A recent commit:

fc582cd26e88 ("io_uring/msg_ring: ensure io_kiocb freeing is deferred for RCU")

fixed an issue with not deferring freeing of io_kiocb structs that
msg_ring allocates to after the current RCU grace period. But this only
covers requests that don't end up in the allocation cache. If a request
goes into the alloc cache, it can get reused before it is sane to do so.
A recent syzbot report would seem to indicate that there's something
there, however it may very well just be because of the KASAN poisoning
that the alloc_cache handles manually.

Rather than attempt to make the alloc_cache sane for that use case, just
drop the usage of the alloc_cache for msg_ring request payload data.

Fixes: 50cf5f3842af ("io_uring/msg_ring: add an alloc cache for io_kiocb entries")
Link: https://lore.kernel.org/io-uring/68cc2687.050a0220.139b6.0005.GAE@google.com/
Reported-by: syzbot+baa2e0f4e02df602583e@syzkaller.appspotmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

---

diff --git a/include/linux/io_uring_types.h b/include/linux/io_uring_types.h
index 80a178f3d896..12f5ee43850e 100644
--- a/include/linux/io_uring_types.h
+++ b/include/linux/io_uring_types.h
@@ -420,9 +420,6 @@ struct io_ring_ctx {
 	struct list_head		defer_list;
 	unsigned			nr_drained;
 
-	struct io_alloc_cache		msg_cache;
-	spinlock_t			msg_lock;
-
 #ifdef CONFIG_NET_RX_BUSY_POLL
 	struct list_head	napi_list;	/* track busy poll napi_id */
 	spinlock_t		napi_lock;	/* napi_list lock */
diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index bcec12256f34..93665cebe9bd 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -290,7 +290,6 @@ static void io_free_alloc_caches(struct io_ring_ctx *ctx)
 	io_alloc_cache_free(&ctx->netmsg_cache, io_netmsg_cache_free);
 	io_alloc_cache_free(&ctx->rw_cache, io_rw_cache_free);
 	io_alloc_cache_free(&ctx->cmd_cache, io_cmd_cache_free);
-	io_alloc_cache_free(&ctx->msg_cache, kfree);
 	io_futex_cache_free(ctx);
 	io_rsrc_cache_free(ctx);
 }
@@ -337,9 +336,6 @@ static __cold struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
 	ret |= io_alloc_cache_init(&ctx->cmd_cache, IO_ALLOC_CACHE_MAX,
 			    sizeof(struct io_async_cmd),
 			    sizeof(struct io_async_cmd));
-	spin_lock_init(&ctx->msg_lock);
-	ret |= io_alloc_cache_init(&ctx->msg_cache, IO_ALLOC_CACHE_MAX,
-			    sizeof(struct io_kiocb), 0);
 	ret |= io_futex_cache_init(ctx);
 	ret |= io_rsrc_cache_init(ctx);
 	if (ret)
diff --git a/io_uring/msg_ring.c b/io_uring/msg_ring.c
index 4c2578f2efcb..5e5b94236d72 100644
--- a/io_uring/msg_ring.c
+++ b/io_uring/msg_ring.c
@@ -11,7 +11,6 @@
 #include "io_uring.h"
 #include "rsrc.h"
 #include "filetable.h"
-#include "alloc_cache.h"
 #include "msg_ring.h"
 
 /* All valid masks for MSG_RING */
@@ -76,13 +75,7 @@ static void io_msg_tw_complete(struct io_kiocb *req, io_tw_token_t tw)
 	struct io_ring_ctx *ctx = req->ctx;
 
 	io_add_aux_cqe(ctx, req->cqe.user_data, req->cqe.res, req->cqe.flags);
-	if (spin_trylock(&ctx->msg_lock)) {
-		if (io_alloc_cache_put(&ctx->msg_cache, req))
-			req = NULL;
-		spin_unlock(&ctx->msg_lock);
-	}
-	if (req)
-		kfree_rcu(req, rcu_head);
+	kfree_rcu(req, rcu_head);
 	percpu_ref_put(&ctx->refs);
 }
 
@@ -104,26 +97,13 @@ static int io_msg_remote_post(struct io_ring_ctx *ctx, struct io_kiocb *req,
 	return 0;
 }
 
-static struct io_kiocb *io_msg_get_kiocb(struct io_ring_ctx *ctx)
-{
-	struct io_kiocb *req = NULL;
-
-	if (spin_trylock(&ctx->msg_lock)) {
-		req = io_alloc_cache_get(&ctx->msg_cache);
-		spin_unlock(&ctx->msg_lock);
-		if (req)
-			return req;
-	}
-	return kmem_cache_alloc(req_cachep, GFP_KERNEL | __GFP_NOWARN | __GFP_ZERO);
-}
-
 static int io_msg_data_remote(struct io_ring_ctx *target_ctx,
 			      struct io_msg *msg)
 {
 	struct io_kiocb *target;
 	u32 flags = 0;
 
-	target = io_msg_get_kiocb(target_ctx);
+	target = kmem_cache_alloc(req_cachep, GFP_KERNEL | __GFP_NOWARN | __GFP_ZERO)  ;
 	if (unlikely(!target))
 		return -ENOMEM;
 
-- 
Jens Axboe