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

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.