[PATCH v5 20/21] arm64: head: avoid relocating the kernel twice for KASLR

Ard Biesheuvel
2022-06-24T15:17:55Z

Currently, when KASLR is in effect, we set up the kernel virtual address
space twice: the first time, the KASLR seed is looked up in the device
tree, and the kernel virtual mapping is torn down and recreated again,
after which the relocations are applied a second time. The latter step
means that statically initialized global pointer variables will be reset
to their initial values, and to ensure that BSS variables are not set to
values based on the initial translation, they are cleared again as well.

All of this is needed because we need the command line (taken from the
DT) to tell us whether or not to randomize the virtual address space
before entering the kernel proper. However, this code has expanded
little by little and now creates global state unrelated to the virtual
randomization of the kernel before the mapping is torn down and set up
again, and the BSS cleared for a second time. This has created some
issues in the past, and it would be better to avoid this little dance if
possible.

So instead, let's use the temporary mapping of the device tree, and
execute the bare minimum of code to decide whether or not KASLR should
be enabled, and what the seed is. Only then, create the virtual kernel
mapping, clear BSS, etc and proceed as normal. This avoids the issues
around inconsistent global state due to BSS being cleared twice, and is
generally more maintainable, as it permits us to defer all the remaining
DT parsing and KASLR initialization to a later time.

This means the relocation fixup code runs only a single time as well,
allowing us to simplify the RELR handling code too, which is not
idempotent and was therefore required to keep track of the offset that
was applied the first time around.

Note that this means we have to clone a pair of FDT library objects, so
that we can control how they are built - we need the stack protector
and other instrumentation disabled so that the code can tolerate being
called this early. Note that only the kernel page tables and the
temporary stack are mapped read-write at this point, which ensures that
the early code does not modify any global state inadvertently.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
arch/arm64/kernel/Makefile | 2 +-
arch/arm64/kernel/head.S | 73 ++++---------
arch/arm64/kernel/image-vars.h | 4 +
arch/arm64/kernel/kaslr.c | 87 ---------------
arch/arm64/kernel/pi/Makefile | 33 ++++++
arch/arm64/kernel/pi/kaslr_early.c | 112 ++++++++++++++++++++
6 files changed, 171 insertions(+), 140 deletions(-)

diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
index fa7981d0d917..88a96511580e 100644
--- a/arch/arm64/kernel/Makefile
+++ b/arch/arm64/kernel/Makefile
@@ -59,7 +59,7 @@ obj-$(CONFIG_ACPI) += acpi.o
obj-$(CONFIG_ACPI_NUMA) += acpi_numa.o
obj-$(CONFIG_ARM64_ACPI_PARKING_PROTOCOL) += acpi_parking_protocol.o
obj-$(CONFIG_PARAVIRT) += paravirt.o
-obj-$(CONFIG_RANDOMIZE_BASE) += kaslr.o
+obj-$(CONFIG_RANDOMIZE_BASE) += kaslr.o pi/
obj-$(CONFIG_HIBERNATION) += hibernate.o hibernate-asm.o
obj-$(CONFIG_ELF_CORE) += elfcore.o
obj-$(CONFIG_KEXEC_CORE) += machine_kexec.o relocate_kernel.o \
diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index 8de346dd4470..5a2ff6466b6b 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -86,15 +86,13 @@
* x21 primary_entry() .. start_kernel() FDT pointer passed at boot in x0
* x22 create_idmap() .. start_kernel() ID map VA of the DT blob
* x23 primary_entry() .. start_kernel() physical misalignment/KASLR offset
- * x24 __primary_switch() .. relocate_kernel() current RELR displacement
+ * x24 __primary_switch() linear map KASLR seed
* x28 create_idmap() callee preserved temp register
*/
SYM_CODE_START(primary_entry)
bl preserve_boot_args
bl init_kernel_el // w0=cpu_boot_mode
mov x20, x0
- adrp x23, __PHYS_OFFSET
- and x23, x23, MIN_KIMG_ALIGN - 1 // KASLR offset, defaults to 0
bl create_idmap

/*
@@ -441,6 +439,10 @@ SYM_FUNC_START_LOCAL(__primary_switched)
bl __pi_memset
dsb ishst // Make zero page visible to PTW

