Re: [PATCH] fuse: virtio_fs: add checks for FUSE protocol compliance

["Darrick J. Wong" <djwong@kernel.org>]

On Wed, Oct 29, 2025 at 01:33:11AM +0530, Brajesh Patil wrote:
> Add validation in virtio-fs to ensure the server follows the FUSE
> protocol for response headers, addressing the existing TODO for
> verifying protocol compliance.
> 
> Add checks for fuse_out_header to verify:
>  - oh->unique matches req->in.h.unique
>  - FUSE_INT_REQ_BIT is not set
>  - error codes are valid
>  - oh->len does not exceed the expected size
> 
> Signed-off-by: Brajesh Patil <brajeshpatil11@gmail.com>
> ---
>  fs/fuse/virtio_fs.c | 30 +++++++++++++++++++++++++-----
>  1 file changed, 25 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
> index 6bc7c97b017d..52e8338bf436 100644
> --- a/fs/fuse/virtio_fs.c
> +++ b/fs/fuse/virtio_fs.c
> @@ -764,14 +764,34 @@ static void virtio_fs_request_complete(struct fuse_req *req,
>  {
>  	struct fuse_args *args;
>  	struct fuse_args_pages *ap;
> -	unsigned int len, i, thislen;
> +	struct fuse_out_header *oh;
> +	unsigned int len, i, thislen, expected_len = 0;
>  	struct folio *folio;
>  
> -	/*
> -	 * TODO verify that server properly follows FUSE protocol
> -	 * (oh.uniq, oh.len)
> -	 */
> +	oh = &req->out.h;
> +
> +	if (oh->unique == 0)
> +		pr_warn_once("notify through fuse-virtio-fs not supported");
> +
> +	if ((oh->unique & ~FUSE_INT_REQ_BIT) != req->in.h.unique)
> +		pr_warn_ratelimited("virtio-fs: unique mismatch, expected: %llu got %llu\n",
> +				    req->in.h.unique, oh->unique & ~FUSE_INT_REQ_BIT);

Er... shouldn't these be rejecting the response somehow?  Instead of
warning that something's amiss but continuing with known bad data?

--D

> +
> +	WARN_ON_ONCE(oh->unique & FUSE_INT_REQ_BIT);
> +
> +	if (oh->error <= -ERESTARTSYS || oh->error > 0)
> +		pr_warn_ratelimited("virtio-fs: invalid error code from server: %d\n",
> +				    oh->error);
> +
>  	args = req->args;
> +
> +	for (i = 0; i < args->out_numargs; i++)
> +		expected_len += args->out_args[i].size;
> +
> +	if (oh->len > sizeof(*oh) + expected_len)
> +		pr_warn("FUSE reply too long! got=%u expected<=%u\n",
> +			oh->len, (unsigned int)(sizeof(*oh) + expected_len));
> +
>  	copy_args_from_argbuf(args, req);
>  
>  	if (args->out_pages && args->page_zeroing) {
> -- 
> 2.43.0
> 
>