CVE-2026-53161

In the Linux kernel, the following vulnerability has been resolved:

misc: fastrpc: fix use-after-free of fastrpc_user in workqueue context

There is a race between fastrpc_device_release() and the workqueue that processes DSP responses. When the user closes the file descriptor, fastrpc_device_release() frees the fastrpc_user structure. Concurrently, an in-flight DSP invocation can complete and fastrpc_rpmsg_callback() schedules context cleanup via schedule_work(&ctx->put_work). If the workqueue runs fastrpc_context_free() in parallel with or after fastrpc_device_release() has freed the user structure, it dereferences the freed fastrpc_user. Depending on the state of the context at the time of the race, any one of the following accesses can be hit:

1. fastrpc_buf_free() calls fastrpc_ipa_to_dma_addr(buf->fl->cctx, …) to strip the SID bits from the stored IOVA before passing the physical address to dma_free_coherent().

2. fastrpc_free_map() reads map->fl->cctx->vmperms[0].vmid to reconstruct the source permission bitmask needed for the qcom_scm_assign_mem() call that returns memory from the DSP VM back to HLOS.

3. fastrpc_free_map() acquires map->fl->lock to safely remove the map node from the fl->maps list.

The resulting use-after-free manifests as:

pc : fastrpc_buf_free+0x38/0x80 [fastrpc] lr : fastrpc_context_free+0xa8/0x1b0 [fastrpc] fastrpc_context_free+0xa8/0x1b0 [fastrpc] fastrpc_context_put_wq+0x78/0xa0 [fastrpc] process_one_work+0x180/0x450 worker_thread+0x26c/0x388

Add kref-based reference counting to fastrpc_user. Have each invoke context take a reference on the user at allocation time and release it when the context is freed. Release the initial reference in fastrpc_device_release() at file close. Move the teardown of the user structure — freeing pending contexts, maps, mmaps, and the channel context reference — into the kref release callback fastrpc_user_free(), so that it runs only when the last reference is dropped, regardless of whether that happens at device close or after the final in-flight context completes.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/5278ccd357e0d7aeeb1e76c0f3e0e02894a9897c
• https://git.kernel.org/stable/c/c6e5c2be09f814377d7f1ce97370a5b7b3e02814
• https://git.kernel.org/stable/c/d42679eef34dd590b694ce3b666c5e2ba10cd4bf
• https://git.kernel.org/stable/c/df08fadcf0e5f3708365ec3b6d30b5aafd98bea1
• https://git.kernel.org/stable/c/e1e3a05efe5954d5bad01157d79429d39a67a7ae
• https://git.kernel.org/stable/c/e85eb5feca8e254905ffa6c57a3c99c89a674a0f
• https://git.kernel.org/stable/c/ecea4967c2bff92c2fafbc59893f711b39f7b152
• https://git.kernel.org/stable/c/fbe0947420eec18a84638d29468c2d563ce4e6a3