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

Kirill A. Shutemov
2022-06-14T12:02:57Z

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.

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 have PageUnaccepted() set on
them. PageUnaccepted(page) 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>
---
arch/x86/mm/unaccepted_memory.c | 36 +++++++++++++++++++++++++
drivers/firmware/efi/libstub/x86-stub.c | 7 +++++
2 files changed, 43 insertions(+)

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.
+ */
+ 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;
+
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;
+
/*
* If unaccepted memory is present allocate a bitmap to track what
* memory has to be accepted before access.
--
2.35.1