[PATCH 6.18.y 6/6] fuse: copy request headers via a stack buffer for io-uring

[Sasha Levin <sashal@kernel.org>]

From: Xiang Mei <xmei5@asu.edu>

[ Upstream commit fd10f40af314f07b6d6e028b1ca25c8b49903aab ]

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!
  Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI
  RIP: 0010:usercopy_abort (mm/usercopy.c:90)
  Call Trace:
   __check_heap_object (mm/slub.c:8268)
   __check_object_size (mm/usercopy.c:197 mm/usercopy.c:258 mm/usercopy.c:223)
   copy_header_to_ring (fs/fuse/dev_uring.c:618)
   fuse_uring_prepare_send (fs/fuse/dev_uring.c:776 fs/fuse/dev_uring.c:785)
   fuse_uring_send_in_task (fs/fuse/dev_uring.c:1306)
   tctx_task_work_run (io_uring/tw.c:96)
   task_work_run (kernel/task_work.c:233)
   io_run_task_work (io_uring/tw.h:84)
   io_cqring_wait (io_uring/wait.c:278)
   __do_sys_io_uring_enter (io_uring/io_uring.c:2685)
   entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)

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: Bernd Schubert <bernd@bsbernd.com>
Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
[ merged declaration of `in_header`/`out_header` alongside pre-existing `ring`/`fc` locals introduced by prior refactors ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 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 95c23c3499e78..fc84ef09a164b 100644
--- a/fs/fuse/dev_uring.c
+++ b/fs/fuse/dev_uring.c
@@ -745,6 +745,7 @@ 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;
@@ -766,8 +767,9 @@ 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,
@@ -901,11 +903,13 @@ static void fuse_uring_commit(struct fuse_ring_ent *ent, struct fuse_req *req,
 {
 	struct fuse_ring *ring = ent->queue->ring;
 	struct fuse_conn *fc = ring->fc;
+	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, fc);
 	if (err) {
-- 
2.53.0