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

Brajesh Patil
2025-10-28T20:03:29Z

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);
+
+ 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