Re: [PATCH v2 1/2] fuse: copy request headers via a stack buffer for io-uring

[Bernd Schubert <bernd@bsbernd.com>]

On 7/7/26 20:44, Xiang Mei wrote:
> The fuse-io-uring transport copies req->in.h out to the ring in
> fuse_uring_copy_to_ring() and req->out.h back in fuse_uring_commit().
> Both headers live inside the fuse_request slab object, whose cache
> (fuse_req_cachep) is created without a usercopy whitelist, so copying
> them directly to/from userspace trips CONFIG_HARDENED_USERCOPY and
> panics:
> 
>   usercopy: Kernel memory exposure attempt detected from SLUB object
>   'fuse_request' (offset 56, size 40)!
>   kernel BUG at mm/usercopy.c:102!
>   RIP: 0010:usercopy_abort+0x6c/0x80
>   Call Trace:
>    __check_heap_object
>    __check_object_size
>    copy_header_to_ring          fs/fuse/dev_uring.c:618
>    fuse_uring_prepare_send
>    fuse_uring_send_in_task
>    ...
>    __do_sys_io_uring_enter
>    entry_SYSCALL_64_after_hwframe
> 
> Bounce both headers through an on-stack copy so the usercopy touches
> stack memory, not the slab object.
> 
> Fixes: c090c8abae4b ("fuse: Add io-uring sqe commit and fetch support")
> Cc: stable@vger.kernel.org
> Reported-by: Weiming Shi <bestswngs@gmail.com>
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Xiang Mei <xmei5@asu.edu>
> Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
> ---
> v2: add: Cc stable and Reviewed-by tags
> 
>  fs/fuse/dev_uring.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/fuse/dev_uring.c b/fs/fuse/dev_uring.c
> index 77c8cec43d9c..0814681eb04b 100644
> --- a/fs/fuse/dev_uring.c
> +++ b/fs/fuse/dev_uring.c
> @@ -744,6 +744,7 @@ static int fuse_uring_copy_to_ring(struct fuse_ring_ent *ent,
>  {
>  	struct fuse_ring_queue *queue = ent->queue;
>  	struct fuse_ring *ring = queue->ring;
> +	struct fuse_in_header in_header;
>  	int err;
>  
>  	err = -EIO;
> @@ -765,8 +766,9 @@ static int fuse_uring_copy_to_ring(struct fuse_ring_ent *ent,
>  	}
>  
>  	/* copy fuse_in_header */
> -	return copy_header_to_ring(ent, FUSE_URING_HEADER_IN_OUT, &req->in.h,
> -				   sizeof(req->in.h));
> +	in_header = req->in.h;
> +	return copy_header_to_ring(ent, FUSE_URING_HEADER_IN_OUT, &in_header,
> +				   sizeof(in_header));
>  }
>  
>  static int fuse_uring_prepare_send(struct fuse_ring_ent *ent,
> @@ -871,11 +873,13 @@ static void fuse_uring_commit(struct fuse_ring_ent *ent, struct fuse_req *req,
>  			      unsigned int issue_flags)
>  {
>  	struct fuse_ring *ring = ent->queue->ring;
> +	struct fuse_out_header out_header;
>  	ssize_t err = -EFAULT;
>  
> -	if (copy_header_from_ring(ent, FUSE_URING_HEADER_IN_OUT, &req->out.h,
> -				  sizeof(req->out.h)))
> +	if (copy_header_from_ring(ent, FUSE_URING_HEADER_IN_OUT, &out_header,
> +				  sizeof(out_header)))
>  		goto out;
> +	req->out.h = out_header;
>  
>  	err = fuse_uring_out_header_has_err(&req->out.h, req);
>  	if (err) {

Reviewed-by: Bernd Schubert <bernd@bsbernd.com>