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

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Message-ID: <35869346-9eed-4120-9e4b-cdc1b04dc844@canonical.com>
Date: Mon, 19 Jan 2026 16:44:35 -0800
From: John Johansen <john.johansen@...onical.com>
To: Zhengmian Hu <huzhengmian@...il.com>, john@...armor.net,
 apparmor@...ts.ubuntu.com
Cc: linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] apparmor: avoid per-cpu hold underflow in
 aa_get_buffer

On 1/19/26 16:03, Zhengmian Hu wrote:
> 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>

thanks, Acked-by: John Johansen <john.johansen@...onical.com>

I have pulled this into apparmor-next

> ---
>   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];