linux-kernel - Re: [PATCH v3 3/3] tomoyo: reduce mmap_sem hold for mm->exe

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Message-Id: <201502252040.IHB78651.OQFSLtFFHOOJMV@I-love.SAKURA.ne.jp>
Date:	Wed, 25 Feb 2015 20:40:07 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	dave@...olabs.net, jmorris@...ei.org
Cc:	akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, takedakn@...data.co.jp,
	linux-security-module@...r.kernel.org,
	tomoyo-dev-en@...ts.sourceforge.jp
Subject: Re: [PATCH v3 3/3] tomoyo: reduce mmap_sem hold for mm->exe_file

Davidlohr Bueso wrote:
> The mm->exe_file is currently serialized with mmap_sem (shared) in order
> to both safely (1) read the file and (2) compute the realpath by calling
> tomoyo_realpath_from_path, making it an absolute overkill. Good users will,
> on the other hand, make use of the more standard get_mm_exe_file(), requiring
> only holding the mmap_sem to read the value, and relying on reference
> 
> Signed-off-by: Davidlohr Bueso <dbueso@...e.de>

Acked-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>

James, will you apply to linux-security.git#next ?
I'm not using publicly accessible git tree for sending pull requests.

> ---
> 
> Changes from v2: remove cleanups and cp initialization.
> 
>  security/tomoyo/util.c | 21 ++++++++++++---------
>  1 file changed, 12 insertions(+), 9 deletions(-)
> 
> diff --git a/security/tomoyo/util.c b/security/tomoyo/util.c
> index 2952ba5..29f3b65 100644
> --- a/security/tomoyo/util.c
> +++ b/security/tomoyo/util.c
> @@ -948,16 +948,19 @@ bool tomoyo_path_matches_pattern(const struct tomoyo_path_info *filename,
>   */
>  const char *tomoyo_get_exe(void)
>  {
> -	struct mm_struct *mm = current->mm;
> -	const char *cp = NULL;
> +       struct file *exe_file;
> +       const char *cp;
> +       struct mm_struct *mm = current->mm;
>  
> -	if (!mm)
> -		return NULL;
> -	down_read(&mm->mmap_sem);
> -	if (mm->exe_file)
> -		cp = tomoyo_realpath_from_path(&mm->exe_file->f_path);
> -	up_read(&mm->mmap_sem);
> -	return cp;
> +       if (!mm)
> +	       return NULL;
> +       exe_file = get_mm_exe_file(mm);
> +       if (!exe_file)
> +	       return NULL;
> +
> +       cp = tomoyo_realpath_from_path(&exe_file->f_path);
> +       fput(exe_file);
> +       return cp;
>  }
>  
>  /**
> -- 
> 2.1.4
> 
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