linux-kernel - Re: KASAN: use-after-free Read in snd_pcm_oss

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Date:   Mon, 31 Aug 2020 11:45:40 +0200
From:   Takashi Iwai <tiwai@...e.de>
To:     Hillf Danton <hdanton@...a.com>
Cc:     syzbot <syzbot+12a3fcd1493ec6585007@...kaller.appspotmail.com>,
        akpm@...ux-foundation.org, alsa-devel@...a-project.org,
        arnd@...db.de, gustavoars@...nel.org, linux-kernel@...r.kernel.org,
        perex@...ex.cz, syzkaller-bugs@...glegroups.com, tiwai@...e.com,
        vbabka@...e.cz, walken@...gle.com
Subject: Re: KASAN: use-after-free Read in snd_pcm_oss_release

On Sat, 29 Aug 2020 05:50:43 +0200,
Hillf Danton wrote:
> 
> 
> Thu, 27 Aug 2020 09:34:15 -0700
> > syzbot found the following issue on:
> > 
> > HEAD commit:    f37be724 Add linux-next specific files for 20200826
> > git tree:       linux-next
> > console output: https://syzkaller.appspot.com/x/log.txt?x=137c7cb9900000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=1e5a1cf036089d95
> > dashboard link: https://syzkaller.appspot.com/bug?extid=12a3fcd1493ec6585007
> > compiler:       gcc (GCC) 10.1.0-syz 20200507
> > 
> > Unfortunately, I don't have any reproducer for this issue yet.
> > 
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+12a3fcd1493ec6585007@...kaller.appspotmail.com
> > 
> > ==================================================================
> > BUG: KASAN: use-after-free in snd_pcm_oss_release_file sound/core/oss/pcm_oss.c:2367 [inline]
> > BUG: KASAN: use-after-free in snd_pcm_oss_release_file sound/core/oss/pcm_oss.c:2361 [inline]
> > BUG: KASAN: use-after-free in snd_pcm_oss_release+0x28d/0x300 sound/core/oss/pcm_oss.c:2548
> > Read of size 8 at addr ffff888026b66480 by task syz-executor.1/27751
> > 
> > CPU: 1 PID: 27751 Comm: syz-executor.1 Not tainted 5.9.0-rc2-next-20200826-syzkaller #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > Call Trace:
> >  __dump_stack lib/dump_stack.c:77 [inline]
> >  dump_stack+0x18f/0x20d lib/dump_stack.c:118
> >  print_address_description.constprop.0.cold+0xae/0x497 mm/kasan/report.c:383
> >  __kasan_report mm/kasan/report.c:513 [inline]
> >  kasan_report.cold+0x1f/0x37 mm/kasan/report.c:530
> >  snd_pcm_oss_release_file sound/core/oss/pcm_oss.c:2367 [inline]
> >  snd_pcm_oss_release_file sound/core/oss/pcm_oss.c:2361 [inline]
> >  snd_pcm_oss_release+0x28d/0x300 sound/core/oss/pcm_oss.c:2548
> >  __fput+0x285/0x920 fs/file_table.c:281
> >  task_work_run+0xdd/0x190 kernel/task_work.c:141
> >  tracehook_notify_resume include/linux/tracehook.h:188 [inline]
> >  exit_to_user_mode_loop kernel/entry/common.c:140 [inline]
> >  exit_to_user_mode_prepare+0x195/0x1c0 kernel/entry/common.c:167
> >  syscall_exit_to_user_mode+0x59/0x2b0 kernel/entry/common.c:242
> >  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> > RIP: 0033:0x416f01
> > Code: 75 14 b8 03 00 00 00 0f 05 48 3d 01 f0 ff ff 0f 83 04 1b 00 00 c3 48 83 ec 08 e8 0a fc ff ff 48 89 04 24 b8 03 00 00 00 0f 05 <48> 8b 3c 24 48 89 c2 e8 53 fc ff ff 48 89 d0 48 83 c4 08 48 3d 01
> > RSP: 002b:00007ffdd39a5e70 EFLAGS: 00000293 ORIG_RAX: 0000000000000003
> > RAX: 0000000000000000 RBX: 0000000000000004 RCX: 0000000000416f01
> > RDX: 0000000000000001 RSI: 0000000001190428 RDI: 0000000000000003
> > RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
> > R10: 00007ffdd39a5f60 R11: 0000000000000293 R12: 0000000001190428
> > R13: 0000000000268c54 R14: ffffffffffffffff R15: 000000000118cfec
> > 
> > Allocated by task 27754:
> >  kasan_save_stack+0x1b/0x40 mm/kasan/common.c:48
> >  kasan_set_track mm/kasan/common.c:56 [inline]
> >  __kasan_kmalloc.constprop.0+0xbf/0xd0 mm/kasan/common.c:461
> >  kmem_cache_alloc_trace+0x16e/0x2c0 mm/slab.c:3550
> >  kmalloc include/linux/slab.h:554 [inline]
> >  kzalloc include/linux/slab.h:666 [inline]
