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Fix DEFER_TASKRUN ring resize flag manipulation

PATCH New Jens Axboe 2 participants 5 messages Mar 11

Subsystems (2)
Stable branch io_uring
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Jens Axboe

author maintainer
#0 lore raw

PATCH v2 1/1 Fix DEFER_TASKRUN ring resize flag manipulation

Hi,

Two patches here:

1) Fix adding local task_work during ring resize. There's a tiny gap where a NULL pointer would be used.

2) Same issue exists in the eventfd handling, so apply the same kind of fix there.

Thanks to Hao-Yu Yang for the report and initial fix attempt, and Pavel for a good suggestion on how best to handle this.

Changes since v1:
- Don't clear -rings_rcu during resize, old rings remain valid until post RCU synchronize anyway (Pavel)

Signature · 1 line
Jens Axboe
author maintainer
#1 lore raw

PATCH 1/2 io_uring: ensure ctx->rings is stable for task work flags manipulation

If DEFER_TASKRUN | SETUP_TASKRUN is used and task work is added while the ring is being resized, it's possible for the OR'ing of IORING_SQ_TASKRUN to happen in the small window of swapping into the new rings and the old rings being freed.

Prevent this by adding a 2nd ->rings pointer, ->rings_rcu, which is protected by RCU. The task work flags manipulation is inside RCU already, and if the resize ring freeing is done post an RCU synchronize, then there's no need to add locking to the fast path of task work additions.

Note: this is only done for DEFER_TASKRUN, as that's the only setup mode that supports ring resizing. If this ever changes, then they too need to use the io_ctx_mark_taskrun() helper.

Link https://lore.kernel.org/io-uring/[email protected]/ open here
Cc [email protected]
Fixes 79cfe9e59c2a ("io_uring/register: add IORING_REGISTER_RESIZE_RINGS")
Reported-by Hao-Yu Yang <[email protected]>
Suggested-by Pavel Begunkov <[email protected]>
Signed-off-by Jens Axboe <[email protected]>
---
 include/linux/io_uring_types.h |  1 +
 io_uring/io_uring.c            |  2 ++
 io_uring/register.c            | 11 +++++++++++
 io_uring/tw.c                  | 21 +++++++++++++++++++--
 4 files changed, 33 insertions(+), 2 deletions(-)
include/linux/io_uring_types.h +1 -0
index 3e4a82a6f817..dd1420bfcb73 100644
@@ -388,6 +388,7 @@ struct io_ring_ctx {
 	 * regularly bounce b/w CPUs.
 	 */
 	struct {
+		struct io_rings	__rcu	*rings_rcu;
 		struct llist_head	work_llist;
 		struct llist_head	retry_llist;
 		unsigned long		check_cq;
io_uring/io_uring.c +2 -0
index ccab8562d273..20fdc442e014 100644
@@ -2066,6 +2066,7 @@ static void io_rings_free(struct io_ring_ctx *ctx)
 	io_free_region(ctx->user, &ctx->sq_region);
 	io_free_region(ctx->user, &ctx->ring_region);
 	ctx->rings = NULL;
+	RCU_INIT_POINTER(ctx->rings_rcu, NULL);
 	ctx->sq_sqes = NULL;
 }
 

@@ -2703,6 +2704,7 @@ static __cold int io_allocate_scq_urings(struct io_ring_ctx *ctx,
 	if (ret)
 		return ret;
 	ctx->rings = rings = io_region_get_ptr(&ctx->ring_region);
+	rcu_assign_pointer(ctx->rings_rcu, rings);
 	if (!(ctx->flags & IORING_SETUP_NO_SQARRAY))
 		ctx->sq_array = (u32 *)((char *)rings + rl->sq_array_offset);
io_uring/register.c +11 -0
index a839b22fd392..6d3e65b17514 100644
@@ -633,7 +633,15 @@ static int io_register_resize_rings(struct io_ring_ctx *ctx, void __user *arg)
 	ctx->sq_entries = p->sq_entries;
 	ctx->cq_entries = p->cq_entries;
 
+	/*
+	 * Just mark any flag we may have missed and that the application
+	 * should act on unconditionally. Worst case it'll be an extra
+	 * syscall.
+	 */
+	atomic_or(IORING_SQ_TASKRUN | IORING_SQ_NEED_WAKEUP, &n.rings->sq_flags);
 	ctx->rings = n.rings;
+	rcu_assign_pointer(ctx->rings_rcu, n.rings);
+
 	ctx->sq_sqes = n.sq_sqes;
 	swap_old(ctx, o, n, ring_region);
 	swap_old(ctx, o, n, sq_region);

@@ -642,6 +650,9 @@ static int io_register_resize_rings(struct io_ring_ctx *ctx, void __user *arg)
 out:
 	spin_unlock(&ctx->completion_lock);
 	mutex_unlock(&ctx->mmap_lock);
+	/* Wait for concurrent io_ctx_mark_taskrun() */
+	if (to_free == &o)
+		synchronize_rcu();
 	io_register_free_rings(ctx, to_free);
 
 	if (ctx->sq_data)
io_uring/tw.c +19 -2
index 1ee2b8ab07c8..0c860a7e6c61 100644
@@ -152,6 +152,20 @@ void tctx_task_work(struct callback_head *cb)
 	WARN_ON_ONCE(ret);
 }
 
