Re: [PATCHv11 6/9] efi/unaccepted: Avoid load_unaligned_zeropad() stepping into unaccepted memory

[Tom Lendacky]

On 5/13/23 17:04, 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.
>
> 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 two 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+unit_size if 'end' is aligned on a unit_size
>     boundary.
> 2. Implicitly extend accept_memory(start, end) to end+unit_size if 'end'
>     is aligned on a unit_size boundary.
>
> Side note: This leads to something strange. Pages which were accepted
> 	   at boot, marked by the firmware as accepted and will never
> 	   _need_ to be accepted might be on unaccepted_pages list
> 	   This is a cue to ensure that the next page is accepted
> 	   before 'page' can be used.
>
> This is an actual, real-world problem which was discovered during TDX
> testing.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Reviewed-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

Reviewed-by: Tom Lendacky <thomas.lendacky@xxxxxxx>

> ---
>   drivers/firmware/efi/unaccepted_memory.c | 35 ++++++++++++++++++++++++
>   1 file changed, 35 insertions(+)
>
> diff --git a/drivers/firmware/efi/unaccepted_memory.c b/drivers/firmware/efi/unaccepted_memory.c
> index bb91c41f76fb..3d1ca60916dd 100644
> --- a/drivers/firmware/efi/unaccepted_memory.c
> +++ b/drivers/firmware/efi/unaccepted_memory.c
> @@ -37,6 +37,34 @@ void accept_memory(phys_addr_t start, phys_addr_t end)
>   	start -= unaccepted->phys_base;
>   	end -= unaccepted->phys_base;
>   
> +	/*
> +	 * 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 two 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+unit_size if 'end' is aligned on a unit_size
> +	 *    boundary.
> +	 *
> +	 * 2. Implicitly extend accept_memory(start, end) to end+unit_size if
> +	 *    'end' is aligned on a unit_size boundary. (immediately following
> +	 *    this comment)
> +	 */
> +	if (!(end % unit_size))
> +		end += unit_size;
> +
>   	/* Make sure not to overrun the bitmap */
>   	if (end > unaccepted->size * unit_size * BITS_PER_BYTE)
>   		end = unaccepted->size * unit_size * BITS_PER_BYTE;
> @@ -84,6 +112,13 @@ bool range_contains_unaccepted_memory(phys_addr_t start, phys_addr_t end)
>   	start -= unaccepted->phys_base;
>   	end -= unaccepted->phys_base;
>   
> +	/*
> +	 * Also consider the unaccepted state of the *next* page. See fix #1 in
> +	 * the comment on load_unaligned_zeropad() in accept_memory().
> +	 */
> +	if (!(end % unit_size))
> +		end += unit_size;
> +
>   	/* Make sure not to overrun the bitmap */
>   	if (end > unaccepted->size * unit_size * BITS_PER_BYTE)
>   		end = unaccepted->size * unit_size * BITS_PER_BYTE;