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Date:   Tue, 16 May 2023 13:17:34 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Michael McCracken <michael.mccracken@gmail.com>
Cc:     linux-kernel@vger.kernel.org, kernel-hardening@lists.openwall.com,
        serge@hallyn.com, tycho@tycho.pizza,
        Luis Chamberlain <mcgrof@kernel.org>,
        Iurii Zaikin <yzaikin@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH] sysctl: add config to make randomize_va_space RO
Message-ID: <202305161312.078E5E7@keescook>
References: <20230504213002.56803-1-michael.mccracken@gmail.com>
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In-Reply-To: <20230504213002.56803-1-michael.mccracken@gmail.com>
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On Thu, May 04, 2023 at 02:30:02PM -0700, Michael McCracken wrote:
> Add config RO_RANDMAP_SYSCTL to set the mode of the randomize_va_space
> sysctl to 0444 to disallow all runtime changes. This will prevent
> accidental changing of this value by a root service.
> 
> The config is disabled by default to avoid surprises.
> 
> Signed-off-by: Michael McCracken <michael.mccracken@gmail.com>
> ---
>  kernel/sysctl.c | 4 ++++
>  mm/Kconfig      | 7 +++++++
>  2 files changed, 11 insertions(+)
> 
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index bfe53e835524..c5aafb734abe 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1913,7 +1913,11 @@ static struct ctl_table kern_table[] = {
>  		.procname	= "randomize_va_space",
>  		.data		= &randomize_va_space,
>  		.maxlen		= sizeof(int),
> +#if defined(CONFIG_RO_RANDMAP_SYSCTL)
> +		.mode		= 0444,
> +#else
>  		.mode		= 0644,
> +#endif

The way we've dealt with this in the past for similarly sensitive
sysctl variables to was set a "locked" position. (e.g. 0==off, 1==on,
2==cannot be turned off). In this case, we could make it, 0, 1, 2,
3==forced on full.

I note that there is actually no min/max (extra1/extra2) for this sysctl,
which is itself a bug, IMO. And there is just a magic "> 1" test that
should be a define or enum:

fs/binfmt_elf.c:        if ((current->flags & PF_RANDOMIZE) && (randomize_va_space > 1)) {

I think much of this should be improved.

Regardless, take a look at yama_dointvec_minmax(), which could, perhaps,
be generalized and used here.

Then we have a run-time way to manage this bit, without needing full
kernel rebuilds, etc, etc.

-Kees

-- 
Kees Cook