+/*
+ * Sets IORING_SQ_TASKRUN in the sq_flags shared with userspace, using the
+ * RCU protected rings pointer to be safe against concurrent ring resizing.
+ * Must be called inside an RCU read-side critical section.
+ */
+static void io_ctx_mark_taskrun(struct io_ring_ctx *ctx)
+{
+	if (ctx->flags & IORING_SETUP_TASKRUN_FLAG) {
+		struct io_rings *rings = rcu_dereference(ctx->rings_rcu);
+
+		atomic_or(IORING_SQ_TASKRUN, &rings->sq_flags);
+	}
+}
+
 void io_req_local_work_add(struct io_kiocb *req, unsigned flags)
 {
 	struct io_ring_ctx *ctx = req->ctx;

@@ -206,8 +220,7 @@ void io_req_local_work_add(struct io_kiocb *req, unsigned flags)
 	 */
 
 	if (!head) {
-		if (ctx->flags & IORING_SETUP_TASKRUN_FLAG)
-			atomic_or(IORING_SQ_TASKRUN, &ctx->rings->sq_flags);
+		io_ctx_mark_taskrun(ctx);
 		if (ctx->has_evfd)
 			io_eventfd_signal(ctx, false);
 	}

@@ -231,6 +244,10 @@ void io_req_normal_work_add(struct io_kiocb *req)
 	if (!llist_add(&req->io_task_work.node, &tctx->task_list))
 		return;
 
+	/*
+	 * Doesn't need to use ->rings_rcu, as resizing isn't supported for
+	 * !DEFER_TASKRUN.
+	 */
 	if (ctx->flags & IORING_SETUP_TASKRUN_FLAG)
 		atomic_or(IORING_SQ_TASKRUN, &ctx->rings->sq_flags);
Signature · 1 line
2.53.0
#2 lore raw

Re: [PATCH 1/2] io_uring: ensure ctx->rings is stable for task work flags manipulation

Quoted 4 lines · Jens Axboe

+/*
+ * Sets IORING_SQ_TASKRUN in the sq_flags shared with userspace, using the
+ * RCU protected rings pointer to be safe against concurrent ring resizing.
+ * Must be called inside an RCU read-side critical section.

You can make the rcu requirement explicit in the code with:

	ASSERT(rcu_read_lock_held());

And debug kernels will catch misuse, too.

Quoted 9 lines

+ */

io_uring/tw.c +8 -0
+static void io_ctx_mark_taskrun(struct io_ring_ctx *ctx)
+{
+	if (ctx->flags & IORING_SETUP_TASKRUN_FLAG) {
+		struct io_rings *rings = rcu_dereference(ctx->rings_rcu);
+
+		atomic_or(IORING_SQ_TASKRUN, &rings->sq_flags);
+	}
+}
author maintainer
#3 lore raw

Re: [PATCH 1/2] io_uring: ensure ctx->rings is stable for task work flags manipulation

Quoted 11 lines · Keith Busch
Quoted 4 lines · Jens Axboe

+/*
+ * Sets IORING_SQ_TASKRUN in the sq_flags shared with userspace, using the
+ * RCU protected rings pointer to be safe against concurrent ring resizing.
+ * Must be called inside an RCU read-side critical section.

You can make the rcu requirement explicit in the code with:

	ASSERT(rcu_read_lock_held());

And debug kernels will catch misuse, too.

We have lockdep_assert_in_rcu_read_lock(), that should do it. Did ponder that, and then I could also kill the comment as it's self documenting by that point.

Signature · 1 line
Jens Axboe
author maintainer
#4 lore raw

PATCH 2/2 io_uring/eventfd: use ctx->rings_rcu for flags checking

In reply to Jens Axboe

Similarly to what commit e78f7b70e837 did for local task work additions, use ->rings_rcu under RCU rather than dereference ->rings directly. See that commit for more details.

Cc [email protected]
Fixes 79cfe9e59c2a ("io_uring/register: add IORING_REGISTER_RESIZE_RINGS")
Signed-off-by Jens Axboe <[email protected]>
---
 io_uring/eventfd.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)
io_uring/eventfd.c +7 -3
index 78f8ab7db104..ab789e1ebe91 100644
@@ -76,11 +76,15 @@ void io_eventfd_signal(struct io_ring_ctx *ctx, bool cqe_event)
 {
 	bool skip = false;
 	struct io_ev_fd *ev_fd;
-
-	if (READ_ONCE(ctx->rings->cq_flags) & IORING_CQ_EVENTFD_DISABLED)
-		return;
+	struct io_rings *rings;
 
 	guard(rcu)();
+
+	rings = rcu_dereference(ctx->rings_rcu);
+	if (!rings)
+		return;
+	if (READ_ONCE(rings->cq_flags) & IORING_CQ_EVENTFD_DISABLED)
+		return;
 	ev_fd = rcu_dereference(ctx->io_ev_fd);
 	/*
 	 * Check again if ev_fd exists in case an io_eventfd_unregister call
Signature · 1 line
2.53.0