Re: [PATCHv7 10/14] x86/mm: Avoid load_unaligned_zeropad() stepping into unaccepted memory

Borislav Petkov
2022-07-26T17:25:44Z

On Tue, Jun 14, 2022 at 03:02:27PM +0300, Kirill A. Shutemov wrote:
> diff --git a/arch/x86/mm/unaccepted_memory.c b/arch/x86/mm/unaccepted_memory.c
> index 1df918b21469..bcd56fe82b9e 100644
> --- a/arch/x86/mm/unaccepted_memory.c
> +++ b/arch/x86/mm/unaccepted_memory.c
> @@ -23,6 +23,38 @@ void accept_memory(phys_addr_t start, phys_addr_t end)
> bitmap = __va(boot_params.unaccepted_memory);
> range_start = start / PMD_SIZE;
>
> + /*
> + * 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.
> + *
> + * But, this approach does not work for unaccepted memory. For TDX, a
> + * load from unaccepted memory will not lead to a recoverable exception
> + * within the guest. The guest will exit to the VMM where the only
> + * recourse is to terminate the guest.
> + *
> + * There are three parts to fix this issue and comprehensively avoid
> + * access to unaccepted memory. Together these ensure that an extra
> + * “guard” page is accepted in addition to the memory that needs to be
> + * used:
> + *
> + * 1. Implicitly extend the range_contains_unaccepted_memory(start, end)
> + * checks up to end+2M if ‘end’ is aligned on a 2M boundary.
> + *
> + * 2. Implicitly extend accept_memory(start, end) to end+2M if ‘end’ is
> + * aligned on a 2M boundary.

Why do we need those unicode quotes and backticks in there?

verify_diff: Warning: Unicode char [“] (0x8220 in line: + * “guard” page is accepted in addition to the memory that needs to be
verify_diff: Warning: Unicode char [‘] (0x8216 in line: + * checks up to end+2M if ‘end’ is aligned on a 2M boundary.
verify_diff: Warning: Unicode char [‘] (0x8216 in line: + * 2. Implicitly extend accept_memory(start, end) to end+2M if ‘end’ is
verify_diff: Warning: Unicode char [‘] (0x8216 in line: + * needs to be done to make ‘page’ usable. That work might include
verify_diff: Warning: Unicode char [‘] (0x8216 in line: + * accepting pages in addition to ‘page’ itself.

--
Regards/Gruss,
Boris.