io_uring/fdinfo: fix OOB read in SQE_MIXED wrap check

__io_uring_show_fdinfo() iterates over pending SQEs and, for 128-byte
SQEs on an IORING_SETUP_SQE_MIXED ring, needs to detect when the second
half of the SQE would be past the end of the sq_sqes array. The current
check tests (++sq_head & sq_mask) == 0, but sq_head is only incremented
when a 128-byte SQE is encountered, not on every iteration. The actual
array index is sq_idx = (i + sq_head) & sq_mask, which can be sq_mask
(the last slot) while the wrap check passes.

Fix by checking sq_idx directly. Keep the sq_head increment so the loop
still skips the second half of the 128-byte SQE on the next iteration.

Fixes: 1cba30b ("io_uring: add support for IORING_SETUP_SQE_MIXED")
Signed-off-by: Nicholas Carlini <nicholas@carlini.com>
Link: https://patch.msgid.link/20260327021823.3138396-1-nicholas@carlini.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: carlini

io_uring/fdinfo.c

@@ -119,12 +119,13 @@ static void __io_uring_show_fdinfo(struct io_ring_ctx *ctx, struct seq_file *m)
 					sq_idx);
 				break;
 			}
-			if ((++sq_head & sq_mask) == 0) {
+			if (sq_idx == sq_mask) {
 				seq_printf(m,
 					"%5u: corrupted sqe, wrapping 128B entry\n",
 					sq_idx);
 				break;
 			}
+			sq_head++;
 			i++;
 			sqe128 = true;
 		}