linux-kernel - Re: [PATCH] mm/migrate: Avoid possible unnecessary ptrace_may_access() call in kernel_move

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Message-ID: <202008191103.D88345E410@keescook>
Date:   Wed, 19 Aug 2020 11:04:24 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org
Subject: Re: [PATCH] mm/migrate: Avoid possible unnecessary
 ptrace_may_access() call in kernel_move_pages()

On Mon, Aug 17, 2020 at 07:59:33AM -0400, Miaohe Lin wrote:
> There is no need to check if this process has the right to modify the
> specified process when they are same.
> 
> Signed-off-by: Hongxiang Lou <louhongxiang@...wei.com>
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  mm/migrate.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 34a842a8eb6a..342c1ce0b433 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1903,7 +1903,7 @@ static int kernel_move_pages(pid_t pid, unsigned long nr_pages,
>  	 * Check if this process has the right to modify the specified
>  	 * process. Use the regular "ptrace_may_access()" checks.
>  	 */
> -	if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {
> +	if (pid && !ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {
>  		rcu_read_unlock();
>  		err = -EPERM;
>  		goto out;

NAK, please don't do this -- the ptrace and security hooks already do
these kinds of self-introspection checks, and I'd like to keep a central
place to perform these kinds of checks.

Is there a specific problem you've encountered that this fixes?

-- 
Kees Cook