From 9b110a9dcecc59516c77cb3c0caf1f492f75df2d Mon Sep 17 00:00:00 2001 From: Yilin Zhang Date: Mon, 31 Aug 2026 12:55:06 +0800 Subject: [PATCH] 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 Fixes: 92ee3c60ec9f ("ALSA: pcm: Fix races among concurrent hw_params and hw_free calls") Signed-off-by: Yilin Zhang Link: https://patch.msgid.link/20260831045506.889070-1-yilinzhang@moonshot.ai Signed-off-by: Takashi Iwai --- sound/core/pcm_native.c | 35 +++++++++++++++++++++++++---------- 1 file changed, 25 insertions(+), 10 deletions(-) diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c index 20ae67949e05..62324282fcae 100644 --- a/sound/core/pcm_native.c +++ b/sound/core/pcm_native.c @@ -4023,20 +4023,33 @@ int snd_pcm_mmap_data(struct snd_pcm_substream *substream, struct file *file, return -EINVAL; } runtime = substream->runtime; - if (runtime->state == SNDRV_PCM_STATE_OPEN) - return -EBADFD; - if (!(runtime->info & SNDRV_PCM_INFO_MMAP)) - return -ENXIO; + /* don't race with buffer reallocation in hw_params/hw_free */ + if (!atomic_inc_unless_negative(&runtime->buffer_accessing)) + return -EBUSY; + if (runtime->state == SNDRV_PCM_STATE_OPEN) { + err = -EBADFD; + goto out; + } + if (!(runtime->info & SNDRV_PCM_INFO_MMAP)) { + err = -ENXIO; + goto out; + } if (runtime->access == SNDRV_PCM_ACCESS_RW_INTERLEAVED || - runtime->access == SNDRV_PCM_ACCESS_RW_NONINTERLEAVED) - return -EINVAL; + runtime->access == SNDRV_PCM_ACCESS_RW_NONINTERLEAVED) { + err = -EINVAL; + goto out; + } size = area->vm_end - area->vm_start; offset = area->vm_pgoff << PAGE_SHIFT; dma_bytes = PAGE_ALIGN(runtime->dma_bytes); - if ((size_t)size > dma_bytes) - return -EINVAL; - if (offset > dma_bytes - size) - return -EINVAL; + if ((size_t)size > dma_bytes) { + err = -EINVAL; + goto out; + } + if (offset > dma_bytes - size) { + err = -EINVAL; + goto out; + } area->vm_ops = &snd_pcm_vm_ops_data; area->vm_private_data = substream; @@ -4046,6 +4059,8 @@ int snd_pcm_mmap_data(struct snd_pcm_substream *substream, struct file *file, err = snd_pcm_lib_default_mmap(substream, area); if (!err) atomic_inc(&substream->mmap_count); +out: + atomic_dec(&runtime->buffer_accessing); return err; } EXPORT_SYMBOL(snd_pcm_mmap_data);