io_uring: add request poisoning

Poison various request fields on free. __io_req_caches_free() is a slow
path, so can be done unconditionally, but gate it on kasan for
io_req_add_to_cache(). Note that some fields are logically retained
between cache allocations and can't be poisoned in
io_req_add_to_cache().

Ideally, it'd be replaced with KASAN'ed caches, but that can't be
enabled because of some synchronisation nuances.

Signed-off-by: Pavel Begunkov <[e-mail]>
Link: https://lore.kernel.org/r/7a78[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

include/linux/poison.h

···
 /********** lib/stackdepot.c **********/
 #define STACK_DEPOT_POISON ((void *)(0xD390 + POISON_POINTER_DELTA))
 
+/********** io_uring/ **********/
+#define IO_URING_PTR_POISON ((void *)(0x1091UL + POISON_POINTER_DELTA))
+
 #endif

io_uring/io_uring.c

···
 };
 #endif
 
+static void io_poison_cached_req(struct io_kiocb *req)
+{
+	req->ctx = IO_URING_PTR_POISON;
+	req->tctx = IO_URING_PTR_POISON;
+	req->file = IO_URING_PTR_POISON;
+	req->creds = IO_URING_PTR_POISON;
+	req->io_task_work.func = IO_URING_PTR_POISON;
+	req->apoll = IO_URING_PTR_POISON;
+}
+
+static void io_poison_req(struct io_kiocb *req)
+{
+	io_poison_cached_req(req);
+	req->async_data = IO_URING_PTR_POISON;
+	req->kbuf = IO_URING_PTR_POISON;
+	req->comp_list.next = IO_URING_PTR_POISON;
+	req->file_node = IO_URING_PTR_POISON;
+	req->link = IO_URING_PTR_POISON;
+}
+
 static inline unsigned int __io_cqring_events(struct io_ring_ctx *ctx)
 {
 	return ctx->cached_cq_tail - READ_ONCE(ctx->rings->cq.head);
···
 
 static inline void io_req_add_to_cache(struct io_kiocb *req, struct io_ring_ctx *ctx)
 {
+	if (IS_ENABLED(CONFIG_KASAN))
+		io_poison_cached_req(req);
 	wq_stack_add_head(&req->comp_list, &ctx->submit_state.free_list);
 }
 
···
 
 	while (!io_req_cache_empty(ctx)) {
 		req = io_extract_req(ctx);
+		io_poison_req(req);
 		kmem_cache_free(req_cachep, req);
 		nr++;
 	}