline]
call_rcu+0x167/0xa70 kernel/rcu/tree.c:3200
context_switch kernel/sched/core.c:5412 [inline]
__schedule+0x17f0/0x4a50 kernel/sched/core.c:6746
__schedule_loop kernel/sched/core.c:6823 [inline]
schedule+0x14b/0x320 kernel/sched/core.c:6838
smpboot_thread_fn+0x61e/0xa30 kernel/smpboot.c:160
kthread+0x2f0/0x390 kernel/kthread.c:388
ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:147
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:243
</TASK>
----------------
Code disassembly (best guess):
0: f5 cmc
1: 90 nop
2: 0f 0b ud2
4: 90 nop
5: 90 nop
6: 90 nop
7: eb c6 jmp 0xffffffcf
9: 0f 1f 40 00 nopl 0x0(%rax)
d: 90 nop
e: 90 nop
f: 90 nop
10: 90 nop
11: 90 nop
12: 90 nop
13: 90 nop
14: 90 nop
15: 90 nop
16: 90 nop
17: 90 nop
18: 90 nop
19: 90 nop
1a: 90 nop
1b: 90 nop
1c: 90 nop
1d: f3 0f 1e fa endbr64
21: 31 c0 xor %eax,%eax
23: 83 3d c7 0f 28 04 00 cmpl $0x0,0x4280fc7(%rip) # 0x4280ff1
* 2a: 74 1e je 0x4a <-- trapping instruction
2c: 83 3d 26 42 28 04 00 cmpl $0x0,0x4284226(%rip) # 0x4284259
33: 74 15 je 0x4a
35: 65 48 8b 0c 25 c0 d3 mov %gs:0x3d3c0,%rcx
3c: 03 00
3e: 31 c0 xor %eax,%eax


---
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[syzbot]

syzbot has found a reproducer for the following issue on:

HEAD commit: 6bd343537461 Add linux-next specific files for 20240415
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=132b923b180000
kernel config: https://syzkaller.appspot.com/x/.config?x=c247afaa437e6409

dashboard link: https://syzkaller.appspot.com/bug?extid=dce04ed6d1438ad69656
compiler: Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40

syz repro: https://syzkaller.appspot.com/x/repro.syz?x=11e7ff77180000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=125e7767180000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/aaf63ca280e5/disk-6bd34353.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/353fdc5b9ee6/vmlinux-6bd34353.xz
kernel image: https://storage.googleapis.com/syzbot-assets/25a7a362b023/bzImage-6bd34353.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+dce04e...@syzkaller.appspotmail.com

=============================
WARNING: suspicious RCU usage

6.9.0-rc4-next-20240415-syzkaller #0 Not tainted

-----------------------------
kernel/rcu/tree.c:276 Illegal rcu_softirq_qs() in RCU read-side critical section!

other info that might help us debug this:


rcu_scheduler_active = 2, debug_locks = 1
1 lock held by ksoftirqd/0/16:

#0: ffffffff8e334160

(rcu_read_lock_sched){....}-{1:2}, at: rcu_lock_acquire include/linux/rcupdate.h:329 [inline]

(rcu_read_lock_sched){....}-{1:2}, at: rcu_read_lock_sched include/linux/rcupdate.h:933 [inline]

(rcu_read_lock_sched){....}-{1:2}, at: pfn_valid include/linux/mmzone.h:2019 [inline]

(rcu_read_lock_sched){....}-{1:2}, at: __virt_addr_valid+0x183/0x520 arch/x86/mm/physaddr.c:65

stack backtrace:

CPU: 0 PID: 16 Comm: ksoftirqd/0 Not tainted 6.9.0-rc4-next-20240415-syzkaller #0

Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/27/2024
Call Trace:
<IRQ>
__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0x241/0x360 lib/dump_stack.c:114
lockdep_rcu_suspicious+0x221/0x340 kernel/locking/lockdep.c:6712
rcu_softirq_qs+0xd9/0x370 kernel/rcu/tree.c:273
__do_softirq+0x5fd/0x980 kernel/softirq.c:568
invoke_softirq kernel/softirq.c:428 [inline]
__irq_exit_rcu+0xf2/0x1c0 kernel/softirq.c:633
irq_exit_rcu+0x9/0x30 kernel/softirq.c:645
instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1043 [inline]
sysvec_apic_timer_interrupt+0xa6/0xc0 arch/x86/kernel/apic/apic.c:1043
</IRQ>
<TASK>
asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:702

