Re: [PATCH] mm/migrate: Avoid possible unnecessary ptrace_may_access() call in kernel_move_pages()

Re: [PATCH] mm/migrate: Avoid possible unnecessary ptrace_may_access() call in kernel_move_pages()

[Kees Cook]

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@huawei.com>
> Signed-off-by: Miaohe Lin <linmiaohe@huawei.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

Re: [PATCH] mm/migrate: Avoid possible unnecessary ptrace_may_access() call in kernel_move_pages()

[linmiaohe]

Kees Cook <keescook@chromium.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@huawei.com>
>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.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.

>--
>Kees Cook

Thanks again.

Re: [PATCH] mm/migrate: Avoid possible unnecessary ptrace_may_access() call in kernel_move_pages()

[Kees Cook]

On Thu, Aug 20, 2020 at 02:18:21AM +0000, linmiaohe wrote:
> Kees Cook <keescook@chromium.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@huawei.com>
> >> Signed-off-by: Miaohe Lin <linmiaohe@huawei.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