selinux: fix sidtab string cache locking

Avoiding taking a lock in an IRQ context is not enough to prevent
deadlocks, as discovered by syzbot:

===
WARNING: SOFTIRQ-safe -> SOFTIRQ-unsafe lock order detected
5.5.0-syzkaller #0 Not tainted
-----------------------------------------------------
syz-executor.0/8927 [HC0[0]:SC0[2]:HE1:SE0] is trying to acquire:
ffff888027c94098 (&(&s->cache_lock)->rlock){+.+.}, at: spin_lock include/linux/spinlock.h:338 [inline]
ffff888027c94098 (&(&s->cache_lock)->rlock){+.+.}, at: sidtab_sid2str_put.part.0+0x36/0x880 security/selinux/ss/sidtab.c:533

and this task is already holding:
ffffffff898639b0 (&(&nf_conntrack_locks[i])->rlock){+.-.}, at: spin_lock include/linux/spinlock.h:338 [inline]
ffffffff898639b0 (&(&nf_conntrack_locks[i])->rlock){+.-.}, at: nf_conntrack_lock+0x17/0x70 net/netfilter/nf_conntrack_core.c:91
which would create a new lock dependency:
 (&(&nf_conntrack_locks[i])->rlock){+.-.} -> (&(&s->cache_lock)->rlock){+.+.}

but this new dependency connects a SOFTIRQ-irq-safe lock:
 (&(&nf_conntrack_locks[i])->rlock){+.-.}

[...]

other info that might help us debug this:

 Possible interrupt unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&(&s->cache_lock)->rlock);
                               local_irq_disable();
                               lock(&(&nf_conntrack_locks[i])->rlock);
                               lock(&(&s->cache_lock)->rlock);
  <Interrupt>
    lock(&(&nf_conntrack_locks[i])->rlock);

 *** DEADLOCK ***
[...]
===

Fix this by simply locking with irqsave/irqrestore and stop giving up on
!in_task(). It makes the locking a bit slower, but it shouldn't make a
big difference in real workloads. Under the scenario from [1] (only
cache hits) it only increased the runtime overhead from the
security_secid_to_secctx() function from ~2% to ~3% (it was ~5-65%
before introducing the cache).

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1733259

Fixes: d97bd23 ("selinux: cache the SID -> context string translation")
Reported-by: syzbot+61cba5033e2072d61806@syzkaller.appspotmail.com
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/ss/sidtab.c

@@ -518,19 +518,13 @@ void sidtab_sid2str_put(struct sidtab *s, struct sidtab_entry *entry,
 			const char *str, u32 str_len)
 {
 	struct sidtab_str_cache *cache, *victim = NULL;
+	unsigned long flags;
 
 	/* do not cache invalid contexts */
 	if (entry->context.len)
 		return;
 
-	/*
-	 * Skip the put operation when in non-task context to avoid the need
-	 * to disable interrupts while holding s->cache_lock.
-	 */
-	if (!in_task())
-		return;
-
-	spin_lock(&s->cache_lock);
+	spin_lock_irqsave(&s->cache_lock, flags);
 
 	cache = rcu_dereference_protected(entry->cache,
 					  lockdep_is_held(&s->cache_lock));
@@ -561,7 +555,7 @@ void sidtab_sid2str_put(struct sidtab *s, struct sidtab_entry *entry,
 	rcu_assign_pointer(entry->cache, cache);
 
 out_unlock:
-	spin_unlock(&s->cache_lock);
+	spin_unlock_irqrestore(&s->cache_lock, flags);
 	kfree_rcu(victim, rcu_member);
 }