RIP: 0010:lock_acquire+0x264/0x550 kernel/locking/lockdep.c:5758
Code: 2b 00 74 08 4c 89 f7 e8 8a fd 88 00 f6 44 24 61 02 0f 85 85 01 00 00 41 f7 c7 00 02 00 00 74 01 fb 48 c7 44 24 40 0e 36 e0 45 <4b> c7 44 25 00 00 00 00 00 43 c7 44 25 09 00 00 00 00 43 c7 44 25
RSP: 0018:ffffc90000157820 EFLAGS: 00000206
RAX: 0000000000000001 RBX: 1ffff9200002af10 RCX: 0000000000000001
RDX: dffffc0000000000 RSI: ffffffff8bcac4e0 RDI: ffffffff8c1f8ee0
RBP: ffffc90000157980 R08: ffffffff92f765a7 R09: 1ffffffff25eecb4
R10: dffffc0000000000 R11: fffffbfff25eecb5 R12: 1ffff9200002af0c
R13: dffffc0000000000 R14: ffffc90000157880 R15: 0000000000000246
rcu_lock_acquire include/linux/rcupdate.h:329 [inline]
rcu_read_lock_sched include/linux/rcupdate.h:933 [inline]
pfn_valid include/linux/mmzone.h:2019 [inline]
__virt_addr_valid+0x1a0/0x520 arch/x86/mm/physaddr.c:65

kasan_addr_to_slab+0xd/0x80 mm/kasan/common.c:37
__kasan_record_aux_stack+0x11/0xc0 mm/kasan/generic.c:526

__call_rcu_common kernel/rcu/tree.c:3102 [inline]
call_rcu+0x167/0xa70 kernel/rcu/tree.c:3206

context_switch kernel/sched/core.c:5412 [inline]
__schedule+0x17f0/0x4a50 kernel/sched/core.c:6746

preempt_schedule_common+0x84/0xd0 kernel/sched/core.c:6925
preempt_schedule+0xe1/0xf0 kernel/sched/core.c:6949
preempt_schedule_thunk+0x1a/0x30 arch/x86/entry/thunk.S:12
smpboot_thread_fn+0x65b/0xa30 kernel/smpboot.c:163
kthread+0x2f0/0x390 kernel/kthread.c:389
ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:147
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244

</TASK>
----------------
Code disassembly (best guess):

0: 2b 00 sub (%rax),%eax
2: 74 08 je 0xc
4: 4c 89 f7 mov %r14,%rdi
7: e8 8a fd 88 00 call 0x88fd96
c: f6 44 24 61 02 testb $0x2,0x61(%rsp)
11: 0f 85 85 01 00 00 jne 0x19c
17: 41 f7 c7 00 02 00 00 test $0x200,%r15d
1e: 74 01 je 0x21
20: fb sti
21: 48 c7 44 24 40 0e 36 movq $0x45e0360e,0x40(%rsp)
28: e0 45
* 2a: 4b c7 44 25 00 00 00 movq $0x0,0x0(%r13,%r12,1) <-- trapping instruction
31: 00 00
33: 43 c7 44 25 09 00 00 movl $0x0,0x9(%r13,%r12,1)
3a: 00 00
3c: 43 rex.XB
3d: c7 .byte 0xc7
3e: 44 rex.R
3f: 25 .byte 0x25


---

[Z qiang]

Cc: Paul

This should be caused by the following commit:
d818cc76e2b4 ("kasan: Record work creation stack trace with interrupts enabled")

Is it possible to make the rcu_softirq_qs() run only in ksoftirqd task?

Thanks
Zqiang

[Z qiang]

use rcu_softirq_qs_periodic() in run_ksoftirqd().

[Paul E. McKenney]

Huh. This statement is supposed to prevent this call to __do_softirq()
from interrupt exit::

if (!IS_ENABLED(CONFIG_PREEMPT_RT) &&
__this_cpu_read(ksoftirqd) == current)

So was the ksoftirqd kthread interrupted at a point where it happens to
have softirq enabled?

Thanx, Paul

[Z qiang]

It should look like this:
schedule()
switch_to ksoftirqd/0
finish_task_switch
->put_task_struct_rcu_user
->call_rcu(&task->rcu, delayed_put_task_struct)
->__kasan_record_aux_stack
->pfn_valid
->rcu_read_lock_sched
<interrupt>
__irq_exit_rcu
->__do_softirq
-> if (!IS_ENABLED(CONFIG_PREEMPT_RT) &&
__this_cpu_read(ksoftirqd) == current)
rcu_softirq_qs
->
RCU_LOCKDEP_WARN(lock_is_held(&rcu_sched_lock_map))

Thanks
Zqiang

[Paul E. McKenney]

So this CPU's ksoftirqd task is running.

