linux-kernel - Re: [PATCH] x86/sev: Check whether SEV or SME is supported first

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Message-ID: <YLZrXEQ8w5ntu7ov@google.com>
Date:   Tue, 1 Jun 2021 17:16:12 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Tom Lendacky <thomas.lendacky@....com>, Pu Wen <puwen@...on.cn>,
        Joerg Roedel <jroedel@...e.de>, x86@...nel.org,
        joro@...tes.org, dave.hansen@...ux.intel.com, peterz@...radead.org,
        tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        sashal@...nel.org, gregkh@...uxfoundation.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] x86/sev: Check whether SEV or SME is supported first

On Tue, Jun 01, 2021, Borislav Petkov wrote:
> Yah, ain't gonna happen. I'm not taking some #GP handler to the early
> code just because some hardware is operating out of spec.

The bug isn't limited to out-of-spec hardware.  At the point of #GP, sme_enable()
has only verified the max leaf is greater than 0x8000001f, it has not verified
that 0x8000001f is actually supported.  The APM itself declares several leafs
between 0x80000000 and 0x8000001f as reserved/unsupported, so we can't argue that
0x8000001f must be supported if the max leaf is greater than 0x8000001f.

The only way to verify that 0x8000001f is supported is to find a non-zero bit,
which is what Pu Wen's patch does.