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Subject[PATCH v3 21/21] arm64: efi: invoke EFI_RNG_PROTOCOL to supply KASLR randomness
Date
Since arm64 does not use a decompressor that supplies an execution
environment where it is feasible to some extent to provide a source of
randomness, the arm64 KASLR kernel depends on the bootloader to supply
some random bits in register x1 upon kernel entry.

On UEFI systems, we can use the EFI_RNG_PROTOCOL, if supplied, to obtain
some random bits. At the same time, use it to randomize the offset of the
kernel Image in physical memory.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
arch/arm64/Kconfig | 5 ++
arch/arm64/kernel/efi-entry.S | 7 +-
drivers/firmware/efi/libstub/arm-stub.c | 17 ++---
drivers/firmware/efi/libstub/arm64-stub.c | 67 +++++++++++++++-----
drivers/firmware/efi/libstub/efi-stub-helper.c | 10 +++
drivers/firmware/efi/libstub/efistub.h | 2 +
6 files changed, 82 insertions(+), 26 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 7fa5b74ee80d..ba347302308b 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -734,6 +734,11 @@ config RANDOMIZE_BASE
It is the bootloader's job to provide entropy, by passing a
random value in x1 at kernel entry.

+ When booting via the UEFI stub, it will invoke the firmware's
+ EFI_RNG_PROTOCOL implementation (if available) to supply entropy
+ to the kernel proper. In addition, it will randomise the physical
+ location of the kernel Image as well.
+
If unsure, say N.


diff --git a/arch/arm64/kernel/efi-entry.S b/arch/arm64/kernel/efi-entry.S
index f82036e02485..f41073dde7e0 100644
--- a/arch/arm64/kernel/efi-entry.S
+++ b/arch/arm64/kernel/efi-entry.S
@@ -110,7 +110,7 @@ ENTRY(entry)
2:
/* Jump to kernel entry point */
mov x0, x20
- mov x1, xzr
+ ldr x1, efi_rnd
mov x2, xzr
mov x3, xzr
br x21
@@ -119,6 +119,9 @@ efi_load_fail:
mov x0, #EFI_LOAD_ERROR
ldp x29, x30, [sp], #32
ret
+ENDPROC(entry)
+
+ENTRY(efi_rnd)
+ .quad 0, 0

entry_end:
-ENDPROC(entry)
diff --git a/drivers/firmware/efi/libstub/arm-stub.c b/drivers/firmware/efi/libstub/arm-stub.c
index 950c87f5d279..c39e04a1e8aa 100644
--- a/drivers/firmware/efi/libstub/arm-stub.c
+++ b/drivers/firmware/efi/libstub/arm-stub.c
@@ -207,14 +207,6 @@ unsigned long efi_entry(void *handle, efi_system_table_t *sys_table,
pr_efi_err(sys_table, "Failed to find DRAM base\n");
goto fail;
}
- status = handle_kernel_image(sys_table, image_addr, &image_size,
- &reserve_addr,
- &reserve_size,
- dram_base, image);
- if (status != EFI_SUCCESS) {
- pr_efi_err(sys_table, "Failed to relocate kernel\n");
- goto fail;
- }

/*
* Get the command line from EFI, using the LOADED_IMAGE
@@ -231,6 +223,15 @@ unsigned long efi_entry(void *handle, efi_system_table_t *sys_table,
if (status != EFI_SUCCESS)
pr_efi_err(sys_table, "Failed to parse EFI cmdline options\n");

+ status = handle_kernel_image(sys_table, image_addr, &image_size,
+ &reserve_addr,
+ &reserve_size,
+ dram_base, image);
+ if (status != EFI_SUCCESS) {
+ pr_efi_err(sys_table, "Failed to relocate kernel\n");
+ goto fail;
+ }
+
/*
* Unauthenticated device tree data is a security hazard, so
* ignore 'dtb=' unless UEFI Secure Boot is disabled.
diff --git a/drivers/firmware/efi/libstub/arm64-stub.c b/drivers/firmware/efi/libstub/arm64-stub.c
index 78dfbd34b6bf..96d43bed098f 100644
--- a/drivers/firmware/efi/libstub/arm64-stub.c
+++ b/drivers/firmware/efi/libstub/arm64-stub.c
@@ -13,6 +13,13 @@
#include <asm/efi.h>
#include <asm/sections.h>

+#include "efistub.h"
+
+extern struct {
+ u64 virt_seed;
+ u64 phys_seed;
+} efi_rnd;
+
efi_status_t __init handle_kernel_image(efi_system_table_t *sys_table_arg,
unsigned long *image_addr,
unsigned long *image_size,
@@ -27,6 +34,22 @@ efi_status_t __init handle_kernel_image(efi_system_table_t *sys_table_arg,
void *old_image_addr = (void *)*image_addr;
unsigned long preferred_offset;

