misc: fastrpc: fix use-after-free in fastrpc_map_attach() error path

[PATCH] misc: fastrpc: fix use-after-free in fastrpc_map_attach() error path

Posted by Yifei Gao 1 month, 3 weeks ago

map->table is set right after the attachment is mapped, before the
len > map->size check. When that check fails and jumps to map_err, the
error path manually calls dma_buf_detach() and dma_buf_put(), then falls
through to fastrpc_map_put() -> fastrpc_free_map().

Since map->table is still non-NULL, fastrpc_free_map() repeats the
cleanup: dma_buf_unmap_attachment_unlocked() dereferences the map->attach
already freed by dma_buf_detach() (use-after-free read), and a second
dma_buf_put() drops an extra reference on map->buf. As the exporting fd is
typically still held by userspace, this imbalance can later lead to
premature destruction of the dma_buf and a use-after-free.

The 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.

Clear map->table in the map_err path so the fastrpc_map_put() fallthrough
does not operate on the already released attachment and buffer.

Fixes: 334f1a1cbe03 ("misc: fastrpc: Use fastrpc_map_put in fastrpc_map_create on fail")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Yifei Gao <gyf161023@gmail.com>
---
 drivers/misc/fastrpc.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
index f3a49384586d..d6be951d5538 100644
--- a/drivers/misc/fastrpc.c
+++ b/drivers/misc/fastrpc.c
@@ -915,6 +915,7 @@ static int fastrpc_map_attach(struct fastrpc_user *fl, int fd,
 
 map_err:
 	dma_buf_detach(map->buf, map->attach);
+	map->table = NULL;
 attach_err:
 	dma_buf_put(map->buf);
 get_err:
-- 
2.43.0

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

Posted by Dmitry Baryshkov 1 month, 3 weeks ago

On Thu, Aug 06, 2026 at 11:50:54PM +0000, Yifei Gao wrote:
> map->table is set right after the attachment is mapped, before the
> len > map->size check. When that check fails and jumps to map_err, the
> error path manually calls dma_buf_detach() and dma_buf_put(), then falls
> through to fastrpc_map_put() -> fastrpc_free_map().
> 
> Since map->table is still non-NULL, fastrpc_free_map() repeats the
> cleanup: dma_buf_unmap_attachment_unlocked() dereferences the map->attach
> already freed by dma_buf_detach() (use-after-free read), and a second
> dma_buf_put() drops an extra reference on map->buf. As the exporting fd is
> typically still held by userspace, this imbalance can later lead to
> premature destruction of the dma_buf and a use-after-free.
> 
> The 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.
> 
> Clear map->table in the map_err path so the fastrpc_map_put() fallthrough
> does not operate on the already released attachment and buffer.
> 
> Fixes: 334f1a1cbe03 ("misc: fastrpc: Use fastrpc_map_put in fastrpc_map_create on fail")
> Cc: stable@vger.kernel.org
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Yifei Gao <gyf161023@gmail.com>
> ---
>  drivers/misc/fastrpc.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
> index f3a49384586d..d6be951d5538 100644
> --- a/drivers/misc/fastrpc.c
> +++ b/drivers/misc/fastrpc.c
> @@ -915,6 +915,7 @@ static int fastrpc_map_attach(struct fastrpc_user *fl, int fd,
>  
>  map_err:
>  	dma_buf_detach(map->buf, map->attach);
> +	map->table = NULL;

This way it will skip dma_buf_unmap_attachment_unlocked() in
fastrpc_map_free(), which is not nice.

>  attach_err:
>  	dma_buf_put(map->buf);
>  get_err:
> -- 
> 2.43.0
> 

-- 
With best wishes
Dmitry

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

Posted by Yifei Gao 1 month, 2 weeks ago

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@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Yifei Gao <gyf161023@gmail.com>
---
v2:
 - Instead of clearing map->table, route the map->table-is-set failure
   paths (len > map->size and the qcom_scm_assign_mem() failure) to
   get_err so fastrpc_free_map() is the single owner of the
   unmap/detach/put sequence. The map_err label is kept for the
   dma_buf_map_attachment_unlocked() failure, where map->table is still
   NULL. Suggested by Dmitry Baryshkov.

 drivers/misc/fastrpc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
index eb6c2a78d3c7..d480a87752a7 100644
--- a/drivers/misc/fastrpc.c
+++ b/drivers/misc/fastrpc.c
@@ -881,7 +881,7 @@ static int fastrpc_map_attach(struct fastrpc_user *fl, int fd,
 		dev_dbg(sess->dev, "Bad size passed len 0x%llx map size 0x%llx\n",
 				len, map->size);
 		err = -EINVAL;
-		goto map_err;
+		goto get_err;
 	}
 	map->va = sg_virt(map->table->sgl);
 	map->len = len;
@@ -904,7 +904,7 @@ static int fastrpc_map_attach(struct fastrpc_user *fl, int fd,
 			dev_err(sess->dev,
 				"Failed to assign memory with dma_addr %pad size 0x%llx err %d\n",
 				&map->dma_addr, map->len, err);
-			goto map_err;
+			goto get_err;
 		}
 	}
 	spin_lock(&fl->lock);
-- 
2.43.0

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

Posted by Srinivas Kandagatla 2 weeks, 1 day ago

On Tue, 11 Aug 2026 22:51:01 +0000, 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.
> 
> [...]

Applied, thanks!

[1/1] misc: fastrpc: fix double-free in fastrpc_map_attach() error path
      commit: 2159430fe26068ca3c89557ddd2d5eb17a5a8cf0

Best regards,
-- 
Srinivas Kandagatla <srini@kernel.org>

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

Posted by Ekansh Gupta 1 month, 1 week ago

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@vger.kernel.org
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Yifei Gao <gyf161023@gmail.com>

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