[PATCH v2] xfrm: prevent policy_hthresh.work from racing with netns teardown

[PATCH v2] xfrm: prevent policy_hthresh.work from racing with netns teardown

[Tahera Fahimi]

An XFRM_MSG_NEWSPDINFO request can queue the per-net work item
policy_hthresh.work onto the system workqueue. xfrm_hash_rebuild()
retrieves the enclosing struct net from the work item, so the callback
can dereference freed memory if it runs after net namespace teardown.

Mainline commit 29fe3a61bcdc ("xfrm: prevent policy_hthresh.work
from racing with netns teardown") fixes this issue using
disable_work_sync(). That API is not available in Linux 6.6, so the
mainline change cannot be directly backported.

Provide the same guarantee with an XFRM-local work_disabled flag. Protect
the flag and schedule_work() with the existing policy_hthresh seqlock so
teardown can atomically stop new queueing attempts. Then use
cancel_work_sync() to synchronize work that was queued before the flag was
set. This ensures policy_hthresh.work cannot outlive its struct net.

Fixes: 880a6fab8f6b ("xfrm: configure policy hash table thresholds by netlink")
Assisted-by: GitHub-Copilot:GPT-5.6 Sol
Signed-off-by: Tahera Fahimi <taherafahimi@linux.microsoft.com>
Reviewed-by: Allen Pais <apais@linux.microsoft.com>
---
changes in v2:
- removing incorrect upstream commit annotation from description

 include/net/netns/xfrm.h |  1 +
 net/xfrm/xfrm_policy.c   | 19 ++++++++++++++++++-
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/include/net/netns/xfrm.h b/include/net/netns/xfrm.h
index 423b52eca908d..99eb5c2ff888b 100644
--- a/include/net/netns/xfrm.h
+++ b/include/net/netns/xfrm.h
@@ -23,6 +23,7 @@ struct xfrm_policy_hash {
 struct xfrm_policy_hthresh {
 	struct work_struct	work;
 	seqlock_t		lock;
+	bool			work_disabled;
 	u8			lbits4;
 	u8			rbits4;
 	u8			lbits6;
diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
index 00d9693c13ae7..cc8f4d4b70875 100644
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -1384,7 +1384,10 @@ static void xfrm_hash_rebuild(struct work_struct *work)
 
 void xfrm_policy_hash_rebuild(struct net *net)
 {
-	schedule_work(&net->xfrm.policy_hthresh.work);
+	write_seqlock(&net->xfrm.policy_hthresh.lock);
+	if (!net->xfrm.policy_hthresh.work_disabled)
+		schedule_work(&net->xfrm.policy_hthresh.work);
+	write_sequnlock(&net->xfrm.policy_hthresh.lock);
 }
 EXPORT_SYMBOL(xfrm_policy_hash_rebuild);
 
@@ -4181,6 +4184,7 @@ static int __net_init xfrm_policy_init(struct net *net)
 	net->xfrm.policy_hthresh.rbits6 = 128;
 
 	seqlock_init(&net->xfrm.policy_hthresh.lock);
+	net->xfrm.policy_hthresh.work_disabled = false;
 
 	INIT_LIST_HEAD(&net->xfrm.policy_all);
 	INIT_LIST_HEAD(&net->xfrm.inexact_bins);
@@ -4206,6 +4210,19 @@ static void xfrm_policy_fini(struct net *net)
 	unsigned int sz;
 	int dir;
 
+	/* Prevent new policy hash rebuilds before draining the work item.
+	 *
+	 * The upstream fix uses disable_work_sync(), which is unavailable
+	 * in v6.6. Protecting work_disabled and schedule_work() with the
+	 * same seqlock closes the check-to-queue race, while the subsequent
+	 * cancel_work_sync() drains work that was queued or running before
+	 * teardown disabled it.
+	 */
+	write_seqlock(&net->xfrm.policy_hthresh.lock);
+	net->xfrm.policy_hthresh.work_disabled = true;
+	write_sequnlock(&net->xfrm.policy_hthresh.lock);
+	cancel_work_sync(&net->xfrm.policy_hthresh.work);
+
 	flush_work(&net->xfrm.policy_hash_work);
 #ifdef CONFIG_XFRM_SUB_POLICY
 	xfrm_policy_flush(net, XFRM_POLICY_TYPE_SUB, false);
-- 
2.43.0

Re: [PATCH v2] xfrm: prevent policy_hthresh.work from racing with netns teardown

[netdev-bot+sinfo]

Hi!

This is an automated message. This series looks like a fix, but its
commit messages seem to be missing some information:

 - Whether the issue was actually triggered, or is only theoretical
   (e.g. found by code inspection). If it was triggered please include
   the symptoms, like the stack trace or error messages.

Please do not repost the series just to address the above. Instead,
reply to this email with the missing information, so that reviewers
can take it into account. If the series needs another revision for
other reasons, please include the information in the commit messages
then.

The evaluation is done by an LLM so it may be wrong, if you think
that is the case please reply and explain.