[PATCH 5.10.y] xfrm: prevent policy_hthresh.work from racing with netns teardown

[PATCH 5.10.y] 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 5.10, 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>
---
 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 47bc0ec051eb5..bafdfdccbd954 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 26bc082139550..c14f495f62b80 100644
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -1378,7 +1378,10 @@ 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);
 
@@ -4129,6 +4132,7 @@ 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);
@@ -4154,6 +4158,19 @@ 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