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ALSA: seq: Break too long mutex context in the write loop

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The fix for the racy writes and ioctls to sequencer widened the
application of client->ioctl_mutex to the whole write loop.  Although
it does unlock/relock for the lengthy operation like the event dup,
the loop keeps the ioctl_mutex for the whole time in other
situations.  This may take quite long time if the user-space would
give a huge buffer, and this is a likely cause of some weird behavior
spotted by syzcaller fuzzer.

This patch puts a simple workaround, just adding a mutex break in the
loop when a large number of events have been processed.  This
shouldn't hit any performance drop because the threshold is set high
enough for usual operations.

Fixes: 7bd8009156 ("ALSA: seq: More protection for concurrent write and ioctl races")
Reported-by: syzbot+97aae04ce27e39cbfca9@syzkaller.appspotmail.com
Reported-by: syzbot+4c595632b98bb8ffcc66@syzkaller.appspotmail.com
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
This commit is contained in:
Takashi Iwai 2019-07-15 22:50:27 +02:00
parent 4b4e0e32e4
commit ede34f397d
1 changed files with 10 additions and 1 deletions
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11
sound/core/seq/seq_clientmgr.c
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@ -1021,7 +1021,7 @@ static ssize_t snd_seq_write(struct file *file, const char __user *buf,
{
struct snd_seq_client *client = file->private_data;
int written = 0, len;
int err;
int err, handled;
struct snd_seq_event event;
if (!(snd_seq_file_flags(file) & SNDRV_SEQ_LFLG_OUTPUT))
@ -1034,6 +1034,8 @@ static ssize_t snd_seq_write(struct file *file, const char __user *buf,
if (!client->accept_output || client->pool == NULL)
return -ENXIO;
repeat:
handled = 0;
/* allocate the pool now if the pool is not allocated yet */
mutex_lock(&client->ioctl_mutex);
if (client->pool->size > 0 && !snd_seq_write_pool_allocated(client)) {
@ -1093,12 +1095,19 @@ static ssize_t snd_seq_write(struct file *file, const char __user *buf,
0, 0, &client->ioctl_mutex);
if (err < 0)
break;
handled++;
__skip_event:
/* Update pointers and counts */
count -= len;
buf += len;
written += len;
/* let's have a coffee break if too many events are queued */
if (++handled >= 200) {
mutex_unlock(&client->ioctl_mutex);
goto repeat;
}
}
out: