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

[Dave Hansen]

On 6/14/22 05:02, 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 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.
> 3. Set PageUnaccepted() on both memory that itself needs to be accepted
>    *and* memory where the next page needs to be accepted. Essentially,
>    make PageUnaccepted(page) a marker for whether work needs to be done
>    to make ‘page’ usable. That work might include accepting pages in
>    addition to ‘page’ itself.
...

That all looks pretty good.

> 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.
> +	 *
> +	 * 3. Set PageUnaccepted() on both memory that itself needs to be
> +	 *    accepted *and* memory where the next page needs to be accepted.
> +	 *    Essentially, make PageUnaccepted(page) a marker for whether work
> +	 *    needs to be done to make ‘page’ usable. That work might include
> +	 *    accepting pages in addition to ‘page’ itself.
> +	 */

One nit with this: I'd much rather add one sentence to these to help tie
the code implementing it with this comment.  Maybe something like:

 * 2. Implicitly extend accept_memory(start, end) to end+2M if ‘end’ is
 *    aligned on a 2M boundary. (immediately following this comment)


> +	if (!(end % PMD_SIZE))
> +		end += PMD_SIZE;
> +
>  	spin_lock_irqsave(&unaccepted_memory_lock, flags);
>  	for_each_set_bitrange_from(range_start, range_end, bitmap,
>  				   DIV_ROUND_UP(end, PMD_SIZE)) {
> @@ -46,6 +78,10 @@ bool range_contains_unaccepted_memory(phys_addr_t start, phys_addr_t end)
>  
>  	bitmap = __va(boot_params.unaccepted_memory);
>  
> +	/* See comment on load_unaligned_zeropad() in accept_memory() */
> +	if (!(end % PMD_SIZE))
> +		end += PMD_SIZE;

It's a wee bit hard to follow this back to the comment that it
references, even with them sitting next to each other in this diff.  How
about adding:

	/*
	 * Also consider the unaccepted state of the *next* page.  See
	 * fix #1 in the comment on load_unaligned_zeropad() in
	 * accept_memory().
	 */

>  	spin_lock_irqsave(&unaccepted_memory_lock, flags);
>  	while (start < end) {
>  		if (test_bit(start / PMD_SIZE, bitmap)) {
> diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
> index b91c89100b2d..bc1110509de4 100644
> --- a/drivers/firmware/efi/libstub/x86-stub.c
> +++ b/drivers/firmware/efi/libstub/x86-stub.c
> @@ -709,6 +709,13 @@ static efi_status_t allocate_unaccepted_memory(struct boot_params *params,
>  		return EFI_SUCCESS;
>  	}
>  
> +	/*
> +	 * range_contains_unaccepted_memory() may need to check one 2M chunk
> +	 * beyond the end of RAM to deal with load_unaligned_zeropad(). Make
> +	 * sure that the bitmap is large enough handle it.
> +	 */
> +	max_addr += PMD_SIZE;

I guess the alternative to this would have been to record 'max_addr',
then special case 'max_addr'+2M in the bitmap checks.  I agree this is
probably nicer.

Also, the changelog needs to at least *mention* this little tidbit.  It
was a bit of a surprise when I got here.

With those fixed:

Reviewed-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>