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

[Borislav Petkov]

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.

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