io_uring/rsrc: reject zero-length fixed buffer import

validate_fixed_range() admits buf_addr at the exact end of the
registered region when len is zero, because the check uses strict
greater-than (buf_end > imu->ubuf + imu->len).  io_import_fixed()
then computes offset == imu->len, which causes the bvec skip logic
to advance past the last bio_vec entry and read bv_offset from
out-of-bounds slab memory.

Return early from io_import_fixed() when len is zero.  A zero-length
import has no data to transfer and should not walk the bvec array
at all.

  BUG: KASAN: slab-out-of-bounds in io_import_reg_buf+0x697/0x7f0
  Read of size 4 at addr ffff888002bcc254 by task poc/103
  Call Trace:
   io_import_reg_buf+0x697/0x7f0
   io_write_fixed+0xd9/0x250
   __io_issue_sqe+0xad/0x710
   io_issue_sqe+0x7d/0x1100
   io_submit_sqes+0x86a/0x23c0
   __do_sys_io_uring_enter+0xa98/0x1590
  Allocated by task 103:
  The buggy address is located 12 bytes to the right of
   allocated 584-byte region [ffff888002bcc000, ffff888002bcc248)

Fixes: 8622b20f23 ("io_uring: add validate_fixed_range() for validate fixed buffer")
Signed-off-by: Qi Tang <tpluszz77@gmail.com>
Link: https://patch.msgid.link/20260329164936.240871-1-tpluszz77@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/rsrc.c

@@ -1062,6 +1062,10 @@ static int io_import_fixed(int ddir, struct iov_iter *iter,
 		return ret;
 	if (!(imu->dir & (1 << ddir)))
 		return -EFAULT;
+	if (unlikely(!len)) {
+		iov_iter_bvec(iter, ddir, NULL, 0, 0);
+		return 0;
+	}
 
 	offset = buf_addr - imu->ubuf;