+#ifdef CONFIG_RANDOMIZE_BASE
+ adrp x5, memstart_offset_seed // Save KASLR linear map seed
+ strh w24, [x5, :lo12:memstart_offset_seed]
+#endif
#if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)
bl kasan_early_init
#endif
@@ -448,16 +450,6 @@ SYM_FUNC_START_LOCAL(__primary_switched)
bl early_fdt_map // Try mapping the FDT early
mov x0, x22 // pass FDT address in x0
bl init_feature_override // Parse cpu feature overrides
-#ifdef CONFIG_RANDOMIZE_BASE
- tst x23, ~(MIN_KIMG_ALIGN - 1) // already running randomized?
- b.ne 0f
- bl kaslr_early_init // parse FDT for KASLR options
- cbz x0, 0f // KASLR disabled? just proceed
- orr x23, x23, x0 // record KASLR offset
- ldp x29, x30, [sp], #16 // we must enable KASLR, return
- ret // to __primary_switch()
-0:
-#endif
mov x0, x20
bl switch_to_vhe // Prefer VHE if possible
ldp x29, x30, [sp], #16
@@ -759,27 +751,17 @@ SYM_FUNC_START_LOCAL(__relocate_kernel)
* entry in x9, the address being relocated by the current address or
* bitmap entry in x13 and the address being relocated by the current
* bit in x14.
- *
- * Because addends are stored in place in the binary, RELR relocations
- * cannot be applied idempotently. We use x24 to keep track of the
- * currently applied displacement so that we can correctly relocate if
- * __relocate_kernel is called twice with non-zero displacements (i.e.
- * if there is both a physical misalignment and a KASLR displacement).
*/
adr_l x9, __relr_start
adr_l x10, __relr_end

- sub x15, x23, x24 // delta from previous offset
- cbz x15, 7f // nothing to do if unchanged
- mov x24, x23 // save new offset
-
2: cmp x9, x10
b.hs 7f
ldr x11, [x9], #8
tbnz x11, #0, 3f // branch to handle bitmaps
add x13, x11, x23
ldr x12, [x13] // relocate address entry
- add x12, x12, x15
+ add x12, x12, x23
str x12, [x13], #8 // adjust to start of bitmap
b 2b

@@ -788,7 +770,7 @@ SYM_FUNC_START_LOCAL(__relocate_kernel)
cbz x11, 6f
tbz x11, #0, 5f // skip bit if not set
ldr x12, [x14] // relocate bit
- add x12, x12, x15
+ add x12, x12, x23
str x12, [x14]

5: add x14, x14, #8 // move to next bit's address
@@ -812,40 +794,27 @@ SYM_FUNC_START_LOCAL(__primary_switch)
adrp x1, reserved_pg_dir
adrp x2, init_idmap_pg_dir
bl __enable_mmu
-
+#ifdef CONFIG_RELOCATABLE
+ adrp x23, __PHYS_OFFSET
+ and x23, x23, MIN_KIMG_ALIGN - 1
+#ifdef CONFIG_RANDOMIZE_BASE
+ mov x0, x22
+ adrp x1, init_pg_end
+ mov sp, x1
+ mov x29, xzr
+ bl __pi_kaslr_early_init
+ and x24, x0, #SZ_2M - 1 // capture memstart offset seed
+ bic x0, x0, #SZ_2M - 1
+ orr x23, x23, x0 // record kernel offset
+#endif
+#endif
bl clear_page_tables
bl create_kernel_mapping

adrp x1, init_pg_dir
load_ttbr1 x1, x1, x2
#ifdef CONFIG_RELOCATABLE
-#ifdef CONFIG_RELR
- mov x24, #0 // no RELR displacement yet
-#endif
bl __relocate_kernel
-#ifdef CONFIG_RANDOMIZE_BASE
- ldr x8, =__primary_switched
- adrp x0, __PHYS_OFFSET
- blr x8
-
- /*
- * If we return here, we have a KASLR displacement in x23 which we need
- * to take into account by discarding the current kernel mapping and
- * creating a new one.
- */
- adrp x1, reserved_pg_dir // Disable translations via TTBR1
- load_ttbr1 x1, x1, x2
- bl clear_page_tables
- bl create_kernel_mapping // Recreate kernel mapping
-
- tlbi vmalle1 // Remove any stale TLB entries
- dsb nsh
- isb
-
- adrp x1, init_pg_dir // Re-enable translations via TTBR1
- load_ttbr1 x1, x1, x2
- bl __relocate_kernel
-#endif
#endif
ldr x8, =__primary_switched
adrp x0, __PHYS_OFFSET
diff --git a/arch/arm64/kernel/image-vars.h b/arch/arm64/kernel/image-vars.h
index 241c86b67d01..0c381a405bf0 100644
--- a/arch/arm64/kernel/image-vars.h
+++ b/arch/arm64/kernel/image-vars.h
@@ -41,6 +41,10 @@ __efistub_dcache_clean_poc = __pi_dcache_clean_poc;
__efistub___memcpy = __pi_memcpy;
__efistub___memmove = __pi_memmove;
__efistub___memset = __pi_memset;
+
+__pi___memcpy = __pi_memcpy;
+__pi___memmove = __pi_memmove;
+__pi___memset = __pi_memset;
#endif

