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Date:   Wed, 15 Jun 2022 20:10:42 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        luto@kernel.org, peterz@infradead.org, ak@linux.intel.com,
        dan.j.williams@intel.com, david@redhat.com, hpa@zytor.com,
        linux-kernel@vger.kernel.org,
        sathyanarayanan.kuppuswamy@linux.intel.com, seanjc@google.com,
        thomas.lendacky@amd.com, x86@kernel.org
Subject: Re: [PATCHv4 3/3] x86/tdx: Handle load_unaligned_zeropad()
 page-cross to a shared page
Message-ID: <20220615171042.oeumb3vs3ttu4rvd@black.fi.intel.com>
References: <20220614120135.14812-1-kirill.shutemov@linux.intel.com>
 <20220614120135.14812-4-kirill.shutemov@linux.intel.com>
 <051fd468-11e6-308b-66c8-4de16ff80deb@intel.com>
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On Wed, Jun 15, 2022 at 08:27:52AM -0700, Dave Hansen wrote:
> 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?

They are different.

The patch by the link addresses issue of load_unaligned_zeropad() stepping
onto unaccepted memory. This was triggered in practice.

This patch address stepping onto MMIO shared memory. I had to force the
situation manually as MMIO memory mapped with ioremap() and it is not next
to normally allocated memory used by load_unaligned_zeropad() (such as
dentry cache).

Although any shared memory (SWIOTLB buffer for instance) can generate EPT
violation #VE if the VMM is malicious enough.

-- 
 Kirill A. Shutemov