audit: move put_tree() to avoid trim_trees refcount underflow and UAF

commit 67d69e9d1a6c889d98951c1d74b19332ce0565af upstream.

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

git bisect fingered commit fb041bb7c0a9	("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 8432c7006297 ("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.

Cc: Jan Kara <>
Cc: Will Deacon <>
Cc: Alexander Viro <>
Cc: Seiji Nishikawa <>
Fixes: 8432c7006297 ("audit: Simplify locking around untag_chunk()")
Signed-off-by: Richard Guy Briggs <>
Reviewed-by: Jan Kara <>
[PM: reformatted/cleaned-up the commit description]
Signed-off-by: Paul Moore <>
Signed-off-by: Greg Kroah-Hartman <>
diff --git a/kernel/audit_tree.c b/kernel/audit_tree.c
index e49c912..9dec631 100644
--- a/kernel/audit_tree.c
+++ b/kernel/audit_tree.c
@@ -595,7 +595,6 @@
-	put_tree(victim);
@@ -604,6 +603,7 @@
 static void prune_one(struct audit_tree *victim)
 	prune_tree_chunks(victim, false);
+	put_tree(victim);
 /* trim the uncommitted chunks from tree */