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.

Reported-by: Kimi Security Team <bug-report@moonshot.ai>
Fixes: 92ee3c60ec9f ("ALSA: pcm: Fix races among concurrent hw_params and hw_free calls")
Signed-off-by: Yilin Zhang <yilinzhang@moonshot.ai>
Link: https://patch.msgid.link/20260831045506.889070-1-yilinzhang@moonshot.ai
Signed-off-by: Takashi Iwai <tiwai@suse.de>

0 files changed, 0 insertions, 0 deletions