[PATCH] efi/libstub/arm64: handle randomized TEXT_OFFSET

[Mark Rutland]

When CONFIG_RANDOMIZE_TEXT_OFFSET is selected, TEXT_OFFSET is an
arbitrary multiple of PAGE_SIZE in the interval [0, 2MB).

The EFI stub doesn't accuont for this, and only handles the case where
TEXT_OFFSET is multiple of EFI_KIMG_ALIGN. This can result in the kernel
being loaded to an erroneous physical alignment. This has been observed
to result in spurious stack overflow reports and failure to make use of
the IRQ stacks, and theoretically could result in a number of other
issues.

We can OR in the low bits of TEXT_OFFSET to ensure that we have the
necessary offset (and hence have the necessary alignment), so let's do
that.

Fixes: 6f26b3671184c36d ("arm64: kaslr: increase randomization granularity")
Reported-by: Kim Phillips <kim.phillips at arm.com>
Signed-off-by: Mark Rutland <mark.rutland at arm.com>
Cc: Ard Biesheuvel <ard.biesheuvel at linaro.org>
Cc: linux-efi at vger.kernel.org
---
 drivers/firmware/efi/libstub/arm64-stub.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/firmware/efi/libstub/arm64-stub.c b/drivers/firmware/efi/libstub/arm64-stub.c
index b9bd827caa22..541b82fdc8a2 100644
--- a/drivers/firmware/efi/libstub/arm64-stub.c
+++ b/drivers/firmware/efi/libstub/arm64-stub.c
@@ -98,6 +98,13 @@ efi_status_t handle_kernel_image(efi_system_table_t *sys_table_arg,
 			     (phys_seed >> 32) & mask : TEXT_OFFSET;
 
 		/*
+		 * With CONFIG_RANDOMIZE_TEXT_OFFSET, TEXT_OFFSET may not be a
+		 * multiple of EFI_KIMG_ALIGN, and we must ensure that we apply
+		 * the offset below EFI_KIMG_ALIGN.
+		 */
+		offset |= (TEXT_OFFSET % EFI_KIMG_ALIGN);
+
+		/*
 		 * If KASLR is enabled, and we have some randomness available,
 		 * locate the kernel at a randomized offset in physical memory.
 		 */
-- 
2.11.0