__efistub__text = _text;
diff --git a/arch/arm64/kernel/kaslr.c b/arch/arm64/kernel/kaslr.c
index 3edee81d8ea7..325455d16dbc 100644
--- a/arch/arm64/kernel/kaslr.c
+++ b/arch/arm64/kernel/kaslr.c
@@ -23,95 +23,8 @@
u64 __ro_after_init module_alloc_base;
u16 __initdata memstart_offset_seed;

-static __init u64 get_kaslr_seed(void *fdt)
-{
- int node, len;
- fdt64_t *prop;
- u64 ret;
-
- node = fdt_path_offset(fdt, "/chosen");
- if (node < 0)
- return 0;
-
- prop = fdt_getprop_w(fdt, node, "kaslr-seed", &len);
- if (!prop || len != sizeof(u64))
- return 0;
-
- ret = fdt64_to_cpu(*prop);
- *prop = 0;
- return ret;
-}
-
struct arm64_ftr_override kaslr_feature_override __initdata;

-/*
- * This routine will be executed with the kernel mapped at its default virtual
- * address, and if it returns successfully, the kernel will be remapped, and
- * start_kernel() will be executed from a randomized virtual offset. The
- * relocation will result in all absolute references (e.g., static variables
- * containing function pointers) to be reinitialized, and zero-initialized
- * .bss variables will be reset to 0.
- */
-u64 __init kaslr_early_init(void)
-{
- void *fdt;
- u64 seed, offset, mask;
- unsigned long raw;
-
- /*
- * Try to map the FDT early. If this fails, we simply bail,
- * and proceed with KASLR disabled. We will make another
- * attempt at mapping the FDT in setup_machine()
- */
- fdt = get_early_fdt_ptr();
- if (!fdt) {
- return 0;
- }
-
- /*
- * Retrieve (and wipe) the seed from the FDT
- */
- seed = get_kaslr_seed(fdt);
-
- /*
- * Check if 'nokaslr' appears on the command line, and
- * return 0 if that is the case.
- */
- if (kaslr_feature_override.val & kaslr_feature_override.mask & 0xf) {
- return 0;
- }
-
- /*
- * Mix in any entropy obtainable architecturally if enabled
- * and supported.
- */
-
- if (arch_get_random_seed_long_early(&raw))
- seed ^= raw;
-
- if (!seed) {
- return 0;
- }
-
- /*
- * OK, so we are proceeding with KASLR enabled. Calculate a suitable
- * kernel image offset from the seed. Let's place the kernel in the
- * middle half of the VMALLOC area (VA_BITS_MIN - 2), and stay clear of
- * the lower and upper quarters to avoid colliding with other
- * allocations.
- * Even if we could randomize at page granularity for 16k and 64k pages,
- * let's always round to 2 MB so we don't interfere with the ability to
- * map using contiguous PTEs
- */
- mask = ((1UL << (VA_BITS_MIN - 2)) - 1) & ~(SZ_2M - 1);
- offset = BIT(VA_BITS_MIN - 3) + (seed & mask);
-
- /* use the top 16 bits to randomize the linear region */
- memstart_offset_seed = seed >> 48;
-
- return offset;
-}
-
static int __init kaslr_init(void)
{
u64 module_range;
diff --git a/arch/arm64/kernel/pi/Makefile b/arch/arm64/kernel/pi/Makefile
new file mode 100644
index 000000000000..839291430cb3
--- /dev/null
+++ b/arch/arm64/kernel/pi/Makefile
@@ -0,0 +1,33 @@
+# SPDX-License-Identifier: GPL-2.0
+# Copyright 2022 Google LLC
+
+KBUILD_CFLAGS := $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS)) -fpie \
+ -Os -DDISABLE_BRANCH_PROFILING $(DISABLE_STACKLEAK_PLUGIN) \
+ $(call cc-option,-mbranch-protection=none) \
+ -I$(srctree)/scripts/dtc/libfdt -fno-stack-protector \
+ -include $(srctree)/include/linux/hidden.h \
+ -D__DISABLE_EXPORTS -ffreestanding -D__NO_FORTIFY \
+ $(call cc-option,-fno-addrsig)
