FKIE_CVE-2026-31584
Vulnerability from fkie_nvd - Published: 2026-04-24 15:16 - Updated: 2026-04-24 17:51
Severity ?
Summary
In the Linux kernel, the following vulnerability has been resolved:
media: mediatek: vcodec: fix use-after-free in encoder release path
The fops_vcodec_release() function frees the context structure (ctx)
without first cancelling any pending or running work in ctx->encode_work.
This creates a race window where the workqueue handler (mtk_venc_worker)
may still be accessing the context memory after it has been freed.
Race condition:
CPU 0 (release path) CPU 1 (workqueue)
--------------------- ------------------
fops_vcodec_release()
v4l2_m2m_ctx_release()
v4l2_m2m_cancel_job()
// waits for m2m job "done"
mtk_venc_worker()
v4l2_m2m_job_finish()
// m2m job "done"
// BUT worker still running!
// post-job_finish access:
other ctx dereferences
// UAF if ctx already freed
// returns (job "done")
kfree(ctx) // ctx freed
Root cause: The v4l2_m2m_ctx_release() only waits for the m2m job
lifecycle (via TRANS_RUNNING flag), not the workqueue lifecycle.
After v4l2_m2m_job_finish() is called, the m2m framework considers
the job complete and v4l2_m2m_ctx_release() returns, but the worker
function continues executing and may still access ctx.
The work is queued during encode operations via:
queue_work(ctx->dev->encode_workqueue, &ctx->encode_work)
The worker function accesses ctx->m2m_ctx, ctx->dev, and other ctx
fields even after calling v4l2_m2m_job_finish().
This vulnerability was confirmed with KASAN by running an instrumented
test module that widens the post-job_finish race window. KASAN detected:
BUG: KASAN: slab-use-after-free in mtk_venc_worker+0x159/0x180
Read of size 4 at addr ffff88800326e000 by task kworker/u8:0/12
Workqueue: mtk_vcodec_enc_wq mtk_venc_worker
Allocated by task 47:
__kasan_kmalloc+0x7f/0x90
fops_vcodec_open+0x85/0x1a0
Freed by task 47:
__kasan_slab_free+0x43/0x70
kfree+0xee/0x3a0
fops_vcodec_release+0xb7/0x190
Fix this by calling cancel_work_sync(&ctx->encode_work) before kfree(ctx).
This ensures the workqueue handler is both cancelled (if pending) and
synchronized (waits for any running handler to complete) before the
context is freed.
Placement rationale: The fix is placed after v4l2_ctrl_handler_free()
and before list_del_init(&ctx->list). At this point, all m2m operations
are done (v4l2_m2m_ctx_release() has returned), and we need to ensure
the workqueue is synchronized before removing ctx from the list and
freeing it.
Note: The open error path does NOT need cancel_work_sync() because
INIT_WORK() only initializes the work structure - it does not schedule
it. Work is only scheduled later during device_run() operations.
References
Impacted products
| Vendor | Product | Version |
|---|
{
"cveTags": [],
"descriptions": [
{
"lang": "en",
"value": "In the Linux kernel, the following vulnerability has been resolved:\n\nmedia: mediatek: vcodec: fix use-after-free in encoder release path\n\nThe fops_vcodec_release() function frees the context structure (ctx)\nwithout first cancelling any pending or running work in ctx-\u003eencode_work.\nThis creates a race window where the workqueue handler (mtk_venc_worker)\nmay still be accessing the context memory after it has been freed.\n\nRace condition:\n\n CPU 0 (release path) CPU 1 (workqueue)\n --------------------- ------------------\n fops_vcodec_release()\n v4l2_m2m_ctx_release()\n v4l2_m2m_cancel_job()\n // waits for m2m job \"done\"\n mtk_venc_worker()\n v4l2_m2m_job_finish()\n // m2m job \"done\"\n // BUT worker still running!\n // post-job_finish access:\n other ctx dereferences\n // UAF if ctx already freed\n // returns (job \"done\")\n kfree(ctx) // ctx freed\n\nRoot cause: The v4l2_m2m_ctx_release() only waits for the m2m job\nlifecycle (via TRANS_RUNNING flag), not the workqueue lifecycle.\nAfter v4l2_m2m_job_finish() is called, the m2m framework considers\nthe job complete and v4l2_m2m_ctx_release() returns, but the worker\nfunction continues executing and may still access ctx.\n\nThe work is queued during encode operations via:\n queue_work(ctx-\u003edev-\u003eencode_workqueue, \u0026ctx-\u003eencode_work)\nThe worker function accesses ctx-\u003em2m_ctx, ctx-\u003edev, and other ctx\nfields even after calling v4l2_m2m_job_finish().\n\nThis vulnerability was confirmed with KASAN by running an instrumented\ntest module that widens the post-job_finish race window. KASAN detected:\n\n BUG: KASAN: slab-use-after-free in mtk_venc_worker+0x159/0x180\n Read of size 4 at addr ffff88800326e000 by task kworker/u8:0/12\n\n Workqueue: mtk_vcodec_enc_wq mtk_venc_worker\n\n Allocated by task 47:\n __kasan_kmalloc+0x7f/0x90\n fops_vcodec_open+0x85/0x1a0\n\n Freed by task 47:\n __kasan_slab_free+0x43/0x70\n kfree+0xee/0x3a0\n fops_vcodec_release+0xb7/0x190\n\nFix this by calling cancel_work_sync(\u0026ctx-\u003eencode_work) before kfree(ctx).\nThis ensures the workqueue handler is both cancelled (if pending) and\nsynchronized (waits for any running handler to complete) before the\ncontext is freed.\n\nPlacement rationale: The fix is placed after v4l2_ctrl_handler_free()\nand before list_del_init(\u0026ctx-\u003elist). At this point, all m2m operations\nare done (v4l2_m2m_ctx_release() has returned), and we need to ensure\nthe workqueue is synchronized before removing ctx from the list and\nfreeing it.\n\nNote: The open error path does NOT need cancel_work_sync() because\nINIT_WORK() only initializes the work structure - it does not schedule\nit. Work is only scheduled later during device_run() operations."
}
],
"id": "CVE-2026-31584",
"lastModified": "2026-04-24T17:51:40.810",
"metrics": {},
"published": "2026-04-24T15:16:33.117",
"references": [
{
"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"url": "https://git.kernel.org/stable/c/93d9a58961a9e09306857e999b3ee76aa4be67f0"
},
{
"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"url": "https://git.kernel.org/stable/c/a8a55913552aed45108525d1851c65e1db0cc25b"
},
{
"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"url": "https://git.kernel.org/stable/c/f1692337c6fa26e04f89b22a4d84bf5b7ada50d1"
},
{
"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"url": "https://git.kernel.org/stable/c/f99353cd0e9f58bf17889049137b8d65fb44ebf1"
}
],
"sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"vulnStatus": "Awaiting Analysis"
}
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Sightings
| Author | Source | Type | Date |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
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