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From: Borislav Petkov <>
To: Kees Cook <>
Cc: Thomas Gleixner <>,
	Zhang Xiaoxu <>,,,,,,
	Matthew Garrett <>
Subject: Re: [PATCH] x86/mtrr: Require CAP_SYS_ADMIN for all access
Date: Tue, 19 Nov 2019 11:07:53 +0100
Message-ID: <20191119100753.GA27787@zn.tnic> (raw)
In-Reply-To: <201911181308.63F06502A1@keescook>

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]
> Reported-by: Zhang Xiaoxu <>
> Signed-off-by: Kees Cook <>
> ---
>  arch/x86/kernel/cpu/mtrr/if.c | 21 ++-------------------
>  1 file changed, 2 insertions(+), 19 deletions(-)

Yap, LGTM, thanks!

Reviewed-by: Borislav Petkov <>

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.



  reply index

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-18 21:09 Kees Cook
2019-11-19 10:07 ` Borislav Petkov [this message]
2019-11-20 20:24   ` James Morris
2019-12-09  8:51 ` [tip: x86/mtrr] " tip-bot2 for Kees Cook

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