ALSA: seq: Fix racy pool initializations

ALSA sequencer core initializes the event pool on demand by invoking
snd_seq_pool_init() when the first write happens and the pool is
empty.  Meanwhile user can reset the pool size manually via ioctl
concurrently, and this may lead to UAF or out-of-bound accesses since
the function tries to vmalloc / vfree the buffer.

A simple fix is to just wrap the snd_seq_pool_init() call with the
recently introduced client->ioctl_mutex; as the calls for
snd_seq_pool_init() from other side are always protected with this
mutex, we can avoid the race.

Reported-by: 范龙飞 <long7573@126.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/core/seq/seq_clientmgr.c b/sound/core/seq/seq_clientmgr.c
index 60db32785f6229773fdf752a93e01f7bf42398eb..04d4db44fae5c9199754aa80066ae0c1220a41d5 100644 (file)
--- a/sound/core/seq/seq_clientmgr.c
+++ b/sound/core/seq/seq_clientmgr.c
@@ -1003,7 +1003,7 @@ 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 = -EINVAL;
+       int err;
        struct snd_seq_event event;
 
        if (!(snd_seq_file_flags(file) & SNDRV_SEQ_LFLG_OUTPUT))
@@ -1018,11 +1018,15 @@ static ssize_t snd_seq_write(struct file *file, const char __user *buf,
 
        /* allocate the pool now if the pool is not allocated yet */ 
        if (client->pool->size > 0 && !snd_seq_write_pool_allocated(client)) {
-               if (snd_seq_pool_init(client->pool) < 0)
+               mutex_lock(&client->ioctl_mutex);
+               err = snd_seq_pool_init(client->pool);
+               mutex_unlock(&client->ioctl_mutex);
+               if (err < 0)
                        return -ENOMEM;
        }
 
        /* only process whole events */
+       err = -EINVAL;
        while (count >= sizeof(struct snd_seq_event)) {
                /* Read in the event header from the user */
                len = sizeof(event);