[PATCH 6.1.y] selinux: fix data race on AVC latest_notif

[Sasha Levin <sashal@kernel.org>]

From: Christian Göttsche <cgzones@googlemail.com>

[ Upstream commit 1ca924044bf0de879a1648be93a56939652267d3 ]

avc_policy_seqno() reads latest_notif without holding notif_lock, while
avc_latest_notif_update() can update it under that lock. KCSAN reports
these concurrent accesses during policy boolean updates and permission
checks.

Use READ_ONCE() for the lockless read and WRITE_ONCE() for the update.
The spinlock continues to serialize the comparison and update against
other writers. Reads made while holding notif_lock cannot race with a
writer and can remain plain accesses.

This marks both sides of the intentional concurrent access and constrains
compiler transformations, without suppressing checking with data_race().

Cc: stable@vger.kernel.org
Fixes: 788e7dd4c22e ("SELinux: Improve read/write performance")
Reported-by: syzbot+a7f7cb0737bd91aa698c@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=a7f7cb0737bd91aa698c
Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Acked-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>
[ sashal: Adapted direct selinux_avc accesses to the existing avc and state->avc pointers. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 security/selinux/avc.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/security/selinux/avc.c b/security/selinux/avc.c
index dcc8e1df4d7c5..e26f9b01b7305 100644
--- a/security/selinux/avc.c
+++ b/security/selinux/avc.c
@@ -589,7 +589,7 @@ static int avc_latest_notif_update(struct selinux_avc *avc,
 		}
 	} else {
 		if (seqno > avc->avc_cache.latest_notif)
-			avc->avc_cache.latest_notif = seqno;
+			WRITE_ONCE(avc->avc_cache.latest_notif, seqno);
 	}
 	spin_unlock_irqrestore(&notif_lock, flag);
 
@@ -1202,7 +1202,8 @@ int avc_has_perm(struct selinux_state *state, u32 ssid, u32 tsid, u16 tclass,
 
 u32 avc_policy_seqno(struct selinux_state *state)
 {
-	return state->avc->avc_cache.latest_notif;
+	/* Writers hold notif_lock; readers only sample the sequence number. */
+	return READ_ONCE(state->avc->avc_cache.latest_notif);
 }
 
 void avc_disable(void)
-- 
2.53.0