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Date:   Wed, 14 Dec 2022 10:53:19 -0800
From:   Kees Cook <keescook@chromium.org>
To:     jeffxu@chromium.org
Cc:     skhan@linuxfoundation.org, akpm@linux-foundation.org,
        dmitry.torokhov@gmail.com, dverkamp@chromium.org, hughd@google.com,
        jeffxu@google.com, jorgelo@chromium.org,
        linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
        linux-mm@kvack.org, jannh@google.com,
        linux-hardening@vger.kernel.org,
        linux-security-module@vger.kernel.org,
        kernel test robot <lkp@intel.com>
Subject: Re: [PATCH v7 3/6] mm/memfd: add MFD_NOEXEC_SEAL and MFD_EXEC
Message-ID: <202212141053.DD016F79B@keescook>
References: <20221209160453.3246150-1-jeffxu@google.com>
 <20221209160453.3246150-4-jeffxu@google.com>
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On Fri, Dec 09, 2022 at 04:04:50PM +0000, jeffxu@chromium.org wrote:
> From: Jeff Xu <jeffxu@google.com>
> 
> The new MFD_NOEXEC_SEAL and MFD_EXEC flags allows application to
> set executable bit at creation time (memfd_create).
> 
> When MFD_NOEXEC_SEAL is set, memfd is created without executable bit
> (mode:0666), and sealed with F_SEAL_EXEC, so it can't be chmod to
> be executable (mode: 0777) after creation.
> 
> when MFD_EXEC flag is set, memfd is created with executable bit
> (mode:0777), this is the same as the old behavior of memfd_create.
> 
> The new pid namespaced sysctl vm.memfd_noexec has 3 values:
> 0: memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL acts like
>         MFD_EXEC was set.
> 1: memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL acts like
>         MFD_NOEXEC_SEAL was set.
> 2: memfd_create() without MFD_NOEXEC_SEAL will be rejected.
> 
> The sysctl allows finer control of memfd_create for old-software
> that doesn't set the executable bit, for example, a container with
> vm.memfd_noexec=1 means the old-software will create non-executable
> memfd by default. Also, the value of memfd_noexec is passed to child
> namespace at creation time. For example, if the init namespace has
> vm.memfd_noexec=2, all its children namespaces will be created with 2.
> 
> Signed-off-by: Jeff Xu <jeffxu@google.com>

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

-- 
Kees Cook