linux-kernel - Re: [PATCH v6 4/6] mm/memfd: Add write seals when apply SEAL

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Message-ID: <202212080827.41F083F@keescook>
Date:   Thu, 8 Dec 2022 08:27:50 -0800
From:   Kees Cook <keescook@...omium.org>
To:     jeffxu@...omium.org
Cc:     skhan@...uxfoundation.org, akpm@...ux-foundation.org,
        dmitry.torokhov@...il.com, dverkamp@...omium.org, hughd@...gle.com,
        jeffxu@...gle.com, jorgelo@...omium.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-mm@...ck.org, jannh@...gle.com,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH v6 4/6] mm/memfd: Add write seals when apply SEAL_EXEC to
 executable memfd

On Wed, Dec 07, 2022 at 03:49:37PM +0000, jeffxu@...omium.org wrote:
> From: Jeff Xu <jeffxu@...gle.com>
> 
> In order to avoid WX mappings, add F_SEAL_WRITE when apply
> F_SEAL_EXEC to an executable memfd, so W^X from start.
> 
> This implys application need to fill the content of the memfd first,
> after F_SEAL_EXEC is applied, application can no longer modify the
> content of the memfd.
> 
> Typically, application seals the memfd right after writing to it.
> For example:
> 1. memfd_create(MFD_EXEC).
> 2. write() code to the memfd.
> 3. fcntl(F_ADD_SEALS, F_SEAL_EXEC) to convert the memfd to W^X.
> 4. call exec() on the memfd.
> 
> Signed-off-by: Jeff Xu <jeffxu@...gle.com>

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook