linux-kernel - Re: [PATCH v2 3/5] mm: Make PR_MDWE_REFUSE_EXEC

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Date:   Mon, 22 May 2023 10:55:30 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Florent Revest <revest@...omium.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Cc:     akpm@...ux-foundation.org, catalin.marinas@....com,
        anshuman.khandual@....com, joey.gouly@....com, mhocko@...e.com,
        keescook@...omium.org, peterx@...hat.com, izbyshev@...ras.ru,
        broonie@...nel.org, szabolcs.nagy@....com, kpsingh@...nel.org,
        gthelen@...gle.com, toiwoton@...il.com
Subject: Re: [PATCH v2 3/5] mm: Make PR_MDWE_REFUSE_EXEC_GAIN an unsigned long

On 17.05.23 17:03, Florent Revest wrote:
> Alexey pointed out that defining a prctl flag as an int is a footgun
> because, under some circumstances, when used as a flag to prctl, it can
> be casted to long with garbage upper bits which would result in
> unexpected behaviors.
> 
> This patch changes the constant to a UL to eliminate these
> possibilities.
> 
> Signed-off-by: Florent Revest <revest@...omium.org>
> Suggested-by: Alexey Izbyshev <izbyshev@...ras.ru>
> ---
>   include/uapi/linux/prctl.h       | 2 +-
>   tools/include/uapi/linux/prctl.h | 2 +-
>   2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> index f23d9a16507f..6e9af6cbc950 100644
> --- a/include/uapi/linux/prctl.h
> +++ b/include/uapi/linux/prctl.h
> @@ -283,7 +283,7 @@ struct prctl_mm_map {
>   
>   /* Memory deny write / execute */
>   #define PR_SET_MDWE			65
> -# define PR_MDWE_REFUSE_EXEC_GAIN	1
> +# define PR_MDWE_REFUSE_EXEC_GAIN	(1UL << 0)
>   
>   #define PR_GET_MDWE			66
>   
> diff --git a/tools/include/uapi/linux/prctl.h b/tools/include/uapi/linux/prctl.h
> index 759b3f53e53f..6e6563e97fef 100644
> --- a/tools/include/uapi/linux/prctl.h
> +++ b/tools/include/uapi/linux/prctl.h
> @@ -283,7 +283,7 @@ struct prctl_mm_map {
>   
>   /* Memory deny write / execute */
>   #define PR_SET_MDWE			65
> -# define PR_MDWE_REFUSE_EXEC_GAIN	1
> +# define PR_MDWE_REFUSE_EXEC_GAIN	(1UL << 0)
>   
>   #define PR_GET_MDWE			66
>   

Both are changing existing uapi, so you'll already have existing user 
space using the old values, that your kernel code has to deal with no?

-- 
Thanks,

David / dhildenb