linux-kernel - Re: [PATCHv4 3/3] x86/tdx: Handle load_unaligned

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Date:   Wed, 15 Jun 2022 08:27:52 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        luto@...nel.org, peterz@...radead.org
Cc:     ak@...ux.intel.com, dan.j.williams@...el.com, david@...hat.com,
        hpa@...or.com, linux-kernel@...r.kernel.org,
        sathyanarayanan.kuppuswamy@...ux.intel.com, seanjc@...gle.com,
        thomas.lendacky@....com, x86@...nel.org
Subject: Re: [PATCHv4 3/3] x86/tdx: Handle load_unaligned_zeropad() page-cross
 to a shared page

On 6/14/22 05:01, Kirill A. Shutemov wrote:
> load_unaligned_zeropad() can lead to unwanted loads across page boundaries.
> The unwanted loads are typically harmless. But, they might be made to
> totally unrelated or even unmapped memory. load_unaligned_zeropad()
> relies on exception fixup (#PF, #GP and now #VE) to recover from these
> unwanted loads.
> 
> In TDX guests, the second page can be shared page and VMM may configure
> it to trigger #VE.
> 
> Kernel assumes that #VE on a shared page is MMIO access and tries to
> decode instruction to handle it. In case of load_unaligned_zeropad() it
> may result in confusion as it is not MMIO access.
> 
> Fix it by detecting split page MMIO accesses and fail them.
> load_unaligned_zeropad() will recover using exception fixups.
> 
> The issue was discovered by analysis. It was not triggered during the
> testing.

I thought this whole exercise was kicked off by hitting this in testing.
 Am I remembering this wrong?

> https://lore.kernel.org/all/20220517153444.11195-10-kirill.shutemov@linux.intel.com/

Says:

> This is an actual, real-world problem which was discovered during TDX
> testing.

Or were you considering this a different problem somehow?