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

[Ard Biesheuvel]

On Sun, 14 May 2023 at 00:04, Kirill A. Shutemov
<kirill.shutemov@xxxxxxxxxxxxxxx> 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.
>

Does this mean that the kernel maps memory before accepting it? As
otherwise, I would assume that such an access would page fault inside
the guest before triggering an exception related to the unaccepted
state.

> 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>
> ---
>  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;
> --
> 2.39.3
>