io_uring/kbuf: sanitize peek buffer setup

Harden the buffer peeking a bit, by adding a sanity check for it having
a valid size. Outside of that, arg->max_len is a size_t, though it's
only ever set to a 32-bit value (as it's governed by MAX_RW_COUNT).
Bump our needed check to a size_t so we know it fits. Finally, cap the
calculated needed iov value to the PEEK_MAX_IMPORT, which is the
maximum number of segments that should be peeked.

Fixes: 35c8711c ("io_uring/kbuf: add helpers for getting/peeking multiple buffers")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/kbuf.c

@@ -218,10 +218,13 @@ static int io_ring_buffers_peek(struct io_kiocb *req, struct buf_sel_arg *arg,
 
 	buf = io_ring_head_to_buf(br, head, bl->mask);
 	if (arg->max_len) {
-		int needed;
+		u32 len = READ_ONCE(buf->len);
+		size_t needed;
 
-		needed = (arg->max_len + buf->len - 1) / buf->len;
-		needed = min(needed, PEEK_MAX_IMPORT);
+		if (unlikely(!len))
+			return -ENOBUFS;
+		needed = (arg->max_len + len - 1) / len;
+		needed = min_not_zero(needed, (size_t) PEEK_MAX_IMPORT);
 		if (nr_avail > needed)
 			nr_avail = needed;
 	}