[syzbot] [sound?] KCSAN: data-race in snd_seq_poll / snd_seq_pool_init

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 48a5eed9ad58 Merge tag 'devicetree-fixes-for-6.14-2' of gi..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1657d8b7980000
kernel config: https://syzkaller.appspot.com/x/.config?x=523b0e2f15224775
dashboard link: https://syzkaller.appspot.com/bug?extid=2d373c9936c00d7e120c
compiler: Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/877a63ac53f2/disk-48a5eed9.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/7b3a614d4df6/vmlinux-48a5eed9.xz
kernel image: https://storage.googleapis.com/syzbot-assets/2ce987fd9e0c/bzImage-48a5eed9.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+2d373c...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in snd_seq_poll / snd_seq_pool_init

write to 0xffff888114535610 of 4 bytes by task 7006 on cpu 1:
snd_seq_pool_init+0x1c1/0x200 sound/core/seq/seq_memory.c:469
snd_seq_write+0x17f/0x500 sound/core/seq/seq_clientmgr.c:1022
vfs_write+0x27d/0x920 fs/read_write.c:677
ksys_write+0xe8/0x1b0 fs/read_write.c:731
__do_sys_write fs/read_write.c:742 [inline]
__se_sys_write fs/read_write.c:739 [inline]
__x64_sys_write+0x42/0x50 fs/read_write.c:739
x64_sys_call+0x287e/0x2dc0 arch/x86/include/generated/asm/syscalls_64.h:2
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xc9/0x1c0 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888114535610 of 4 bytes by task 7005 on cpu 0:
snd_seq_total_cells sound/core/seq/seq_memory.h:83 [inline]
snd_seq_write_pool_allocated sound/core/seq/seq_clientmgr.c:95 [inline]
snd_seq_poll+0x103/0x170 sound/core/seq/seq_clientmgr.c:1139
vfs_poll include/linux/poll.h:82 [inline]
__io_arm_poll_handler+0x1e5/0xd50 io_uring/poll.c:582
io_arm_poll_handler+0x464/0x5b0 io_uring/poll.c:707
io_queue_async+0x89/0x320 io_uring/io_uring.c:1925
io_queue_sqe io_uring/io_uring.c:1954 [inline]
io_req_task_submit+0xb9/0xc0 io_uring/io_uring.c:1373
io_handle_tw_list+0x1b9/0x200 io_uring/io_uring.c:1059
tctx_task_work_run+0x6e/0x1c0 io_uring/io_uring.c:1123
tctx_task_work+0x40/0x80 io_uring/io_uring.c:1141
task_work_run+0x13a/0x1a0 kernel/task_work.c:227
get_signal+0xe78/0x1000 kernel/signal.c:2809
arch_do_signal_or_restart+0x95/0x4b0 arch/x86/kernel/signal.c:337
exit_to_user_mode_loop kernel/entry/common.c:111 [inline]
exit_to_user_mode_prepare include/linux/entry-common.h:329 [inline]
__syscall_exit_to_user_mode_work kernel/entry/common.c:207 [inline]
syscall_exit_to_user_mode+0x62/0x120 kernel/entry/common.c:218
do_syscall_64+0xd6/0x1c0 arch/x86/entry/common.c:89
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x00000000 -> 0x000001f4

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 7005 Comm: syz.2.1455 Tainted: G W 6.14.0-rc5-syzkaller-00016-g48a5eed9ad58 #0
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2025
==================================================================


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[Lizhi Xu]

syzbot reported a data-race in snd_seq_poll / snd_seq_pool_init. [1]

Just use atomic_set/atomic_read for handling this case.

[1]

Reported-by: syzbot+2d373c...@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=2d373c9936c00d7e120c
Signed-off-by: Lizhi Xu <lizh...@windriver.com>
---
sound/core/seq/seq_memory.c | 12 ++++++------
sound/core/seq/seq_memory.h | 6 +++---
2 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/sound/core/seq/seq_memory.c b/sound/core/seq/seq_memory.c
index 20155e3e87c6..4a5c771ce8f9 100644
--- a/sound/core/seq/seq_memory.c
+++ b/sound/core/seq/seq_memory.c
@@ -21,7 +21,7 @@

static inline int snd_seq_pool_available(struct snd_seq_pool *pool)
{
- return pool->total_elements - atomic_read(&pool->counter);
+ return atomic_read(&pool->total_elements) - atomic_read(&pool->counter);
}

static inline int snd_seq_output_ok(struct snd_seq_pool *pool)
@@ -353,7 +353,7 @@ int snd_seq_event_dup(struct snd_seq_pool *pool, struct snd_seq_event *event,
extlen = event->data.ext.len & ~SNDRV_SEQ_EXT_MASK;
ncells = DIV_ROUND_UP(extlen, sizeof(struct snd_seq_event));
}
- if (ncells >= pool->total_elements)
+ if (ncells >= atomic_read(&pool->total_elements))
return -ENOMEM;

err = snd_seq_cell_alloc(pool, &cell, nonblock, file, mutexp);
@@ -466,7 +466,7 @@ int snd_seq_pool_init(struct snd_seq_pool *pool)

