linux-kernel - Re: [PATCH] x86/mtrr: Require CAP_SYS

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Date:   Tue, 19 Nov 2019 11:07:53 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Kees Cook <keescook@...omium.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Zhang Xiaoxu <zhangxiaoxu5@...wei.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        tyhicks@...onical.com, colin.king@...onical.com,
        linux-security-module@...r.kernel.org,
        Matthew Garrett <mjg59@...gle.com>
Subject: Re: [PATCH] x86/mtrr: Require CAP_SYS_ADMIN for all access

On Mon, Nov 18, 2019 at 01:09:21PM -0800, Kees Cook wrote:
> Zhang Xiaoxu noted that physical address locations for MTRR were
> visible to non-root users, which could be considered an information
> leak. In discussing[1] the options for solving this, it sounded like
> just moving the capable check into open() was the first step. If this
> breaks userspace, then we will have a test case for the more conservative
> approaches discussed in the thread. In summary:
> 
> - MTRR should check capabilities at open time (or retain the
>   checks on the opener's permissions for later checks).
> 
> - changing the DAC permissions might break something that expects to
>   open mtrr when not uid 0.
> 
> - if we leave the DAC permissions alone and just move the capable check
>   to the opener, we should get the desired protection. (i.e. check
>   against CAP_SYS_ADMIN not just the wider uid 0.)
> 
> - if that still breaks things, as in userspace expects to be able to
>   read other parts of the file as non-uid-0 and non-CAP_SYS_ADMIN, then
>   we need to censor the contents using the opener's permissions. For
>   example, as done in other /proc cases, like commit 51d7b120418e
>   ("/proc/iomem: only expose physical resource addresses to privileged
>   users").
> 
> [1] https://lore.kernel.org/lkml/201911110934.AC5BA313@keescook/
> 
> Reported-by: Zhang Xiaoxu <zhangxiaoxu5@...wei.com>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  arch/x86/kernel/cpu/mtrr/if.c | 21 ++-------------------
>  1 file changed, 2 insertions(+), 19 deletions(-)

Yap, LGTM, thanks!

Reviewed-by: Borislav Petkov <bp@...e.de>

However, as it has a user-visible impact and it is not an urgent thing
to have in the tree, I'd not queue this now but after the merge window
is done so that we have a maximum time of exposure in linux-next and we
can have ample time to addres fallout.

/me puts it on the list for after the merge window.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette