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 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.