Patch "RDMAVT: Fix synchronization around percpu_ref" has been added to the 4.14-stable tree

This is a note to let you know that I've just added the patch titled

RDMAVT: Fix synchronization around percpu_ref

to the 4.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
rdmavt-fix-synchronization-around-percpu_ref.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Tejun Heo <[email protected]>
Date: Wed, 14 Mar 2018 12:10:18 -0700
Subject: RDMAVT: Fix synchronization around percpu_ref

From: Tejun Heo <[email protected]>

commit 74b44bbe80b4c62113ac1501482ea1ee40eb9d67 upstream.

rvt_mregion uses percpu_ref for reference counting and RCU to protect accesses from lkey_table. When a rvt_mregion needs to be freed, it first gets unregistered from lkey_table and then rvt_check_refs() is called to wait for in-flight usages before the rvt_mregion is freed.

rvt_check_refs() seems to have a couple issues.

* It has a fast exit path which tests percpu_ref_is_zero(). However, a percpu_ref reading zero doesn't mean that the object can be released. In fact, the ->release() callback might not even have started executing yet. Proceeding with freeing can lead to use-after-free.

* lkey_table is RCU protected but there is no RCU grace period in the free path. percpu_ref uses RCU internally but it's sched-RCU whose grace periods are different from regular RCU. Also, it generally isn't a good idea to depend on internal behaviors like this.

To address the above issues, this patch removes the fast exit and adds an explicit synchronize_rcu().

---
 drivers/infiniband/sw/rdmavt/mr.c |   10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

@@ -489,11 +489,13 @@ static int rvt_check_refs(struct rvt_mre
 	unsigned long timeout;
 	struct rvt_dev_info *rdi = ib_to_rvt(mr->pd->device);
 
-	if (percpu_ref_is_zero(&mr->refcount))
-		return 0;
-	/* avoid dma mr */
-	if (mr->lkey)
+	if (mr->lkey) {
+		/* avoid dma mr */
 		rvt_dereg_clean_qps(mr);
+		/* @mr was indexed on rcu protected @lkey_table */
+		synchronize_rcu();
+	}
+
 	timeout = wait_for_completion_timeout(&mr->comp, 5 * HZ);
 	if (!timeout) {
 		rvt_pr_err(rdi,

Patches currently in stable-queue which might be from [email protected] are

queue-4.14/rdmavt-fix-synchronization-around-percpu_ref.patch queue-4.14/fs-aio-add-explicit-rcu-grace-period-when-freeing-kioctx.patch queue-4.14/fs-aio-use-rcu-accessors-for-kioctx_table-table.patch