linux-kernel - [PATCH AUTOSEL 6.6 107/294] ALSA: seq: Improve data consistency at polling

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Message-Id: <20250505225634.2688578-107-sashal@kernel.org>
Date: Mon,  5 May 2025 18:53:27 -0400
From: Sasha Levin <sashal@...nel.org>
To: linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Cc: Takashi Iwai <tiwai@...e.de>,
	syzbot+2d373c9936c00d7e120c@...kaller.appspotmail.com,
	Sasha Levin <sashal@...nel.org>,
	perex@...ex.cz,
	tiwai@...e.com,
	viro@...iv.linux.org.uk,
	linux-sound@...r.kernel.org
Subject: [PATCH AUTOSEL 6.6 107/294] ALSA: seq: Improve data consistency at polling

From: Takashi Iwai <tiwai@...e.de>

[ Upstream commit e3cd33ab17c33bd8f1a9df66ec83a15dd8f7afbb ]

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@...kaller.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@...e.de>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 sound/core/seq/seq_clientmgr.c | 5 +----
 sound/core/seq/seq_memory.c    | 1 +
 2 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/sound/core/seq/seq_clientmgr.c b/sound/core/seq/seq_clientmgr.c
index 6195fe9dda179..bc1338dc34851 100644
--- a/sound/core/seq/seq_clientmgr.c
+++ b/sound/core/seq/seq_clientmgr.c
@@ -1155,8 +1155,7 @@ 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;
 	}
 
@@ -2570,8 +2569,6 @@ 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;
diff --git a/sound/core/seq/seq_memory.c b/sound/core/seq/seq_memory.c
index b603bb93f8960..692860deec0c3 100644
--- a/sound/core/seq/seq_memory.c
+++ b/sound/core/seq/seq_memory.c
@@ -429,6 +429,7 @@ 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);
 }
 
-- 
2.39.5