Re: [PATCH] ALSA: seq: KASAN: use-after-free Read in delete_and_unsubscribe_port

[Takashi Iwai]

On Sat, 01 Aug 2020 08:24:03 +0200,
<qiang.zhang@xxxxxxxxxxxxx> wrote:
> 
> From: Zhang Qiang <qiang.zhang@xxxxxxxxxxxxx>
> 
> There is a potential race window,when a task acquire "src->list_mutex"
> write sem,traverse the linked list to find "subs" objects through
> parameter "info" in snd_seq_port_disconnect and then release this
> write sem, before this task acquire write sem again,this write sem
> may be acquired by another task, and get the same "subs" object through
> the same "info" before, it could happen "use-after-free" later, so a
> simple solution is to delete the object from the linked list when it
> is found.
> 
> BUG: KASAN: use-after-free in list_empty include/linux/list.h:282 [inline]
> BUG: KASAN: use-after-free in delete_and_unsubscribe_port+0x8b/0x450
> sound/core/seq/seq_ports.c:530
> Read of size 8 at addr ffff888098523060 by task syz-executor.0/7202
> 
> Call Trace:
>  __dump_stack lib/dump_stack.c:77 [inline]
>  dump_stack+0x1f0/0x31e lib/dump_stack.c:118
>  print_address_description+0x66/0x5a0 mm/kasan/report.c:383
>  __kasan_report mm/kasan/report.c:513 [inline]
>  kasan_report+0x132/0x1d0 mm/kasan/report.c:530
>  list_empty include/linux/list.h:282 [inline]
>  delete_and_unsubscribe_port+0x8b/0x450 sound/core/seq/seq_ports.c:530
>  snd_seq_port_disconnect+0x568/0x610 sound/core/seq/seq_ports.c:612
>  snd_seq_ioctl_unsubscribe_port+0x349/0x6c0 sound/core/seq/seq_clientmgr.c:1525
>  snd_seq_oss_midi_close+0x397/0x620 sound/core/seq/oss/seq_oss_midi.c:405
>  snd_seq_oss_synth_reset+0x335/0x8b0 sound/core/seq/oss/seq_oss_synth.c:406
>  snd_seq_oss_reset+0x5b/0x250 sound/core/seq/oss/seq_oss_init.c:435
>  snd_seq_oss_ioctl+0x5c2/0x1090 sound/core/seq/oss/seq_oss_ioctl.c:93
>  odev_ioctl+0x51/0x70 sound/core/seq/oss/seq_oss.c:174
>  vfs_ioctl fs/ioctl.c:48 [inline]
>  ksys_ioctl fs/ioctl.c:753 [inline]
>  __do_sys_ioctl fs/ioctl.c:762 [inline]
>  __se_sys_ioctl+0xf9/0x160 fs/ioctl.c:760
>  do_syscall_64+0x73/0xe0 arch/x86/entry/common.c:384
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> Allocated by task 7202:
>  save_stack mm/kasan/common.c:48 [inline]
>  set_track mm/kasan/common.c:56 [inline]
>  __kasan_kmalloc+0x103/0x140 mm/kasan/common.c:494
>  kmem_cache_alloc_trace+0x234/0x300 mm/slab.c:3551
>  kmalloc include/linux/slab.h:555 [inline]
>  kzalloc include/linux/slab.h:669 [inline]
>  snd_seq_port_connect+0x66/0x460 sound/core/seq/seq_ports.c:553
>  snd_seq_ioctl_subscribe_port+0x349/0x6c0 sound/core/seq/seq_clientmgr.c:1484
>  snd_seq_oss_midi_open+0x4db/0x830 sound/core/seq/oss/seq_oss_midi.c:364
>  snd_seq_oss_synth_setup_midi+0x108/0x510 sound/core/seq/oss/seq_oss_synth.c:269
>  snd_seq_oss_open+0x899/0xe90 sound/core/seq/oss/seq_oss_init.c:261
>  odev_open+0x5e/0x90 sound/core/seq/oss/seq_oss.c:125
>  chrdev_open+0x498/0x580 fs/char_dev.c:414
>  do_dentry_open+0x813/0x1070 fs/open.c:828
>  do_open fs/namei.c:3243 [inline]
>  path_openat+0x278d/0x37f0 fs/namei.c:3360
>  do_filp_open+0x191/0x3a0 fs/namei.c:3387
>  do_sys_openat2+0x463/0x770 fs/open.c:1179
>  do_sys_open fs/open.c:1195 [inline]
>  __do_sys_openat fs/open.c:1209 [inline]
>  __se_sys_openat fs/open.c:1204 [inline]
>  __x64_sys_openat+0x1c8/0x1f0 fs/open.c:1204