> ->put_task_struct_rcu_user
> ->call_rcu(&task->rcu, delayed_put_task_struct)
> ->__kasan_record_aux_stack
> ->pfn_valid
> ->rcu_read_lock_sched
> <interrupt>
> __irq_exit_rcu
> ->__do_softirq
> -> if (!IS_ENABLED(CONFIG_PREEMPT_RT) &&
> __this_cpu_read(ksoftirqd) == current)

But we are also running __do_softirq() from return from interrupt. While
running in this mode, we are not supposed to invoke rcu_softirq_qs().
But the "__this_cpu_read(ksoftirqd) == current" check yields "true",
so we do call rcu_softirq_qs() anyway. That is a bug.

We need to upgrade or replace that check to something that returns true
only if called at process level from ksoftirqd.

Any thoughts on a good way to do that? For example, would adding "&&
in_task()" do the trick, or are there other unfortunate corner cases?

(And good job tracking this down, by the way!)

Thanx, Paul

[Z qiang]

The rcu_softirq_qs() is invoked in softirq_handle_begin/end() critical section,
in softirqd/0 task context, the "in_task()" should return false, will miss
qs report in softirqd/0 task context.

diff --git a/kernel/softirq.c b/kernel/softirq.c
index b315b21fb28c..9b8f0c0f7675 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -563,10 +563,6 @@ asmlinkage __visible void __softirq_entry
__do_softirq(void)
pending >>= softirq_bit;
}

- if (!IS_ENABLED(CONFIG_PREEMPT_RT) &&
- __this_cpu_read(ksoftirqd) == current)
- rcu_softirq_qs();
-
local_irq_disable();

pending = local_softirq_pending();
@@ -915,6 +911,8 @@ static int ksoftirqd_should_run(unsigned int cpu)

static void run_ksoftirqd(unsigned int cpu)
{
+ unsigned long last_qs = jiffies;
+
ksoftirqd_run_begin();
if (local_softirq_pending()) {
/*
@@ -923,6 +921,7 @@ static void run_ksoftirqd(unsigned int cpu)
*/
__do_softirq();
ksoftirqd_run_end();
+ rcu_softirq_qs_periodic(last_qs);
cond_resched();
return;
}

Any thoughts?

Thanks
Zqiang

[Paul E. McKenney]

Unfortunately, we need the quiescent state to be within __do_softirq().

> cond_resched();
> return;
> }
>
> Any thoughts?

Can we mask the return value from preempt_count(), for example, as shown
in the CONFIG_PREEMPTION=n version of rcu_flavor_sched_clock_irq()?

The trick is that we should be able to ignore SOFTIRQ_MASK because
__do_softirq() should not be invoked when softirqs are disabled.
Emphasis on "should". ;-)

Thanx, Paul

[Z qiang]

Does it mean this?

diff --git a/kernel/softirq.c b/kernel/softirq.c
index b315b21fb28c..315b717ec944 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -517,6 +517,8 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
bool in_hardirq;
__u32 pending;
int softirq_bit;
+ bool preempt_enabled = (IS_ENABLED(CONFIG_PREEMPT_COUNT) &&
+ !(preempt_count() & PREEMPT_MASK));

/*
* Mask out PF_MEMALLOC as the current task context is borrowed for the
@@ -564,7 +566,8 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)

}

if (!IS_ENABLED(CONFIG_PREEMPT_RT) &&
- __this_cpu_read(ksoftirqd) == current)

+ __this_cpu_read(ksoftirqd) == current &&
+ preempt_enabled && !rcu_preempt_depth())
rcu_softirq_qs();

local_irq_disable();

For built with CONFIG_PREEMPTION=n and CONFIG_COUNT=n kernels,
the preempt_enabled is always false.

or how about this?

diff --git a/kernel/softirq.c b/kernel/softirq.c
index b315b21fb28c..80b9fb4ee562 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -66,6 +66,8 @@ const char * const softirq_to_name[NR_SOFTIRQS] = {
"TASKLET", "SCHED", "HRTIMER", "RCU"
};

+static DEFINE_PER_CPU(bool, ksoftirqd_work);
+
/*
* we cannot loop indefinitely here to avoid userspace starvation,
* but we also don't want to introduce a worst case 1/HZ latency
@@ -404,10 +406,12 @@ static inline void softirq_handle_end(void)
static inline void ksoftirqd_run_begin(void)
{
local_irq_disable();
+ per_cpu(ksoftirqd_work, smp_processor_id()) = true;
}

static inline void ksoftirqd_run_end(void)
{
+ per_cpu(ksoftirqd_work, smp_processor_id()) = false;
local_irq_enable();
}

