From: Kees Cook <keescook@chromium.org>
To: Borislav Petkov <bp@alien8.de>
Cc: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>,
mingo@redhat.com, hpa@zytor.com, x86@kernel.org,
tyhicks@canonical.com, colin.king@canonical.com,
tglx@linutronix.de, linux-kernel@vger.kernel.org,
linux-security-module@vger.kernel.org,
Matthew Garrett <mjg59@google.com>
Subject: Re: [PATCH] x86/mtrr: only administrator can read the configurations.
Date: Tue, 12 Nov 2019 14:35:43 -0800 [thread overview]
Message-ID: <201911121434.FF26FF3FE@keescook> (raw)
In-Reply-To: <20191112174956.GB32336@zn.tnic>
On Tue, Nov 12, 2019 at 06:49:56PM +0100, Borislav Petkov wrote:
> On Mon, Nov 11, 2019 at 09:56:16AM -0800, Kees Cook wrote:
> > Some recap from being accidentally offlist:
> >
> > - this patch 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
> > read 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 intent of the original patch. (i.e.
> > check against CAP_SYS_ADMIN not just the wider uid 0.)
> >
> > - *this may still break things* if userspace expects to be able to
> > read other parts of the file as non-uid-0 and non-CAP_SYS_ADMIN.
> > If *that* is the case, then we need to censor the contents using
> > the opener's permissions (as done in other /proc cases).
> >
> > I think the most cautious way forward is something like
> > 51d7b120418e ("/proc/iomem: only expose physical resource addresses to
> > privileged users"). Untested (and should likely be expanded to know
> > about read vs write for lockdown interaction):
>
> I'm back'n'forth on this.
>
> So tglx and I agree that it doesn't make a whole lotta sense for
> non-privileged luserspace to be able to read /proc/mtrr because it is a
> small leak and normal users shouldn't care about the caching attributes
> of memory regions in the first place.
>
> So maybe we should do the second variant.
>
> But then we're not supposed to break luserspace.
>
> But then we can revert it if we do...
>
> Ugh.
Shall I send a patch for just moving the capable() checks into open()
and if someone yells we switch to the other option on the assumption
that then we'll have a real-world case we can test the other solution
against?
--
Kees Cook
next prev parent reply other threads:[~2019-11-12 22:35 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20191105071714.27376-1-zhangxiaoxu5@huawei.com>
[not found] ` <201911081236.57A127A@keescook>
[not found] ` <20191108205031.GH4503@zn.tnic>
[not found] ` <201911081320.5D3CD1A4CD@keescook>
[not found] ` <20191108213307.GI4503@zn.tnic>
2019-11-11 17:56 ` [PATCH] x86/mtrr: only administrator can read the configurations Kees Cook
2019-11-12 17:49 ` Borislav Petkov
2019-11-12 22:35 ` Kees Cook [this message]
2019-11-13 21:47 ` Thomas Gleixner
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