CVE-2026-74388: ALSA: seq: oss: Fix UAF at handling events with embedded SysEx data
In the Linux kernel, the following vulnerability has been resolved:
ALSA: seq: oss: Fix UAF at handling events with embedded SysEx data
The OSS sequencer processes the input MIDI bytes into a sequencer event to be dispatched later (in sndseqossmidiputc() called from sndseqossprocessevent()). When it's a SysEx data, the event record contains data.ext.ptr pointer to the original SysEx bytes, and the referred data is copied into the pool afterwards at dispatching. The problem is that, if the sequencer port gets closed concurrently before the dispatch, the OSS sequencer core also releases the resources (in sndseqossmidicheckexitport()), while the pending event may hold a stale pointer, eventually leading to a UAF at a later dispatch.
Fortunately, there is already a refcounting mechanism (snduselockt) for the OSS MIDI device access, and for addressing the issue above, we just need to extend the refcount until the event gets dispatched.
This patch extends sndseqossprocessevent() to give back the refcount object, which is in turn released after calling the sequencer dispatcher with the given event in the caller side.
According to the original report, KASAN report as below:
KASAN slab-use-after-free in sndseqeventdup+0x40c/0x470 RIP: 0033:0x7f2cb66a6340 Read of size 6 Call trace: dumpstacklvl+0x73/0xb0 (?:?) printreport+0xd1/0x650 (?:?) srsoaliasreturnthunk+0x5/0xfbef5 (?:?) virtaddrvalid+0x1a7/0x340 (?:?) kasancompletemodereportinfo+0x64/0x200 (?:?) kasanreport+0xf7/0x130 (?:?) sndseqeventdup+0x40c/0x470 (?:?) kasancheckrange+0x10c/0x1c0 (?:?) asanmemcpy+0x27/0x70 (?:?) sndseqeventdup+0x9/0x470 (?:?) sndseqclientenqueueevent+0x139/0x240 (?:?) rawspinunlockirqrestore+0x4b/0x60 (?:?) sndseqkernelclientenqueue+0x102/0x120 (?:?) sndseqosswrite+0x416/0x4e0 (?:?) apparmorfilepermission+0x20/0x30 (?:?) odevwrite+0x3b/0x60 (?:?) vfswrite+0x1ce/0x850 (?:?) lockrelease+0xc8/0x2a0 (?:?) kasancheckwrite+0x18/0x20 (?:?) mutexunlockslowpath+0x129/0x510 (?:?) ksyswrite+0xe1/0x180 (?:?) mutexunlock+0x16/0x20 (?:?) odevioctl+0x65/0xc0 (?:?) x64syswrite+0x46/0x60 (?:?) x64syscall+0x7d/0x20d0 (?:?) dosyscall64+0xc1/0x360 (arch/x86/entry/syscall64.c:87) entrySYSCALL64afterhwframe+0x77/0x7f (?:?)