[ghak-trim PATCH v1] audit: move put_tree() to avoid trim_trees refcount underflow and UAF

[Richard Guy Briggs]

AUDIT_TRIM is expected to be idempotent, but multiple executions resulted in a
refcount underflow and use-after-free.

git bisect fingered commit fb041bb7c0a918b95c6889fc965cdc4a75b4c0ca (2019-11)
	("locking/refcount: Consolidate implementations of refcount_t")
but this patch with its more thorough checking that wasn't in the x86 assembly
code merely exposed a previously existing tree refcount imbalance in the case
of tree trimming code that was refactored with prune_one() to remove a tree
introduced in commit 8432c70062978d9a57bde6715496d585ec520c3e (2018-11)
	("audit: Simplify locking around untag_chunk()")

Move the put_tree() to cover only the prune_one() case.

Passes audit-testsuite and 3 passes of "auditctl -t" with at least one
directory watch.

Fixes: 8432c7006297 ("audit: Simplify locking around untag_chunk()")
Signed-off-by: Richard Guy Briggs <rgb@xxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Seiji Nishikawa <snishika@xxxxxxxxxx>
---
 kernel/audit_tree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/audit_tree.c b/kernel/audit_tree.c
index b2be4e978ba3..2cd7b5694422 100644
--- a/kernel/audit_tree.c
+++ b/kernel/audit_tree.c
@@ -593,7 +593,6 @@ static void prune_tree_chunks(struct audit_tree *victim, bool tagged)
 		spin_lock(&hash_lock);
 	}
 	spin_unlock(&hash_lock);
-	put_tree(victim);
 }
 
 /*
@@ -602,6 +601,7 @@ static void prune_tree_chunks(struct audit_tree *victim, bool tagged)
 static void prune_one(struct audit_tree *victim)
 {
 	prune_tree_chunks(victim, false);
+	put_tree(victim);
 }
 
 /* trim the uncommitted chunks from tree */
-- 
2.27.0