data-race in snd_seq_oss_midi_check_exit_port / snd_seq_oss_midi_setup

[Abhishek Shah]

Hi all,

We found a race involving the *max_midi_devs* variable. We see an
interleaving where the following check here
<https://elixir.bootlin.com/linux/v5.18-rc5/source/sound/core/seq/oss/seq_oss_init.c#L200>
passes before the *snd_seq_oss_midi_check_exit_port*() finishes, but this
check should not pass if *max_midi_devs* will become zero, but we are not
sure of its implications in terms of security impact. Please let us know
what you think.

Thanks!

*-------------------Report---------------------*

*write* to 0xffffffff88382f80 of 4 bytes by task 6541 on cpu 0:
 snd_seq_oss_midi_check_exit_port+0x1a6/0x270
sound/core/seq/oss/seq_oss_midi.c:237
 receive_announce+0x193/0x1b0 sound/core/seq/oss/seq_oss_init.c:143
 snd_seq_deliver_single_event+0x30d/0x4e0 sound/core/seq/seq_clientmgr.c:640
 deliver_to_subscribers sound/core/seq/seq_clientmgr.c:695 [inline]
 snd_seq_deliver_event+0x38c/0x490 sound/core/seq/seq_clientmgr.c:830
 snd_seq_kernel_client_dispatch+0x189/0x1a0
sound/core/seq/seq_clientmgr.c:2339
 snd_seq_system_broadcast+0x98/0xd0 sound/core/seq/seq_system.c:86
 snd_seq_ioctl_delete_port+0x9a/0xc0 sound/core/seq/seq_clientmgr.c:1356
 snd_seq_ioctl+0x198/0x2d0 sound/core/seq/seq_clientmgr.c:2173
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:870 [inline]
 __se_sys_ioctl+0xe1/0x150 fs/ioctl.c:856
 __x64_sys_ioctl+0x43/0x50 fs/ioctl.c:856
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

*read* to 0xffffffff88382f80 of 4 bytes by task 6542 on cpu 1:
 snd_seq_oss_midi_setup+0x1b/0x40 sound/core/seq/oss/seq_oss_midi.c:273
 snd_seq_oss_open+0x364/0x900 sound/core/seq/oss/seq_oss_init.c:198
 odev_open+0x55/0x70 sound/core/seq/oss/seq_oss.c:128
 soundcore_open+0x315/0x3a0 sound/sound_core.c:593
 chrdev_open+0x373/0x3f0 fs/char_dev.c:414
 do_dentry_open+0x543/0x8f0 fs/open.c:824
 vfs_open+0x47/0x50 fs/open.c:958
 do_open fs/namei.c:3476 [inline]
 path_openat+0x1906/0x1dc0 fs/namei.c:3609
 do_filp_open+0xef/0x200 fs/namei.c:3636
 do_sys_openat2+0xa5/0x2a0 fs/open.c:1213
 do_sys_open fs/open.c:1229 [inline]
 __do_sys_openat fs/open.c:1245 [inline]
 __se_sys_openat fs/open.c:1240 [inline]
 __x64_sys_openat+0xf0/0x120 fs/open.c:1240
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 6542 Comm: syz-executor2-n Not tainted 5.18.0-rc5+ #107
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1
04/01/2014


*Reproducing Inputs*

Input CPU 0:
r0 = openat$sndseq(0xffffffffffffff9c,
&(0x7f0000000040)='/dev/snd/seq\x00', 0x0)
ioctl$SNDRV_SEQ_IOCTL_CREATE_PORT(r0, 0xc0a85320,
&(0x7f0000000240)={{0x80}, 'port1\x00', 0x10})
ioctl$SNDRV_SEQ_IOCTL_SET_CLIENT_POOL(r0, 0x40a85321,
&(0x7f0000000100)={0x80})

Input CPU 1:
r0 = openat$sequencer2(0xffffff9c, &(0x7f0000000000)='/dev/sequencer2\x00',
0x0, 0x0)
ioctl$SNDCTL_SYNTH_INFO(r0, 0xc08c5102,
&(0x7f0000000200)={"02961a3ce6d4828f8b5559726313251b55fa11d8d65406f1f33c9af8e3f8",
0xffffffff})