Re: [PATCH 1/2] misc: fastrpc: fix NULL rpdev dereference when session alloc fails

[Ekansh Gupta <ekansh.gupta@oss.qualcomm.com>]

On 01-10-2026 16:31, Vinayak Katoch wrote:
> When a fastrpc client process dies abnormally without closing its file
> descriptor, fastrpc_device_release() is never called and the compute-cb
> session slot is never freed. The abnormal client death on the DSP side
> triggers a glink channel teardown which propagates up through the rpmsg
> bus and calls fastrpc_rpmsg_remove().
> 
> fastrpc_rpmsg_remove() nulls cctx->rpdev before calling
> misc_deregister(). If a concurrent open() enters fastrpc_device_open()
> in this window and fastrpc_session_alloc() returns NULL (sessions
> exhausted by the leaked slot), the error path dereferences
> cctx->rpdev->dev causing a NULL pointer dereference:
> 
>   Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
>   pc : fastrpc_device_open+0x1b8/0x258 [fastrpc]
>   Call trace:
>    fastrpc_device_open+0x1b8/0x258 [fastrpc] (P)
>    misc_open+0xd8/0x1a0
> 
> Fix by moving misc_deregister() before rpdev = NULL. misc_open() and
> misc_deregister() both take misc_mtx, so misc_deregister() will either
> block until fastrpc_device_open() completes or prevent new opens from
> entering it entirely.
> 
> Fixes: 7c11df42d0c7 ("misc: fastrpc: Fix device_open when no session is available")
> Signed-off-by: Vinayak Katoch <vinayak.katoch@oss.qualcomm.com>

Reviewed-by: Ekansh Gupta <ekansh.gupta@oss.qualcomm.com>