linux-kernel - [PATCH v2 1/1] apparmor: avoid per-cpu hold underflow in aa_get

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Message-ID: <20260120000307.369587-2-huzhengmian@gmail.com>
Date: Mon, 19 Jan 2026 19:03:07 -0500
From: Zhengmian Hu <huzhengmian@...il.com>
To: john.johansen@...onical.com,
	john@...armor.net,
	apparmor@...ts.ubuntu.com
Cc: linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Zhengmian Hu <huzhengmian@...il.com>
Subject: [PATCH v2 1/1] 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.

Signed-off-by: Zhengmian Hu <huzhengmian@...il.com>
---
 security/apparmor/lsm.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
index 9b6c2f157..a6c884ba6 100644
--- a/security/apparmor/lsm.c
+++ b/security/apparmor/lsm.c
@@ -1868,7 +1868,8 @@ 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];
-- 
2.52.0