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ALSA: seq: Improve data consistency at polling

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snd_seq_poll() calls snd_seq_write_pool_allocated() that reads out a
field in client->pool object, while it can be updated concurrently via
ioctls, as reported by syzbot.  The data race itself is harmless, as
it's merely a poll() call, and the state is volatile.  OTOH, the read
out of poll object info from the caller side is fragile, and we can
leave it better in snd_seq_pool_poll_wait() alone.

A similar pattern is seen in snd_seq_kernel_client_write_poll(), too,
which is called from the OSS sequencer.

This patch drops the pool checks from the caller side and add the
pool->lock in snd_seq_pool_poll_wait() for better data consistency.

Reported-by: syzbot+2d373c9936c00d7e120c@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/67c88903.050a0220.15b4b9.0028.GAE@google.com
Link: https://patch.msgid.link/20250307084246.29271-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>
This commit is contained in:
Takashi Iwai 2025-03-07 09:42:42 +01:00
parent aa85822c61
commit e3cd33ab17
2 changed files with 2 additions and 4 deletions
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5
sound/core/seq/seq_clientmgr.c
View file

@ -1130,8 +1130,7 @@ 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;
}
@ -2566,8 +2565,6 @@ int snd_seq_kernel_client_write_poll(int clientid, struct file *file, poll_table
if (client == NULL)
return -ENXIO;
if (! snd_seq_write_pool_allocated(client))
return 1;
if (snd_seq_pool_poll_wait(client->pool, file, wait))
return 1;
return 0;

1
sound/core/seq/seq_memory.c
View file

@ -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);
}