[PATCH v2 1/2] efi/libstub: arm64: Force Image reallocation if BSS was not reserved

[Ard Biesheuvel <ardb@kernel.org>]

Distro versions of GRUB replace the usual LoadImage/StartImage calls
used to load the kernel image with some local code that fails to honor
the allocation requirements described in the PE/COFF header, as it
does not account for the image's BSS section at all: it fails to
allocate space for it, and fails to zero initialize it.

Since the EFI stub itself is allocated in the .init segment, which is
in the middle of the image, its BSS section is not impacted by this,
and the main consequence of this omission is that the BSS section may
overlap with memory regions that are already used by the firmware.

So let's warn about this condition, and force image reallocation to
occur in this case, which works around the problem.

Fixes: 82046702e288 ("efi/libstub/arm64: Replace 'preferred' offset with alignment check")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 drivers/firmware/efi/libstub/arm64-stub.c | 49 +++++++++++++++++++-
 1 file changed, 48 insertions(+), 1 deletion(-)

diff --git a/drivers/firmware/efi/libstub/arm64-stub.c b/drivers/firmware/efi/libstub/arm64-stub.c
index 7bf0a7acae5e..3698c1ce2940 100644
--- a/drivers/firmware/efi/libstub/arm64-stub.c
+++ b/drivers/firmware/efi/libstub/arm64-stub.c
@@ -34,6 +34,51 @@ efi_status_t check_platform_features(void)
 	return EFI_SUCCESS;
 }
 
+/*
+ * Distro versions of GRUB may ignore the BSS allocation entirely (i.e., fail
+ * to provide space, and fail to zero it). Check for this condition by double
+ * checking that the first and the last byte of the image are covered by the
+ * same EFI memory map entry.
+ */
+static bool check_image_region(u64 base, u64 size)
+{
+	unsigned long map_size, desc_size, buff_size;
+	efi_memory_desc_t *memory_map;
+	struct efi_boot_memmap map;
+	efi_status_t status;
+	bool ret = false;
+	int map_offset;
+
+	map.map =	&memory_map;
+	map.map_size =	&map_size;
+	map.desc_size =	&desc_size;
+	map.desc_ver =	NULL;
+	map.key_ptr =	NULL;
+	map.buff_size =	&buff_size;
+
+	status = efi_get_memory_map(&map);
+	if (status != EFI_SUCCESS)
+		return false;
+
+	for (map_offset = 0; map_offset < map_size; map_offset += desc_size) {
+		efi_memory_desc_t *md = (void *)memory_map + map_offset;
+		u64 end = md->phys_addr + md->num_pages * EFI_PAGE_SIZE;
+
+		/*
+		 * Find the region that covers base, and return whether
+		 * it covers base+size bytes.
+		 */
+		if (base >= md->phys_addr && base < end) {
+			ret = (base + size) <= end;
+			break;
+		}
+	}
+
+	efi_bs_call(free_pool, memory_map);
+
+	return ret;
+}
+
 /*
  * Although relocatable kernels can fix up the misalignment with respect to
  * MIN_KIMG_ALIGN, the resulting virtual text addresses are subtly out of
@@ -92,7 +137,9 @@ efi_status_t handle_kernel_image(unsigned long *image_addr,
 	}
 
 	if (status != EFI_SUCCESS) {
-		if (IS_ALIGNED((u64)_text, min_kimg_align())) {
+		if (!check_image_region((u64)_text, kernel_memsize)) {
+			efi_err("FIRMWARE BUG: Image BSS overlaps adjacent EFI memory region\n");
+		} else if (IS_ALIGNED((u64)_text, min_kimg_align())) {
 			/*
 			 * Just execute from wherever we were loaded by the
 			 * UEFI PE/COFF loader if the alignment is suitable.
-- 
2.20.1