+
+# remove SCS flags from all objects in this directory
+KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_SCS), $(KBUILD_CFLAGS))
+# disable LTO
+KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_LTO), $(KBUILD_CFLAGS))
+
+GCOV_PROFILE := n
+KASAN_SANITIZE := n
+KCSAN_SANITIZE := n
+UBSAN_SANITIZE := n
+KCOV_INSTRUMENT := n
+
+$(obj)/%.pi.o: OBJCOPYFLAGS := --prefix-symbols=__pi_ \
+ --remove-section=.note.gnu.property \
+ --prefix-alloc-sections=.init
+$(obj)/%.pi.o: $(obj)/%.o FORCE
+ $(call if_changed,objcopy)
+
+$(obj)/lib-%.o: $(srctree)/lib/%.c FORCE
+ $(call if_changed_rule,cc_o_c)
+
+obj-y := kaslr_early.pi.o lib-fdt.pi.o lib-fdt_ro.pi.o
+extra-y := $(patsubst %.pi.o,%.o,$(obj-y))
diff --git a/arch/arm64/kernel/pi/kaslr_early.c b/arch/arm64/kernel/pi/kaslr_early.c
new file mode 100644
index 000000000000..6c3855e69395
--- /dev/null
+++ b/arch/arm64/kernel/pi/kaslr_early.c
@@ -0,0 +1,112 @@
+// SPDX-License-Identifier: GPL-2.0-only
+// Copyright 2022 Google LLC
+// Author: Ard Biesheuvel <ardb@google.com>
+
+// NOTE: code in this file runs *very* early, and is not permitted to use
+// global variables or anything that relies on absolute addressing.
+
+#include <linux/libfdt.h>
+#include <linux/init.h>
+#include <linux/linkage.h>
+#include <linux/types.h>
+#include <linux/sizes.h>
+#include <linux/string.h>
+
+#include <asm/archrandom.h>
+#include <asm/memory.h>
+
+/* taken from lib/string.c */
+static char *__strstr(const char *s1, const char *s2)
+{
+ size_t l1, l2;
+
+ l2 = strlen(s2);
+ if (!l2)
+ return (char *)s1;
+ l1 = strlen(s1);
+ while (l1 >= l2) {
+ l1--;
+ if (!memcmp(s1, s2, l2))
+ return (char *)s1;
+ s1++;
+ }
+ return NULL;
+}
+static bool cmdline_contains_nokaslr(const u8 *cmdline)
+{
+ const u8 *str;
+
+ str = __strstr(cmdline, "nokaslr");
+ return str == cmdline || (str > cmdline && *(str - 1) == ' ');
+}
+
+static bool is_kaslr_disabled_cmdline(void *fdt)
+{
+ if (!IS_ENABLED(CONFIG_CMDLINE_FORCE)) {
+ int node;
+ const u8 *prop;
+
+ node = fdt_path_offset(fdt, "/chosen");
+ if (node < 0)
+ goto out;
+
+ prop = fdt_getprop(fdt, node, "bootargs", NULL);
+ if (!prop)
+ goto out;
+
+ if (cmdline_contains_nokaslr(prop))
+ return true;
+
+ if (IS_ENABLED(CONFIG_CMDLINE_EXTEND))
+ goto out;
+
+ return false;
+ }
+out:
+ return cmdline_contains_nokaslr(CONFIG_CMDLINE);
+}
+
+static u64 get_kaslr_seed(void *fdt)
+{
+ int node, len;
+ fdt64_t *prop;
+ u64 ret;
+
+ node = fdt_path_offset(fdt, "/chosen");
+ if (node < 0)
+ return 0;
+
+ prop = fdt_getprop_w(fdt, node, "kaslr-seed", &len);
+ if (!prop || len != sizeof(u64))
+ return 0;
+
+ ret = fdt64_to_cpu(*prop);
+ *prop = 0;
+ return ret;
+}
+
+asmlinkage u64 kaslr_early_init(void *fdt)
+{
+ u64 seed;
+
+ if (is_kaslr_disabled_cmdline(fdt))
+ return 0;
+
+ seed = get_kaslr_seed(fdt);
+ if (!seed) {
+#ifdef CONFIG_ARCH_RANDOM
+ if (!__early_cpu_has_rndr() ||
+ !__arm64_rndr((unsigned long *)&seed))
+#endif
+ return 0;
+ }
+
+ /*
+ * OK, so we are proceeding with KASLR enabled. Calculate a suitable
+ * kernel image offset from the seed. Let's place the kernel in the
+ * middle half of the VMALLOC area (VA_BITS_MIN - 2), and stay clear of
+ * the lower and upper quarters to avoid colliding with other
+ * allocations.
+ */
+ return BIT(VA_BITS_MIN - 3) + (seed & GENMASK(VA_BITS_MIN - 3, 0));
+}