From: Zhengmian Hu <huzhengmian@gmail.com>
Date: Tue, 20 Jan 2026 00:03:07 +0000 (-0500)
Subject: apparmor: avoid per-cpu hold underflow in aa_get_buffer
X-Git-Tag: v7.0-rc1~35^2~9
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=640cf2f09575c9dc344b3f7be2498d31e3923ead;p=thirdparty%2Flinux.git

apparmor: avoid per-cpu hold underflow in aa_get_buffer

When aa_get_buffer() pulls from the per-cpu list it unconditionally
decrements cache->hold. If hold reaches 0 while count is still non-zero,
the unsigned decrement wraps to UINT_MAX. This keeps hold non-zero for a
very long time, so aa_put_buffer() never returns buffers to the global
list, which can starve other CPUs and force repeated kmalloc(aa_g_path_max)
allocations.

Guard the decrement so hold never underflows.
Fixes: ea9bae12d028 ("apparmor: cache buffers on percpu list if there is lock contention")

Signed-off-by: Zhengmian Hu <huzhengmian@gmail.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>
---

diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
index 8d5d9a966b71..9175fd677ef3 100644
--- a/security/apparmor/lsm.c
+++ b/security/apparmor/lsm.c
@@ -2137,7 +2137,8 @@ char *aa_get_buffer(bool in_atomic)
 	if (!list_empty(&cache->head)) {
 		aa_buf = list_first_entry(&cache->head, union aa_buffer, list);
 		list_del(&aa_buf->list);
-		cache->hold--;
+		if (cache->hold)
+			cache->hold--;
 		cache->count--;
 		put_cpu_ptr(&aa_local_buffers);
 		return &aa_buf->buffer[0];