linux-kernel - Re: [PATCH v3 4/5] mm: Add a NO_INHERIT flag to the PR_SET

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Message-ID: <ZOtLWxokPxBCg0ao@arm.com>
Date:   Sun, 27 Aug 2023 16:10:51 +0300
From:   Catalin Marinas <catalin.marinas@....com>
To:     Florent Revest <revest@...omium.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, anshuman.khandual@....com,
        joey.gouly@....com, mhocko@...e.com, keescook@...omium.org,
        david@...hat.com, peterx@...hat.com, izbyshev@...ras.ru,
        broonie@...nel.org, szabolcs.nagy@....com, kpsingh@...nel.org,
        gthelen@...gle.com, toiwoton@...il.com
Subject: Re: [PATCH v3 4/5] mm: Add a NO_INHERIT flag to the PR_SET_MDWE prctl

On Tue, Jul 04, 2023 at 05:36:28PM +0200, Florent Revest wrote:
>  static inline int prctl_set_mdwe(unsigned long bits, unsigned long arg3,
>  				 unsigned long arg4, unsigned long arg5)
>  {
> +	unsigned long current_bits;
> +
>  	if (arg3 || arg4 || arg5)
>  		return -EINVAL;
>  
> -	if (bits & ~(PR_MDWE_REFUSE_EXEC_GAIN))
> +	if (bits & ~(PR_MDWE_REFUSE_EXEC_GAIN | PR_MDWE_NO_INHERIT))
> +		return -EINVAL;
> +
> +	/* NO_INHERIT only makes sense with REFUSE_EXEC_GAIN */
> +	if (bits & PR_MDWE_NO_INHERIT && !(bits & PR_MDWE_REFUSE_EXEC_GAIN))
>  		return -EINVAL;
>  
> +	current_bits = get_current_mdwe();
> +	if (current_bits && current_bits != bits)
> +		return -EPERM; /* Cannot unset the flags */
> +
> +	if (bits & PR_MDWE_NO_INHERIT)
> +		set_bit(MMF_HAS_MDWE_NO_INHERIT, &current->mm->flags);
>  	if (bits & PR_MDWE_REFUSE_EXEC_GAIN)
>  		set_bit(MMF_HAS_MDWE, &current->mm->flags);
> -	else if (test_bit(MMF_HAS_MDWE, &current->mm->flags))
> -		return -EPERM; /* Cannot unset the flag */
>  
>  	return 0;
>  }

This works for me, it's easier to read. I don't expect a use-case where
the app starts with MDWE no-inherit, forks some new processes and wants
to turn inherit on for further forking.

> @@ -2384,9 +2406,7 @@ static inline int prctl_get_mdwe(unsigned long arg2, unsigned long arg3,
>  {
>  	if (arg2 || arg3 || arg4 || arg5)
>  		return -EINVAL;
> -
> -	return test_bit(MMF_HAS_MDWE, &current->mm->flags) ?
> -		PR_MDWE_REFUSE_EXEC_GAIN : 0;
> +	return (int)get_current_mdwe();

Nitpick: the type conversion should be handled by the compiler as
prctl_get_mdwe() returns an int already.

Reviewed-by: Catalin Marinas <catalin.marinas@....com>