A divide error bug in snd_pcm_write

26 Sep 2022


      Hi, there is a divide error bug in snd_pcm_write in
sound/core/pcm_native.c in the latest kernel.
##Root Cause
When open the device of /dev/snd/pcmC#D#p, there would attach a
runtime to pcm->substream via snd_pcm_open_substream. see the code
below:
```
int snd_pcm_attach_substream(struct snd_pcm *pcm, int stream,
    struct file *file,
    struct snd_pcm_substream **rsubstream)
{
......
runtime = kzalloc(sizeof(*runtime), GFP_KERNEL);
if (runtime == NULL)
return -ENOMEM;
size = PAGE_ALIGN(sizeof(struct snd_pcm_mmap_status));
runtime->status = alloc_pages_exact(size, GFP_KERNEL);
if (runtime->status == NULL) {
kfree(runtime);
return -ENOMEM;
}
memset(runtime->status, 0, size);
size = PAGE_ALIGN(sizeof(struct snd_pcm_mmap_control));
runtime->control = alloc_pages_exact(size, GFP_KERNEL);
if (runtime->control == NULL) {
free_pages_exact(runtime->status,
      PAGE_ALIGN(sizeof(struct snd_pcm_mmap_status)));
kfree(runtime);
return -ENOMEM;
}
memset(runtime->control, 0, size);
init_waitqueue_head(&runtime->sleep);
init_waitqueue_head(&runtime->tsleep);
runtime->status->state = SNDRV_PCM_STATE_OPEN;
mutex_init(&runtime->buffer_mutex);
atomic_set(&runtime->buffer_accessing, 0);
substream->runtime = runtime;
substream->private_data = pcm->private_data;
substream->ref_count = 1;
substream->f_flags = file->f_flags;
substream->pid = get_pid(task_pid(current));
pstr->substream_opened++;
*rsubstream = substream;
return 0;
}
```
It would kzmalloc a new runtime. And initialize runtime simply.
If we write some data to the device. it would call snd_pcm_write or
snd_pcm_writev. and read some date from the device, it would call
snd_pcm_read or snd_pcm_readv.
Anyway, the four function would use data of runtime, but some data of
runtime is NULL not be initialized, see the code below:
```
static ssize_t snd_pcm_write(struct file *file, const char __user *buf,
    size_t count, loff_t * offset)
{
struct snd_pcm_file *pcm_file;
struct snd_pcm_substream *substream;
struct snd_pcm_runtime *runtime;
snd_pcm_sframes_t result;
pcm_file = file->private_data;
substream = pcm_file->substream;
if (PCM_RUNTIME_CHECK(substream))
return -ENXIO;
runtime = substream->runtime;
if (runtime->status->state == SNDRV_PCM_STATE_OPEN ||
   runtime->status->state == SNDRV_PCM_STATE_DISCONNECTED)
return -EBADFD;
if (!frame_aligned(runtime, count))    ///////  [1]
return -EINVAL;
count = bytes_to_frames(runtime, count);    /////// [2]
result = snd_pcm_lib_write(substream, buf, count);
if (result > 0)
result = frames_to_bytes(runtime, result);
return result;
}
```
[1] call frame_aligned to aligned.
```
static inline int frame_aligned(struct snd_pcm_runtime *runtime, ssize_t bytes)
{
return bytes % runtime->byte_align == 0;
}
```
but runtime->byte_align is NULL.
[2] call bytes_to_frames.
```
static inline ssize_t frames_to_bytes(struct snd_pcm_runtime *runtime,
snd_pcm_sframes_t size)
{
return size * runtime->frame_bits / 8;
}
```
but runtime->frame_bits is NULL.
##reproduce it
[ 1189.305083][ T4656] divide error: 0000 [#1] PREEMPT SMP
[ 1189.305600][ T4656] CPU: 1 PID: 4656 Comm: snd_pcm_write Not
tainted 6.0.0-rc7 #16
[ 1189.306157][ T4656] Hardware name: QEMU Standard PC (i440FX + PIIX,
1996), BIOS 1.13.0-1ubuntu1 04/01/2014
[ 1189.306760][ T4656] RIP: 0010:snd_pcm_write+0x33/0xa0
[ 1189.307155][ T4656] Code: 8b 38 48 85 ff 74 72 48 8b 9f c0 00 00 00
48 85 db 74 66 48 8b 83 00 01 00 00 f7 00 f7 ff ff ff 74 6b 48 89 d0
48 89 d1 31 d2 <48> f7 b3 91
[ 1189.308553][ T4656] RSP: 0018:ffffc9000adc7e68 EFLAGS: 00010246
[ 1189.309034][ T4656] RAX: 0000000000000000 RBX: ffff888048ec2000
RCX: 0000000000000000
[ 1189.309583][ T4656] RDX: 0000000000000000 RSI: 0000000000000000
RDI: ffff888046fc9c00
[ 1189.310163][ T4656] RBP: 0000000000000000 R08: 0000000000000000
R09: 0000000000020026
[ 1189.310679][ T4656] R10: 0000000000000001 R11: 0000000000000000
R12: 0000000000000000
[ 1189.311226][ T4656] R13: ffffc9000adc7f08 R14: 0000000000000000
R15: 0000000000000000
[ 1189.311754][ T4656] FS:  00000000012d8880(0000)
GS:ffff88807ec00000(0000) knlGS:0000000000000000
[ 1189.312350][ T4656] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1189.312780][ T4656] CR2: 0000000020000000 CR3: 000000004b496000
CR4: 00000000000006e0
[ 1189.313234][ T4656] Call Trace:
[ 1189.313424][ T4656]  <TASK>
[ 1189.313597][ T4656]  vfs_write+0xe6/0x4d0
[ 1189.313836][ T4656]  ksys_write+0x60/0xe0
[ 1189.314071][ T4656]  do_syscall_64+0x35/0xb0
[ 1189.314324][ T4656]  entry_SYSCALL_64_after_hwframe+0x63/0xcd
[ 1189.314734][ T4656] RIP: 0033:0x44dc3d
[ 1189.315007][ T4656] Code: 28 c3 e8 36 29 00 00 66 0f 1f 44 00 00 f3
0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b
4c 24 08 0f 05 <48> 3d 01 f8
[ 1189.316338][ T4656] RSP: 002b:00007ffcbdf5ef38 EFLAGS: 00000246
ORIG_RAX: 0000000000000001
[ 1189.317012][ T4656] RAX: ffffffffffffffda RBX: 0000000000400530
RCX: 000000000044dc3d
[ 1189.317693][ T4656] RDX: 0000000000000000 RSI: 0000000000000000
RDI: 0000000000000003
[ 1189.318172][ T4656] RBP: 00007ffcbdf5ef50 R08: 0000000082000000
R09: 0000000000000000
[ 1189.318648][ T4656] R10: 000000000000ffff R11: 0000000000000246
R12: 00000000004030a0
[ 1189.319182][ T4656] R13: 0000000000000000 R14: 00000000004c5018
R15: 0000000000000000
Regrads,
 butt3rflyh4ck.
-- 
Active Defense Lab of Venustech