[PATCH 5.14 11/11] audit: move put_tree() to avoid trim_trees refcount underflow and UAF

From: Greg Kroah-Hartman
Date: Wed Sep 01 2021 - 08:55:12 EST

From: Richard Guy Briggs <rgb@xxxxxxxxxx>

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 <jack@xxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Seiji Nishikawa <snishika@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Fixes: 8432c7006297 ("audit: Simplify locking around untag_chunk()")
Signed-off-by: Richard Guy Briggs <rgb@xxxxxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
[PM: reformatted/cleaned-up the commit description]
Signed-off-by: Paul Moore <paul@xxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
kernel/audit_tree.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/audit_tree.c
+++ b/kernel/audit_tree.c
@@ -593,7 +593,6 @@ static void prune_tree_chunks(struct aud
- put_tree(victim);

@@ -602,6 +601,7 @@ static void prune_tree_chunks(struct aud
static void prune_one(struct audit_tree *victim)
prune_tree_chunks(victim, false);
+ put_tree(victim);

/* trim the uncommitted chunks from tree */