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

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Message-ID: <202008201420.D9F0CB3E@keescook>
Date:   Thu, 20 Aug 2020 14:21:45 -0700
From:   Kees Cook <keescook@...omium.org>
To:     linmiaohe <linmiaohe@...wei.com>
Cc:     "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-security-module@...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 Thu, Aug 20, 2020 at 02:18:21AM +0000, linmiaohe wrote:
> Kees Cook <keescook@...omium.org> wrote:
> >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>
> >
> >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.
> >
> 
> Many thanks for your reply.
> We also avoid get_task_struct/ put_task_struct pair of atomic ops, rcu_lock, task_lock and so on this way.
> 
> >Is there a specific problem you've encountered that this fixes?
> >
> 
> I'am sorry but there's no specific problem. I do this mainly to skip the unnecessary ptrace and security hooks.

Cool. Let's keep this as-is so we continue to have centralized
instrumentation of these things in the LSM. :)

Thanks for your attention to performance!

-- 
Kees Cook