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Date: Tue, 16 May 2023 13:17:34 -0700
From: Kees Cook <keescook@...omium.org>
To: Michael McCracken <michael.mccracken@...il.com>
Cc: linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com,
serge@...lyn.com, tycho@...ho.pizza,
Luis Chamberlain <mcgrof@...nel.org>,
Iurii Zaikin <yzaikin@...gle.com>,
Andrew Morton <akpm@...ux-foundation.org>,
linux-fsdevel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] sysctl: add config to make randomize_va_space RO
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@...il.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
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