Re: [PATCH v2] misc: fastrpc: fix double-free in fastrpc_map_attach() error path

[Ekansh Gupta]

On 12-08-2026 04:21, Yifei Gao wrote:
> map->table is assigned right after dma_buf_map_attachment_unlocked()
> succeeds. The two failure checks that follow, the len > map->size test
> and, where subsystem VMIDs are configured, a failed qcom_scm_assign_mem(),
> jump to map_err with map->table already set.
> 
> map_err manually calls dma_buf_detach() and dma_buf_put() and then falls
> through to fastrpc_map_put(). Since that change the error path tail is
> fastrpc_map_put() -> fastrpc_free_map(), and fastrpc_free_map() already
> unmaps, detaches and puts the dma-buf whenever map->table is set.
> The two operations therefore run twice: the second dma_buf_put() drops an
> extra reference on map->buf, and dma_buf_unmap_attachment_unlocked()
> dereferences the map->attach already freed by the manual dma_buf_detach().
> kref_init() sets the refcount to 1 with no intervening get, so the final
> fastrpc_map_put() frees the map synchronously and the redundant cleanup is
> deterministic.
> 
> The len > map->size branch is reachable by an unprivileged process via
> FASTRPC_IOCTL_MEM_MAP with an fd whose dma-buf is smaller than the
> requested length, before any DSP invocation.
> 
> Route both map->table-is-set failure branches to get_err instead of
> map_err, so fastrpc_free_map() is the single owner of the
> unmap/detach/put sequence. map_err is retained for the
> dma_buf_map_attachment_unlocked() failure, which is reached with
> map->table still NULL and an attachment that fastrpc_free_map() will not
> clean up, so its dma_buf_detach()/dma_buf_put() must still run manually.
> 
> Fixes: 334f1a1cbe03 ("misc: fastrpc: Use fastrpc_map_put in fastrpc_map_create on fail")
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Yifei Gao <gyf161023@xxxxxxxxx>

Reviewed-by: Ekansh Gupta <ekansh.gupta@xxxxxxxxxxxxxxxx>