fuse: don't BUG when the copy buffer is exhausted

fuse_copy_fill() hits BUG_ON(!err) once the iterator backing the copy
runs out of pages.  Over io-uring the amount copied is not bounded by the
payload buffer the server registered, so an unprivileged user can drain
the iterator and panic the host - mount a FUSE filesystem with small
payload buffers, then setxattr() a larger value:

  kernel BUG at fs/fuse/dev.c:1064!
  Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI
  CPU: 1 UID: 0 PID: 148 Comm: exploit Not tainted 7.3.0-rc1 #2
  RIP: 0010:fuse_copy_fill (fs/fuse/dev.c:1033)
  Call Trace:
   <TASK>
   fuse_copy_args (fs/fuse/dev.c:1354 fs/fuse/dev.c:1380)
   fuse_uring_args_to_ring (fs/fuse/dev_uring.c:891)
   fuse_uring_prepare_send (fs/fuse/dev_uring.c:940 fs/fuse/dev_uring.c:1057)
   fuse_uring_send_in_task (fs/fuse/dev_uring.c:1745)
   tctx_task_work_run (io_uring/tw.c:96)
   tctx_task_work (io_uring/tw.c:133)
   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:2676)
   do_syscall_64 (arch/x86/entry/syscall_64.c:61)
   entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)

The reply path can drain it the same way, via the server-supplied
payload_sz in fuse_uring_copy_from_ring().

Return -EIO instead.  All three fuse_copy_fill() callers already
propagate the error.

Fixes: c090c8abae4b ("fuse: Add io-uring sqe commit and fetch support")
Cc: stable@vger.kernel.org
Reported-by: Weiming Shi <bestswngs@gmail.com>
Suggested-by: Joanne Koong <joannelkoong@gmail.com>
Assisted-by: Claude:claude-opus-4-8
Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Xiang Mei <xmei5@asu.edu>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

0 files changed, 0 insertions, 0 deletions