CVE-2026-98116

In the Linux kernel, the following vulnerability has been resolved:

ALSA: pcm: Serialize PCM mmap with buffer reallocation to fix page UAF

snd_pcm_hw_params() and snd_pcm_hw_free() guard buffer reallocation with an mmap_count check performed under the PCM stream lock, but the lock is released long before the buffer is actually freed: snd_pcm_sync_stop(), constraint refinement and do_free_pages() all happen in between. snd_pcm_mmap_data(), on the other hand, takes no lock at all: it validates against the old buffers state and dma_bytes, remaps its pages into the VMA, and only then increments mmap_count.

A concurrent mmap() can therefore slip in between the check and the free. remap_pfn_range() installs writable PTEs for the old buffers pages without taking page references, and the subsequent do_free_pages() returns those pages to the page allocator while the VMA still maps them. This leaves a stale, writable mapping of freed pages: a page-level use-after-free that can be leveraged for local privilege escalation.

Make snd_pcm_mmap_data() participate in the buffer-access scheme introduced for hw_params/hw_free: acquire runtime->buffer_accessing before validating and remapping, and release it afterwards. Buffer reallocation already fails with -EBUSY while accessors are active, and the mmap side now fails with -EBUSY while a reallocation is in progress, so the validate/remap sequence and the check/free sequence can no longer interleave.

A reproducer that turns this race into a stale writable mapping of the freed DMA buffer pages is available on request.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/8c1882dfee8f404d118020664b73eb4592172226
• https://git.kernel.org/stable/c/9857a75714bb5df4781e4d8b1a1753b1e17bf07a
• https://git.kernel.org/stable/c/9b110a9dcecc59516c77cb3c0caf1f492f75df2d
• https://git.kernel.org/stable/c/cbadf2575d25e7dcc0d4da2717817ae569fbc99b
• https://git.kernel.org/stable/c/cbc7ec97601d09b74e05e44470fa6b0bcdabf3f5
• https://git.kernel.org/stable/c/e6cdd2a0470d64073349a970dc1691433804b271
• https://git.kernel.org/stable/c/fd137bf8149bc6460f9b7b1fc292025da04cb9ee