[PATCH v8 2/2] x86/boot/KASLR: Restrict kernel to be randomized in mirror regions

Baoquan He
2017-08-04T07:27:13Z

Currently KASLR will parse all e820 entries of RAM type and add all
candidate position into slots array. Then we will choose one slot
randomly as the new position which kernel will be decompressed into
and run at.

On system with EFI enabled, e820 memory regions are coming from EFI
memory regions by combining adjacent regions. While these EFI memory
regions have more attributes to mark their different use. Mirror
attribute is such kind. The physical memory region whose descriptors
in EFI memory map has EFI_MEMORY_MORE_RELIABLE attribute (bit: 16) are
mirrored. The address range mirroring feature of kernel arranges such
mirror region into normal zone and other region into movable zone. And
with mirroring feature enabled, the code and date of kernel can only be
located in more reliable mirror region. However, the current KASLR code
doesn't check EFI memory entries, and could choose new position in
non-mirrored region. This will break the functionality of the address
range mirroring feature.

So if EFI is detected, iterate EFI memory map and pick the mirror region
to process for adding candidate of randomization slot. If EFI is disabled
or no mirror region found, still process e820 memory map.

Signed-off-by: Baoquan He <bhe@redhat.com>
---
arch/x86/boot/compressed/kaslr.c | 68 ++++++++++++++++++++++++++++++++++++++--
1 file changed, 66 insertions(+), 2 deletions(-)

diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
index 99c7194f7ea6..3d3ce762dc98 100644
--- a/arch/x86/boot/compressed/kaslr.c
+++ b/arch/x86/boot/compressed/kaslr.c
@@ -37,7 +37,9 @@
#include <linux/uts.h>
#include <linux/utsname.h>
#include <linux/ctype.h>
+#include <linux/efi.h>
#include <generated/utsrelease.h>
+#include <asm/efi.h>

/* Macros used by the included decompressor code below. */
#define STATIC
@@ -558,6 +560,65 @@ static void process_mem_region(struct mem_vector *entry,
}
}

+#ifdef CONFIG_EFI
+/*
+ * Returns true if mirror region found (and must have been processed
+ * for slots adding)
+ */
+static bool
+process_efi_entries(unsigned long minimum, unsigned long image_size)
+{
+ struct efi_info *e = &boot_params->efi_info;
+ bool efi_mirror_found = false;
+ struct mem_vector region;
+ efi_memory_desc_t *md;
+ unsigned long pmap;
+ char *signature;
+ u32 nr_desc;
+ int i;
+
+ signature = (char *)&e->efi_loader_signature;
+ if (strncmp(signature, EFI32_LOADER_SIGNATURE, 4) &&
+ strncmp(signature, EFI64_LOADER_SIGNATURE, 4))
+ return false;
+
+#ifdef CONFIG_X86_32
+ /* Can't handle data above 4GB at this time */
+ if (e->efi_memmap_hi) {
+ warn("EFI memmap is above 4GB, can't be handled now on x86_32. EFI should be disabled.\n");
+ return false;
+ }
+ pmap = e->efi_memmap;
+#else
+ pmap = (e->efi_memmap | ((__u64)e->efi_memmap_hi << 32));
+#endif
+
+ nr_desc = e->efi_memmap_size / e->efi_memdesc_size;
+ for (i = 0; i < nr_desc; i++) {
+ md = efi_memdesc_ptr(pmap, e->efi_memdesc_size, i);
+ if (md->attribute & EFI_MEMORY_MORE_RELIABLE) {
+ region.start = md->phys_addr;
+ region.size = md->num_pages << EFI_PAGE_SHIFT;
+ process_mem_region(&region, minimum, image_size);
+ efi_mirror_found = true;
+
+ if (slot_area_index == MAX_SLOT_AREA) {
+ debug_putstr("Aborted EFI scan (slot_areas full)!\n");
+ break;
+ }
+ }
+ }
+
+ return efi_mirror_found;
+}
+#else
+static inline bool
+process_efi_entries(unsigned long minimum, unsigned long image_size)
+{
+ return false;
+}
+#endif
+
static void process_e820_entries(unsigned long minimum,
unsigned long image_size)
{
@@ -586,13 +647,16 @@ static unsigned long find_random_phys_addr(unsigned long minimum,
{
/* Check if we had too many memmaps. */
if (memmap_too_large) {
- debug_putstr("Aborted e820 scan (more than 4 memmap= args)!\n");
+ debug_putstr("Aborted memory entries scan (more than 4 memmap= args)!\n");
return 0;
}

/* Make sure minimum is aligned. */
minimum = ALIGN(minimum, CONFIG_PHYSICAL_ALIGN);

+ if (process_efi_entries(minimum, image_size))
+ return slots_fetch_random();
+
process_e820_entries(minimum, image_size);
return slots_fetch_random();
}
@@ -652,7 +716,7 @@ void choose_random_location(unsigned long input,
*/
min_addr = min(*output, 512UL << 20);

- /* Walk e820 and find a random address. */
+ /* Walk available memory entries to find a random address. */
random_addr = find_random_phys_addr(min_addr, output_size);
if (!random_addr) {
warn("Physical KASLR disabled: no suitable memory region!");
--
2.5.5