[PATCH v2 1/2] x86/boot/KASLR: exclude EFI_BOOT_SERVICES_{CODE|DATA} from KASLR's choice

Naoya Horiguchi
2017-07-07T07:22:53Z

KASLR chooses kernel location from E820_TYPE_RAM regions by walking over
e820 entries now. E820_TYPE_RAM includes EFI_BOOT_SERVICES_CODE and
EFI_BOOT_SERVICES_DATA, so those regions can be the target. According to
UEFI spec, all memory regions marked as EfiBootServicesCode and
EfiBootServicesData are available for free memory after the first call
of ExitBootServices(). So such regions should be usable for kernel on
spec basis.

In x86, however, we have some workaround for broken firmware, where we
keep such regions reserved until SetVirtualAddressMap() is done.
See the following code in should_map_region():

static bool should_map_region(efi_memory_desc_t *md)
{
...
/*
* Map boot services regions as a workaround for buggy
* firmware that accesses them even when they shouldn't.
*
* See efi_{reserve,free}_boot_services().
*/
if (md->type == EFI_BOOT_SERVICES_CODE ||
md->type == EFI_BOOT_SERVICES_DATA)
return false;

This workaround suppressed a boot crash, but potential issues still
remain because no one prevents the regions from overlapping with kernel
image by KASLR.

So let's make sure that EFI_BOOT_SERVICES_{CODE|DATA} regions are never
chosen as kernel memory for the workaround to work fine.

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
---
v1 -> v2:
- switch efi_mirror_found to local variable
- insert break when EFI_MEMORY_MORE_RELIABLE found
---
arch/x86/boot/compressed/kaslr.c | 46 +++++++++++++++++++++++++++++-----------
1 file changed, 34 insertions(+), 12 deletions(-)

diff --git next-20170705/arch/x86/boot/compressed/kaslr.c next-20170705_patched/arch/x86/boot/compressed/kaslr.c
index 94f08fd..29f5c4e 100644
--- next-20170705/arch/x86/boot/compressed/kaslr.c
+++ next-20170705_patched/arch/x86/boot/compressed/kaslr.c
@@ -560,10 +560,8 @@ static void process_mem_region(struct mem_vector *entry,
}
}

-/* Marks if efi mirror regions have been found and handled. */
-static bool efi_mirror_found;
-
-static void process_efi_entry(unsigned long minimum, unsigned long image_size)
+/* Returns true if we really enter efi memmap walk, otherwise returns false. */
+static bool process_efi_entry(unsigned long minimum, unsigned long image_size)
{
struct efi_info *e = &boot_params->efi_info;
struct mem_vector region;
@@ -572,18 +570,18 @@ static void process_efi_entry(unsigned long minimum, unsigned long image_size)
char *signature;
u32 nr_desc;
int i;
-
+ bool efi_mirror_found;

signature = (char *)&boot_params->efi_info.efi_loader_signature;
if (strncmp(signature, EFI32_LOADER_SIGNATURE, 4) &&
strncmp(signature, EFI64_LOADER_SIGNATURE, 4))
- return;
+ return false;

#ifdef CONFIG_X86_32
/* Can't handle data above 4GB at this time */
if (e->efi_memmap_hi) {
warn("Memory map is above 4GB, EFI should be disabled.\n");
- return;
+ return false;
}
pmap = e->efi_memmap;
#else
@@ -594,12 +592,37 @@ static void process_efi_entry(unsigned long minimum, unsigned long image_size)
for (i = 0; i < nr_desc; i++) {
md = (efi_memory_desc_t *)(pmap + (i * e->efi_memdesc_size));
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;
+ break;
}
}
+
+ for (i = 0; i < nr_desc; i++) {
+ md = (efi_memory_desc_t *)(pmap + (i * e->efi_memdesc_size));
+
+ /*
+ * EFI_BOOT_SERVICES_{CODE|DATA} are avoided because boot
+ * services regions could be accessed after ExitBootServices()
+ * due to the workaround for buggy firmware.
+ */
+ if (!(md->type == EFI_LOADER_CODE ||
+ md->type == EFI_LOADER_DATA ||
+ md->type == EFI_CONVENTIONAL_MEMORY))
+ continue;
+
+ if (efi_mirror_found &&
+ !(md->attribute & EFI_MEMORY_MORE_RELIABLE))
+ continue;
+
+ region.start = md->phys_addr;
+ region.size = md->num_pages << EFI_PAGE_SHIFT;
+ process_mem_region(&region, minimum, image_size);
+ if (slot_area_index == MAX_SLOT_AREA) {
+ debug_putstr("Aborted EFI scan (slot_areas full)!\n");
+ break;
+ }
+ }
+ return true;
}

static void process_e820_entry(unsigned long minimum, unsigned long image_size)
@@ -637,8 +660,7 @@ static unsigned long find_random_phys_addr(unsigned long minimum,
minimum = ALIGN(minimum, CONFIG_PHYSICAL_ALIGN);

#ifdef CONFIG_EFI
- process_efi_entry(minimum, image_size);
- if (efi_mirror_found)
+ if (process_efi_entry(minimum, image_size))
return slots_fetch_random();
#endif

--
2.7.0