Hi Takashi,
Makes sense, we'll note this race as benign for our future reference.
Thanks for taking the time to look at this!
Best,
Gabe
On Fri, Aug 19, 2022 at 3:41 AM Takashi Iwai tiwai@suse.de wrote:
On Fri, 19 Aug 2022 03:00:00 +0200, Abhishek Shah wrote:
Hi all,
We found a race involving the max_midi_devs variable. We see an interleaving where the following check here 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.
Through a quick glance, I guess it's rather harmless (although a bit fragile from the code sanity POV).
A MIDI port could be closed at any time, and the dp->max_mididevs holds locally the upper bound of currently possibly accessible ports. The actual access to each port is done via get_mdev() in seq_oss_midi.c, which is a sort of refcount managed, and it should be fine that a port disappears meanwhile.
That said, it'd be even feasible just dropping dp->max_mididevs field and scan all MIDI ports at each time, but it won't bring much benefit, either.
thanks,
Takashi
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})