linux-kernel - Re: [RFC PATCH] x86/msr: Filter MSR writes

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Message-ID: <20200612164602.GC22660@zn.tnic>
Date:   Fri, 12 Jun 2020 18:46:02 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     x86-ml <x86@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] x86/msr: Filter MSR writes

On Fri, Jun 12, 2020 at 09:34:06AM -0700, Sean Christopherson wrote:
> The kernel should be tainted if the WRMSR is attempted, regardless of
> whether it succeeds, and it should happen before the WRMSR.  E.g. pointing
> MSR_IA32_DS_AREA at a bad address will likely cause an OOPS on the #PF

If the MSR write fails, MSR_IA32_DS_AREA won't have the bad address. If
the writes fail, nothing has been changed.

> This can be 0600, or maybe 0644, i.e. allow the user to enable/disable
> writes after the module has been loaded.

What for?

crw------- 1 root root 202, 0 Jun 10 19:54 /dev/cpu/0/msr

You need root to write to the chrdev.

Also, the intent is *not* to open it more but to close it so that the
incentive to design proper interfaces is there.

-- 
Regards/Gruss,
    Boris.

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