Messages in this thread | | | Date | Wed, 15 Jun 2022 20:10:42 +0300 | From | "Kirill A. Shutemov" <> | Subject | Re: [PATCHv4 3/3] x86/tdx: Handle load_unaligned_zeropad() page-cross to a shared page |
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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
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