io_uring/query: cap number of queries

If a query chain forms a cycle, it'll be looping in the kernel until the
process is killed. It might be fine as any such mistake can be easily
uncovered during testing, but it's still nicer to let it break out of
the syscall if it executed too many queries.

Suggested-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: isilence

io_uring/query.c

@@ -6,6 +6,7 @@
 #include "io_uring.h"
 
 #define IO_MAX_QUERY_SIZE		(sizeof(struct io_uring_query_opcode))
+#define IO_MAX_QUERY_ENTRIES		1000
 
 static ssize_t io_query_ops(void *data)
 {
@@ -74,7 +75,7 @@ int io_query(struct io_ring_ctx *ctx, void __user *arg, unsigned nr_args)
 {
 	char entry_buffer[IO_MAX_QUERY_SIZE];
 	void __user *uhdr = arg;
-	int ret;
+	int ret, nr = 0;
 
 	memset(entry_buffer, 0, sizeof(entry_buffer));
 
@@ -89,6 +90,9 @@ int io_query(struct io_ring_ctx *ctx, void __user *arg, unsigned nr_args)
 			return ret;
 		uhdr = u64_to_user_ptr(next_hdr);
 
+		/* Have some limit to avoid a potential cycle */
+		if (++nr >= IO_MAX_QUERY_ENTRIES)
+			return -ERANGE;
 		if (fatal_signal_pending(current))
 			return -EINTR;
 		cond_resched();