/* init statistics */
pool->max_used = 0;
- pool->total_elements = pool->size;
+ atomic_set(&pool->total_elements, pool->size);
return 0;
}

@@ -499,7 +499,7 @@ int snd_seq_pool_done(struct snd_seq_pool *pool)
ptr = pool->ptr;
pool->ptr = NULL;
pool->free = NULL;
- pool->total_elements = 0;
+ atomic_set(&pool->total_elements, 0);
}

kvfree(ptr);
@@ -523,7 +523,7 @@ struct snd_seq_pool *snd_seq_pool_new(int poolsize)
spin_lock_init(&pool->lock);
pool->ptr = NULL;
pool->free = NULL;
- pool->total_elements = 0;
+ atomic_set(&pool->total_elements, 0);
atomic_set(&pool->counter, 0);
pool->closing = 0;
init_waitqueue_head(&pool->output_sleep);
@@ -555,7 +555,7 @@ void snd_seq_info_pool(struct snd_info_buffer *buffer,
{
if (pool == NULL)
return;
- snd_iprintf(buffer, "%sPool size : %d\n", space, pool->total_elements);
+ snd_iprintf(buffer, "%sPool size : %d\n", space, atomic_read(&pool->total_elements));
snd_iprintf(buffer, "%sCells in use : %d\n", space, atomic_read(&pool->counter));
snd_iprintf(buffer, "%sPeak cells in use : %d\n", space, pool->max_used);
snd_iprintf(buffer, "%sAlloc success : %d\n", space, pool->event_alloc_success);
diff --git a/sound/core/seq/seq_memory.h b/sound/core/seq/seq_memory.h
index 7f7a2c0b187d..812edbe8cee0 100644
--- a/sound/core/seq/seq_memory.h
+++ b/sound/core/seq/seq_memory.h
@@ -44,7 +44,7 @@ struct snd_seq_pool {
struct snd_seq_event_cell *ptr; /* pointer to first event chunk */
struct snd_seq_event_cell *free; /* pointer to the head of the free list */

- int total_elements; /* pool size actually allocated */
+ atomic_t total_elements; /* pool size actually allocated */
atomic_t counter; /* cells free */

int size; /* pool size to be allocated */
@@ -74,13 +74,13 @@ int snd_seq_event_dup(struct snd_seq_pool *pool, struct snd_seq_event *event,
/* return number of unused (free) cells */
static inline int snd_seq_unused_cells(struct snd_seq_pool *pool)
{
- return pool ? pool->total_elements - atomic_read(&pool->counter) : 0;
+ return pool ? atomic_read(&pool->total_elements) - atomic_read(&pool->counter) : 0;
}

/* return total number of allocated cells */
static inline int snd_seq_total_cells(struct snd_seq_pool *pool)
{
- return pool ? pool->total_elements : 0;
+ return pool ? atomic_read(&pool->total_elements) : 0;
}

/* init pool - allocate events */
--
2.43.0

[Takashi Iwai]

This is harmless as it's only a reference in poll() and that's rather
volatile. So changing the whole with atomic_t is an overkill only for
that.

OTOH, the check of pool size in the caller side is fragile, and it can
purely rely on snd_seq_pool_poll_wait(). And, there, it should take
the pool->lock for the data consistency.

So, if any, an alternative fix would be something like below.


thanks,

Takashi

-- 8< --
--- a/sound/core/seq/seq_clientmgr.c
+++ b/sound/core/seq/seq_clientmgr.c
@@ -1150,8 +1150,7 @@ static __poll_t snd_seq_poll(struct file *file, poll_table * wait)
if (snd_seq_file_flags(file) & SNDRV_SEQ_LFLG_OUTPUT) {

/* check if data is available in the pool */
- if (!snd_seq_write_pool_allocated(client) ||
- snd_seq_pool_poll_wait(client->pool, file, wait))
+ if (snd_seq_pool_poll_wait(client->pool, file, wait))
mask |= EPOLLOUT | EPOLLWRNORM;
}

--- a/sound/core/seq/seq_memory.c
+++ b/sound/core/seq/seq_memory.c
@@ -427,6 +427,7 @@ int snd_seq_pool_poll_wait(struct snd_seq_pool *pool, struct file *file,
poll_table *wait)
{
poll_wait(file, &pool->output_sleep, wait);
+ guard(spinlock_irq)(&pool->lock);
return snd_seq_output_ok(pool);
}