CVE-2026-98116: ALSA: pcm: Serialize PCM mmap with buffer reallocation to fix page UAF

Severity: high (CVSS 7.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.1% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-09-25 · updated 2026-10-28
Weakness: CWE-416

Description

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 products

Linux kernel: from 4.14.279 before 4.15; from 4.19.243 before 4.20; from 5.4.193 before 5.5; from 5.10.109 before 5.11; from 5.15.32 before 5.15.222; from 5.16.18 through 5.17.1 — fixed in 4.15, 4.20, 5.5, 5.11, 5.15.222, 6.12.111
Linux kernel: from 5.15.32 before 5.15.222; from 4.14.279 before 4.15; from 4.19.243 before 4.20; from 5.4.193 before 5.5; from 5.10.109 before 5.11; from 5.16.18 before 5.17 — fixed in 4.15, 4.20, 5.11, 5.15.222, 5.17, 5.18

How to fix

Upgrade to a fixed version: Linux kernel 4.15, 4.20, 5.5, 5.11; Linux kernel 4.15, 4.20, 5.11, 5.15.222. Check the vendor advisory for mitigations.

References

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