Re: [PATCHv4 3/3] x86/tdx: Handle load_unaligned_zeropad() page-cross to a shared page

[Dave Hansen]

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@xxxxxxxxxxxxxxx/

Says:

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

Or were you considering this a different problem somehow?