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| H A D | unaccepted_memory.c | c211c19e Tue Jun 06 09:26:34 CDT 2023 Kirill A. Shutemov <kirill.shutemov@linux.intel.com> efi/unaccepted: Avoid load_unaligned_zeropad() stepping into unaccepted memory
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@linux.intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20230606142637.5171-7-kirill.shutemov@linux.intel.com
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