@@ -564,7 +568,7 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)

}

if (!IS_ENABLED(CONFIG_PREEMPT_RT) &&
- __this_cpu_read(ksoftirqd) == current)

+ __this_cpu_read(ksoftirqd) == current && ksoftirqd_work)
rcu_softirq_qs();

local_irq_disable();
@@ -970,6 +974,10 @@ static struct smp_hotplug_thread softirq_threads = {

static __init int spawn_ksoftirqd(void)
{
+ int cpu;
+
+ for_each_possible_cpu(cpu)
+ per_cpu(ksoftirqd_work, cpu) = false;
cpuhp_setup_state_nocalls(CPUHP_SOFTIRQ_DEAD, "softirq:dead", NULL,
takeover_tasklets);
BUG_ON(smpboot_register_percpu_thread(&softirq_threads));


Thanks
Zqiang

[Z qiang]

use
__this_cpu_read(ksoftirqd_work)

[Paul E. McKenney]

Assuming that this works, it is preferred over the next one.

Thanx, Paul

[Z qiang]

But for built with CONFIG_PREEMPTION=n and CONFIG_COUNT=n kernels,
will miss invoke rcu_softirq_qs(), even if in the ksoftirqd task context.

Thanks
Zqiang

[Thomas Gleixner]

On Fri, Apr 19 2024 at 13:50, Z qiang wrote:
>> On Thu, Apr 18, 2024 at 05:49:38PM +0800, Z qiang wrote:
> static __init int spawn_ksoftirqd(void)
> {
> + int cpu;
> +
> + for_each_possible_cpu(cpu)
> + per_cpu(ksoftirqd_work, cpu) = false;

First of all that initialization is pointless, but why making all of
this complex as hell?

Thanks,

tglx
---
diff --git a/kernel/softirq.c b/kernel/softirq.c
index b315b21fb28c..e991d735be0d 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -508,7 +508,7 @@ static inline bool lockdep_softirq_start(void) { return false; }
static inline void lockdep_softirq_end(bool in_hardirq) { }
#endif

-asmlinkage __visible void __softirq_entry __do_softirq(void)
+static void handle_softirqs(bool kirqd)
{
unsigned long end = jiffies + MAX_SOFTIRQ_TIME;
unsigned long old_flags = current->flags;
@@ -563,8 +563,7 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)

pending >>= softirq_bit;
}

- if (!IS_ENABLED(CONFIG_PREEMPT_RT) &&
- __this_cpu_read(ksoftirqd) == current)

+ if (!IS_ENABLED(CONFIG_PREEMPT_RT) && kirqd)
rcu_softirq_qs();

local_irq_disable();
@@ -584,6 +583,11 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
current_restore_flags(old_flags, PF_MEMALLOC);
}

+asmlinkage __visible void __softirq_entry __do_softirq(void)
+{
+ handle_softirqs(false);
+}
+
/**
* irq_enter_rcu - Enter an interrupt context with RCU watching
*/
@@ -921,7 +925,7 @@ static void run_ksoftirqd(unsigned int cpu)
* We can safely run softirq on inline stack, as we are not deep
* in the task stack here.
*/
- __do_softirq();
+ handle_softirqs(true);
ksoftirqd_run_end();
cond_resched();
return;

[Z qiang]

Thanks, this is good for me :),
Paul, what do you think?

Thanks
Zqiang

> ksoftirqd_run_end();
> cond_resched();
> return;

[Paul E. McKenney]

This looks quite nice to me, especially given that it avoids changing
all the other calls to __do_softirq(). Some architectures might want
to directly call handle_softirqs(), but if so, they can send the patches.

Thanx, Paul

[Z qiang]

Hello, Thomas
Can you send an official patch? or I will send and add you to
Co-developed-by tags.

Thanks
Zqiang

[Thomas Gleixner]

On Fri, Apr 26 2024 at 10:09, Z qiang wrote:
>> This looks quite nice to me, especially given that it avoids changing
>> all the other calls to __do_softirq(). Some architectures might want
>> to directly call handle_softirqs(), but if so, they can send the patches.
>
>

> Can you send an official patch? or I will send and add you to
> Co-developed-by tags.

Just send it along with a nice changelog and add:

Suggested-by: Thomas Gleixner <tg...@linutronix.de>

That's good enough.