> >  snd_pcm_oss_open_file sound/core/oss/pcm_oss.c:2389 [inline]
> >  snd_pcm_oss_open.part.0+0x55e/0x1290 sound/core/oss/pcm_oss.c:2491
> >  snd_pcm_oss_open+0x3c/0x50 sound/core/oss/pcm_oss.c:2455
> >  soundcore_open+0x445/0x600 sound/sound_core.c:593
> >  chrdev_open+0x266/0x770 fs/char_dev.c:414
> >  do_dentry_open+0x4b9/0x11b0 fs/open.c:817
> >  do_open fs/namei.c:3251 [inline]
> >  path_openat+0x1b9a/0x2730 fs/namei.c:3368
> >  do_filp_open+0x17e/0x3c0 fs/namei.c:3395
> >  do_sys_openat2+0x16d/0x420 fs/open.c:1168
> >  do_sys_open fs/open.c:1184 [inline]
> >  __do_sys_openat fs/open.c:1200 [inline]
> >  __se_sys_openat fs/open.c:1195 [inline]
> >  __x64_sys_openat+0x13f/0x1f0 fs/open.c:1195
> >  do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
> >  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> > 
> > Freed by task 27754:
> >  kasan_save_stack+0x1b/0x40 mm/kasan/common.c:48
> >  kasan_set_track+0x1c/0x30 mm/kasan/common.c:56
> >  kasan_set_free_info+0x1b/0x30 mm/kasan/generic.c:355
> >  __kasan_slab_free+0xd8/0x120 mm/kasan/common.c:422
> >  __cache_free mm/slab.c:3418 [inline]
> >  kfree+0x103/0x2c0 mm/slab.c:3756
> >  snd_pcm_oss_release_file sound/core/oss/pcm_oss.c:2371 [inline]
> >  snd_pcm_oss_release_file sound/core/oss/pcm_oss.c:2361 [inline]
> >  snd_pcm_oss_release+0x17e/0x300 sound/core/oss/pcm_oss.c:2548
> >  __fput+0x285/0x920 fs/file_table.c:281
> >  task_work_run+0xdd/0x190 kernel/task_work.c:141
> >  tracehook_notify_resume include/linux/tracehook.h:188 [inline]
> >  exit_to_user_mode_loop kernel/entry/common.c:140 [inline]
> >  exit_to_user_mode_prepare+0x195/0x1c0 kernel/entry/common.c:167
> >  syscall_exit_to_user_mode+0x59/0x2b0 kernel/entry/common.c:242
> >  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> > 
> > The buggy address belongs to the object at ffff888026b66480
> >  which belongs to the cache kmalloc-32 of size 32
> > The buggy address is located 0 bytes inside of
> >  32-byte region [ffff888026b66480, ffff888026b664a0)
> > The buggy address belongs to the page:
> > page:0000000076bbbb49 refcount:1 mapcount:0 mapping:0000000000000000 index:0xffff888026b66fc1 pfn:0x26b66
> > flags: 0xfffe0000000200(slab)
> > raw: 00fffe0000000200 ffffea0002385208 ffffea0000782448 ffff8880aa040100
> > raw: ffff888026b66fc1 ffff888026b66000 000000010000002e 0000000000000000
> > page dumped because: kasan: bad access detected
> > 
> > Memory state around the buggy address:
> >  ffff888026b66380: fa fb fb fb fc fc fc fc fb fb fb fb fc fc fc fc
> >  ffff888026b66400: 00 fc fc fc fc fc fc fc 00 00 00 00 fc fc fc fc
> > >ffff888026b66480: fa fb fb fb fc fc fc fc fb fb fb fb fc fc fc fc
> >                    ^
> >  ffff888026b66500: 00 fc fc fc fc fc fc fc 00 fc fc fc fc fc fc fc
> >  ffff888026b66580: fa fb fb fb fc fc fc fc fb fb fb fb fc fc fc fc
> > ==================================================================
> 
> Hard to see why file is released more than once though it's not
> that hard to prepare a fix.

Well, it's really the primary question why the release got called for
the same file->private_data at all.  Might it be a red herring, since
it was caught not at the first access of pcm_oss_file in the release
code path.  It's referred already in the code snippet below, although
the UAF got detected at a much later point (snd_pcm_oss_release_file()
call)...


Takashi

> 
> --- a/sound/core/oss/pcm_oss.c
> +++ b/sound/core/oss/pcm_oss.c
> @@ -2535,6 +2535,10 @@ static int snd_pcm_oss_release(struct in
>  	struct snd_pcm_oss_file *pcm_oss_file;
>  
>  	pcm_oss_file = file->private_data;
> +	if (!pcm_oss_file)
> +		return 0;
> +	file->private_data = NULL;
> +
>  	substream = pcm_oss_file->streams[SNDRV_PCM_STREAM_PLAYBACK];
>  	if (substream == NULL)
>  		substream = pcm_oss_file->streams[SNDRV_PCM_STREAM_CAPTURE];
>