CVE-2026-98116

Summary

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 buffer's 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 buffer's 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.

Affected Software

Weaknesses

References

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

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