linux-kernel - Re: [PATCH] fix: Prevent memory leak by checking for NULL buffer before calling css

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Date: Mon, 3 Jun 2024 10:31:38 -0700
From: Axel Rasmussen <axelrasmussen@...gle.com>
To: Geunsik Lim <geunsik.lim@...il.com>
Cc: Vlastimil Babka <vbabka@...e.cz>, Ingo Molnar <mingo@...e.hu>, 
	Andrew Morton <akpm@...ux-foundation.org>, Thomas Gleixner <tglx@...utronix.de>, 
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, Hugh Dickins <hughd@...gle.com>, 
	"H. Peter Anvin" <hpa@...or.com>, Steven Rostedt <rostedt@...dmis.org>, 
	linux-kernel <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH] fix: Prevent memory leak by checking for NULL buffer
 before calling css_put()

On Mon, Jun 3, 2024 at 5:33 AM Geunsik Lim <geunsik.lim@...il.com> wrote:
>
> This commit addresses a potential memory leak in the
> `get_mm_memcg_path()` function
> by explicitly checking if the allocated buffer (`buf`) is NULL before
> calling the
> `css_put()` function. The prefix 'css' means abbreviation of cgroup_subsys_state
>
> Previously, the code would directly call `css_put()` without checking
> the value of
> `buf`, which could lead to a memory leak if the buffer allocation failed.
> This commit introduces a conditional check to ensure that `css_put()`
> is only called
> if `buf` is not NULL.
>
> This change enhances the code's robustness and prevents memory leaks, improving
> overall system stability.
>
> **Specific Changes:**
>
> * In the `out_put` label, an `if` statement is added to check
>   if `buf` is not NULL before calling `css_put()`.
>
> **Benefits:**
>
> * Prevents potential memory leaks
> * Enhances code robustness
> * Improves system stability
>
> Signed-off-by: Geunsik Lim <leemgs@...il.com>
> Signed-off-by: Geunsik Lim <geunsik.lim@...sung.com>
> ---
>  mm/mmap_lock.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c
> index 1854850b4b89..7314045b0e3b 100644
> --- a/mm/mmap_lock.c
> +++ b/mm/mmap_lock.c
> @@ -213,7 +213,8 @@ static const char *get_mm_memcg_path(struct mm_struct *mm)
>         cgroup_path(memcg->css.cgroup, buf, MEMCG_PATH_BUF_SIZE);
>
>  out_put:
> -       css_put(&memcg->css);
> +        if (buf != NULL)
> +                css_put(&memcg->css);
>  out:
>         return buf;
>  }

I think the existing code is correct, and this change actually
introduces a memory leak where there was none before.

In the case where get_memcg_path_buf() returns NULL, we *still* need
to css_put() what we got from get_mem_cgroup_from_mm() before.

NAK, unless I'm missing something.

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