>  do_syscall_64+0x73/0xe0 arch/x86/entry/common.c:384
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> Freed by task 7203:
>  save_stack mm/kasan/common.c:48 [inline]
>  set_track mm/kasan/common.c:56 [inline]
>  kasan_set_free_info mm/kasan/common.c:316 [inline]
>  __kasan_slab_free+0x114/0x170 mm/kasan/common.c:455
>  __cache_free mm/slab.c:3426 [inline]
>  kfree+0x10a/0x220 mm/slab.c:3757
>  snd_seq_port_disconnect+0x570/0x610 sound/core/seq/seq_ports.c:614
>  snd_seq_ioctl_unsubscribe_port+0x349/0x6c0 sound/core/seq/seq_clientmgr.c:1525
>  snd_seq_oss_midi_close+0x397/0x620 sound/core/seq/oss/seq_oss_midi.c:405
>  snd_seq_oss_synth_reset+0x335/0x8b0 sound/core/seq/oss/seq_oss_synth.c:406
>  snd_seq_oss_reset+0x5b/0x250 sound/core/seq/oss/seq_oss_init.c:435
>  snd_seq_oss_ioctl+0x5c2/0x1090 sound/core/seq/oss/seq_oss_ioctl.c:93
>  odev_ioctl+0x51/0x70 sound/core/seq/oss/seq_oss.c:174
>  vfs_ioctl fs/ioctl.c:48 [inline]
>  ksys_ioctl fs/ioctl.c:753 [inline]
>  __do_sys_ioctl fs/ioctl.c:762 [inline]
>  __se_sys_ioctl+0xf9/0x160 fs/ioctl.c:760
>  do_syscall_64+0x73/0xe0 arch/x86/entry/common.c:384
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> The buggy address belongs to the object at ffff888098523000
>  which belongs to the cache kmalloc-128 of size 128
> The buggy address is located 96 bytes inside of
>  128-byte region [ffff888098523000, ffff888098523080)
> The buggy address belongs to the page:
> page:ffffea00026148c0 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0
> flags: 0xfffe0000000200(slab)
> raw: 00fffe0000000200 ffffea0002613988 ffffea000262c648 ffff8880aa400700
> raw: 0000000000000000 ffff888098523000 0000000100000010 0000000000000000
> page dumped because: kasan: bad access detected
> 
> Memory state around the buggy address:
>  ffff888098522f00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>  ffff888098522f80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> >ffff888098523000: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>                                                        ^
>  ffff888098523080: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>  ffff888098523100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ==================================================================
> 
> Reported-by: syzbot+1a54a94bd32716796edd@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Zhang Qiang <qiang.zhang@xxxxxxxxxxxxx>

Thanks for the patch.  But I'm afraid that this change would break the
existing behavior and might have a bad side-effect.

It's likely the same issue as reported in another syzkaller report
("KASAN: invalid-free in snd_seq_port_disconnect"), and Hillf's patch
below should covert this as well.  Could you check whether it works?


thanks,

Takashi

---
--- a/sound/core/seq/oss/seq_oss.c
+++ b/sound/core/seq/oss/seq_oss.c
@@ -167,11 +167,17 @@ odev_write(struct file *file, const char
 static long
 odev_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 {
+	long rc;
 	struct seq_oss_devinfo *dp;
+
+	mutex_lock(&register_mutex);
 	dp = file->private_data;
 	if (snd_BUG_ON(!dp))
-		return -ENXIO;
-	return snd_seq_oss_ioctl(dp, cmd, arg);
+		rc = -ENXIO;
+	else
+		rc = snd_seq_oss_ioctl(dp, cmd, arg);
+	mutex_unlock(&register_mutex);
+	return rc; 
 }
 
 #ifdef CONFIG_COMPAT