Re: [PATCH] [PATCH v2] virtiofs: Strengthen FUSE protocol validation for server responses

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Mon, Mar 23, 2026 at 04:14:54PM +0800, Li Wang wrote:
> 1. We should not trust the length value provided by the server;
> 2. Only replies have an out header/argbuf data to parse;
> 3. Some minor refinements for performance and log throttling.
> 
> Signed-off-by: Li Wang <liwang@kylinos.cn>
> ---
>  fs/fuse/virtio_fs.c | 28 ++++++++++++++++++----------
>  1 file changed, 18 insertions(+), 10 deletions(-)

CCing Yuto Ohnuki since this touches their recent work.

> 
> diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
> index 2f7485ffac52..c3f7f472f517 100644
> --- a/fs/fuse/virtio_fs.c
> +++ b/fs/fuse/virtio_fs.c
> @@ -759,23 +759,31 @@ static void copy_args_from_argbuf(struct fuse_args *args, struct fuse_req *req)
>  }
>  
>  /* Verify that the server properly follows the FUSE protocol */
> -static bool virtio_fs_verify_response(struct fuse_req *req, unsigned int len)
> +static bool virtio_fs_verify_response(struct fuse_req *req)
>  {
> +	unsigned int num_out, cap;
> +	struct fuse_args *args;
>  	struct fuse_out_header *oh = &req->out.h;
>  
> -	if (len < sizeof(*oh)) {
> -		pr_warn("virtio-fs: response too short (%u)\n", len);
> +	if (!test_bit(FR_ISREPLY, &req->flags))
>  		return false;
> -	}
> -	if (oh->len != len) {
> -		pr_warn("virtio-fs: oh.len mismatch (%u != %u)\n", oh->len, len);
> +	if (unlikely(oh->unique != req->in.h.unique)) {
> +		pr_warn_ratelimited("virtio-fs: bad reply unique %llu (expected %llu)\n",
> +					oh->unique, req->in.h.unique);
>  		return false;
>  	}
> -	if (oh->unique != req->in.h.unique) {
> -		pr_warn("virtio-fs: oh.unique mismatch (%llu != %llu)\n",
> -			oh->unique, req->in.h.unique);
> +	args = req->args;
> +	num_out = args->out_numargs - args->out_pages;
> +	cap = sizeof(req->out.h);
> +	cap += fuse_len_args(num_out, args->out_args);
> +	if (args->out_pages)
> +		cap += args->out_args[args->out_numargs - 1].size;
> +	if (unlikely(oh->len < sizeof(*oh) || oh->len > cap)) {
> +		pr_warn_ratelimited("virtio-fs: bad reply len %u (cap %u)\n",
> +					oh->len, cap);
>  		return false;
>  	}
> +
>  	return true;
>  }
>  
> @@ -841,7 +849,7 @@ static void virtio_fs_requests_done_work(struct work_struct *work)
>  		virtqueue_disable_cb(vq);
>  
>  		while ((req = virtqueue_get_buf(vq, &len)) != NULL) {
> -			if (!virtio_fs_verify_response(req, len)) {
> +			if (test_bit(FR_ISREPLY, &req->flags) && !virtio_fs_verify_response(req)) {
>  				req->out.h.error = -EIO;
>  				req->out.h.len = sizeof(struct fuse_out_header);
>  			}
> -- 
> 2.34.1
> 

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