+ if (IS_ENABLED(CONFIG_RANDOMIZE_BASE)) {
+ if (!__nokaslr) {
+ status = efi_get_random_bytes(sys_table_arg,
+ sizeof(efi_rnd),
+ (u8 *)&efi_rnd);
+ if (status == EFI_NOT_FOUND) {
+ pr_efi(sys_table_arg, "EFI_RNG_PROTOCOL unavailable, no randomness supplied\n");
+ } else if (status != EFI_SUCCESS) {
+ pr_efi_err(sys_table_arg, "efi_get_random_bytes() failed\n");
+ return status;
+ }
+ } else {
+ pr_efi(sys_table_arg, "KASLR disabled on kernel command line\n");
+ }
+ }
+
/*
* The preferred offset of the kernel Image is TEXT_OFFSET bytes beyond
* a 2 MB aligned base, which itself may be lower than dram_base, as
@@ -36,13 +59,22 @@ efi_status_t __init handle_kernel_image(efi_system_table_t *sys_table_arg,
if (preferred_offset < dram_base)
preferred_offset += SZ_2M;

- /* Relocate the image, if required. */
kernel_size = _edata - _text;
- if (*image_addr != preferred_offset) {
- kernel_memsize = kernel_size + (_end - _edata);
+ kernel_memsize = kernel_size + (_end - _edata);
+
+ if (IS_ENABLED(CONFIG_RANDOMIZE_BASE) && efi_rnd.phys_seed != 0) {
+ /*
+ * If KASLR is enabled, and we have some randomness available,
+ * locate the kernel at a randomized offset in physical memory.
+ */
+ status = efi_random_alloc(sys_table_arg, kernel_size,
+ ilog2(MIN_KIMG_ALIGN), reserve_addr,
+ efi_rnd.phys_seed);

+ *image_addr = *reserve_addr + TEXT_OFFSET;
+ } else {
/*
- * First, try a straight allocation at the preferred offset.
+ * Else, try a straight allocation at the preferred offset.
* This will work around the issue where, if dram_base == 0x0,
* efi_low_alloc() refuses to allocate at 0x0 (to prevent the
* address of the allocation to be mistaken for a FAIL return
@@ -52,27 +84,30 @@ efi_status_t __init handle_kernel_image(efi_system_table_t *sys_table_arg,
* Mustang), we can still place the kernel at the address
* 'dram_base + TEXT_OFFSET'.
*/
+ if (*image_addr == preferred_offset)
+ return EFI_SUCCESS;
+
*image_addr = *reserve_addr = preferred_offset;
nr_pages = round_up(kernel_memsize, EFI_ALLOC_ALIGN) /
EFI_PAGE_SIZE;
status = efi_call_early(allocate_pages, EFI_ALLOCATE_ADDRESS,
EFI_LOADER_DATA, nr_pages,
(efi_physical_addr_t *)reserve_addr);
- if (status != EFI_SUCCESS) {
- kernel_memsize += TEXT_OFFSET;
- status = efi_low_alloc(sys_table_arg, kernel_memsize,
- SZ_2M, reserve_addr);
+ }

- if (status != EFI_SUCCESS) {
- pr_efi_err(sys_table_arg, "Failed to relocate kernel\n");
- return status;
- }
- *image_addr = *reserve_addr + TEXT_OFFSET;
+ if (status != EFI_SUCCESS) {
+ kernel_memsize += TEXT_OFFSET;
+ status = efi_low_alloc(sys_table_arg, kernel_memsize,
+ SZ_2M, reserve_addr);
+
+ if (status != EFI_SUCCESS) {
+ pr_efi_err(sys_table_arg, "Failed to relocate kernel\n");
+ return status;
}
- memcpy((void *)*image_addr, old_image_addr, kernel_size);
- *reserve_size = kernel_memsize;
+ *image_addr = *reserve_addr + TEXT_OFFSET;
}
-
+ memcpy((void *)*image_addr, old_image_addr, kernel_size);
+ *reserve_size = kernel_memsize;

return EFI_SUCCESS;
}
diff --git a/drivers/firmware/efi/libstub/efi-stub-helper.c b/drivers/firmware/efi/libstub/efi-stub-helper.c
index 2a7a3015d7e0..e8a3b8cd53cc 100644
--- a/drivers/firmware/efi/libstub/efi-stub-helper.c
+++ b/drivers/firmware/efi/libstub/efi-stub-helper.c
@@ -32,6 +32,10 @@

static unsigned long __chunk_size = EFI_READ_CHUNK_SIZE;

+#ifdef CONFIG_RANDOMIZE_BASE
+bool __nokaslr;
+#endif
+
/*
* Allow the platform to override the allocation granularity: this allows
* systems that have the capability to run with a larger page size to deal
@@ -317,6 +321,12 @@ efi_status_t efi_parse_options(char *cmdline)
{
char *str;

+#ifdef CONFIG_RANDOMIZE_BASE
+ str = strstr(cmdline, "nokaslr");
+ if (str && (str == cmdline || *(str - 1) == ' '))
+ __nokaslr = true;
+#endif
+
/*
* If no EFI parameters were specified on the cmdline we've got
* nothing to do.
diff --git a/drivers/firmware/efi/libstub/efistub.h b/drivers/firmware/efi/libstub/efistub.h
index 7a38e29da53d..250ed4737298 100644
--- a/drivers/firmware/efi/libstub/efistub.h
+++ b/drivers/firmware/efi/libstub/efistub.h
@@ -5,6 +5,8 @@
/* error code which can't be mistaken for valid address */
#define EFI_ERROR (~0UL)

+extern bool __nokaslr;
+
void efi_char16_printk(efi_system_table_t *, efi_char16_t *);

efi_status_t efi_open_volume(efi_system_table_t *sys_table_arg, void *__image,
--
2.5.0
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