linux-kernel - Re: [PATCH v2 2/7] x86/boot: Delay sev_verify

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Message-ID: <Y8lDN73cNOmNuciV@zn.tnic>
Date:   Thu, 19 Jan 2023 14:18:47 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Peter Zijlstra <peterz@...radead.org>,
        Jörg Rödel <joro@...tes.org>
Cc:     x86@...nel.org, Joan Bruguera <joanbrugueram@...il.com>,
        linux-kernel@...r.kernel.org, Juergen Gross <jgross@...e.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        xen-devel <xen-devel@...ts.xenproject.org>,
        Jan Beulich <jbeulich@...e.com>,
        Roger Pau Monne <roger.pau@...rix.com>,
        Kees Cook <keescook@...omium.org>, mark.rutland@....com,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 2/7] x86/boot: Delay sev_verify_cbit() a bit

On Mon, Jan 16, 2023 at 03:25:35PM +0100, Peter Zijlstra wrote:
> Per the comment it is important to call sev_verify_cbit() before the
> first RET instruction, this means we can delay calling this until more

Make that "... this means that this can be delayed until... "

And I believe this is not about the first RET insn but about the *next* RET
which will pop poisoned crap from the unencrypted stack and do shits with it.

Also, there's this over sev_verify_cbit():

 * sev_verify_cbit() is called before switching to a new long-mode page-table
 * at boot.

so you can't move it under the

	movq    %rax, %cr3

Looking at this more, there's a sme_enable() call on the BSP which is already in
C.

So, can we do that C-bit verification once on the BSP, *in C* which would be a
lot easier, and be done with it?

Once it is verified there, the bit is the same on all APs so all good.

Right?

joro?

-- 
Regards/Gruss,
    Boris.

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