[RFC PATCH v2 09/18] arm64: head: relocate kernel only a single time if KASLR is enabled
Ard Biesheuvel
ardb at kernel.org
Wed Mar 30 08:41:56 PDT 2022
We have accumulated a small pile of hacks to allow us to run C code and
parse command line options and DT properties before actually entering
the kernel in C mode. This was 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 mapping 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 create a temporary mapping of the device tree before
even mapping the kernel, 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.
Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
---
arch/arm64/include/asm/memory.h | 2 +
arch/arm64/kernel/Makefile | 2 +-
arch/arm64/kernel/head.S | 87 +++++++------
arch/arm64/kernel/kaslr.c | 71 ++---------
arch/arm64/kernel/pi/Makefile | 33 +++++
arch/arm64/kernel/pi/kaslr_early.c | 128 ++++++++++++++++++++
arch/arm64/kernel/setup.c | 2 +
7 files changed, 216 insertions(+), 109 deletions(-)
diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
index 0af70d9abede..2f1a48be11cf 100644
--- a/arch/arm64/include/asm/memory.h
+++ b/arch/arm64/include/asm/memory.h
@@ -191,6 +191,8 @@ static inline unsigned long kaslr_offset(void)
return kimage_vaddr - KIMAGE_VADDR;
}
+void kaslr_init(void *);
+
/*
* Allow all memory at the discovery stage. We will clip it later.
*/
diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
index 986837d7ec82..45f7a0e2d35e 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 5d4cb481e42f..f3b096daf1c5 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -83,14 +83,11 @@
*
* Register Scope Purpose
* x21 primary_entry() .. start_kernel() FDT pointer passed at boot in x0
- * x23 primary_entry() .. start_kernel() physical misalignment/KASLR offset
- * x24 __primary_switch() .. relocate_kernel() current RELR displacement
+ * x23 __primary_switch() .. relocate_kernel() physical misalignment/KASLR offset
*/
SYM_CODE_START(primary_entry)
bl preserve_boot_args
bl init_kernel_el // w0=cpu_boot_mode
- adrp x23, __PHYS_OFFSET
- and x23, x23, MIN_KIMG_ALIGN - 1 // KASLR offset, defaults to 0
bl set_cpu_boot_mode_flag
bl create_idmap
@@ -368,6 +365,27 @@ SYM_FUNC_START_LOCAL(create_kernel_mapping)
ret
SYM_FUNC_END(create_kernel_mapping)
+#ifdef CONFIG_RANDOMIZE_BASE
+// Create a temporary mapping of the device tree blob in the kernel page tables
+// so that we can grab the KASLR seed and command line options before mapping
+// the kernel at its randomized address.
+SYM_FUNC_START_LOCAL(create_temp_fdt_mapping)
+ adrp x2, init_pg_dir
+ mov_q x5, KIMAGE_VADDR
+ mov x4, PTRS_PER_PGD
+ bic x3, x21, #SZ_4M - 1 // map a 4 MiB block
+ add x6, x5, #SZ_4M
+ mov x7, SWAPPER_MM_MMUFLAGS
+
+ sub x0, x21, x3 // return the mapped address in x0
+ add x0, x0, x5
+
+ map_memory x2, x1, x5, x6, x7, x3, x4, x10, x11, x12, x13, x14
+
+ ret
+SYM_FUNC_END(create_temp_fdt_mapping)
+#endif
+
/*
* Initialize CPU registers with task-specific and cpu-specific context.
*
@@ -430,16 +448,6 @@ SYM_FUNC_START_LOCAL(__primary_switched)
mov x0, x21 // pass FDT address in x0
bl early_fdt_map // Try mapping the FDT early
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
bl switch_to_vhe // Prefer VHE if possible
ldp x29, x30, [sp], #16
bl start_kernel
@@ -785,29 +793,19 @@ 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).
*/
ldr w9, =__relr_offset // offset to reloc table
ldr w10, =__relr_size // size of reloc table
add x9, x9, x11 // __va(.relr)
add x10, x9, x10 // __va(.relr) + sizeof(.relr)
- 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
@@ -816,7 +814,7 @@ 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
@@ -840,38 +838,35 @@ SYM_FUNC_START_LOCAL(__primary_switch)
adrp x1, reserved_pg_dir
bl __enable_mmu
bl clear_page_tables
- bl create_kernel_mapping
- adrp x1, init_pg_dir
- load_ttbr1 x1, x2
#ifdef CONFIG_RELOCATABLE
-#ifdef CONFIG_RELR
- mov x24, #0 // no RELR displacement yet
-#endif
- bl __relocate_kernel
+ adrp x23, __PHYS_OFFSET
+ and x23, x23, MIN_KIMG_ALIGN - 1
#ifdef CONFIG_RANDOMIZE_BASE
- ldr x8, =__primary_switched
- adrp x0, __PHYS_OFFSET
- blr x8
+ bl create_temp_fdt_mapping
+ adrp x1, init_pg_dir
+ load_ttbr1 x1, x2
- /*
- * 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
+ adrp x1, init_stack
+ add sp, x1, #THREAD_SIZE
+ mov x29, xzr
+ bl __pi_kaslr_early_init
+ orr x23, x23, x0 // record KASLR offset
+
+ adrp x1, reserved_pg_dir
load_ttbr1 x1, x2
- bl clear_page_tables
- bl create_kernel_mapping // Recreate kernel mapping
tlbi vmalle1 // Remove any stale TLB entries
dsb nsh
isb
+#endif
+#endif
+ bl create_kernel_mapping
- adrp x1, init_pg_dir // Re-enable translations via TTBR1
+ adrp x1, init_pg_dir
load_ttbr1 x1, x2
+#ifdef CONFIG_RELOCATABLE
bl __relocate_kernel
-#endif
#endif
ldr x8, =__primary_switched
adrp x0, __PHYS_OFFSET
diff --git a/arch/arm64/kernel/kaslr.c b/arch/arm64/kernel/kaslr.c
index 3b12715642ce..78339a2aa3f8 100644
--- a/arch/arm64/kernel/kaslr.c
+++ b/arch/arm64/kernel/kaslr.c
@@ -24,7 +24,6 @@ enum kaslr_status {
KASLR_ENABLED,
KASLR_DISABLED_CMDLINE,
KASLR_DISABLED_NO_SEED,
- KASLR_DISABLED_FDT_REMAP,
};
static enum kaslr_status __initdata kaslr_status;
@@ -52,18 +51,9 @@ static __init u64 get_kaslr_seed(void *fdt)
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 __init kaslr_init(void *fdt)
{
- void *fdt;
- u64 seed, offset, mask, module_range;
+ u64 seed, module_range;
unsigned long raw;
/*
@@ -72,17 +62,6 @@ u64 __init kaslr_early_init(void)
*/
module_alloc_base = (u64)_etext - MODULES_VSIZE;
- /*
- * 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) {
- kaslr_status = KASLR_DISABLED_FDT_REMAP;
- return 0;
- }
-
/*
* Retrieve (and wipe) the seed from the FDT
*/
@@ -94,7 +73,7 @@ u64 __init kaslr_early_init(void)
*/
if (kaslr_feature_override.val & kaslr_feature_override.mask & 0xf) {
kaslr_status = KASLR_DISABLED_CMDLINE;
- return 0;
+ return;
}
/*
@@ -105,41 +84,14 @@ u64 __init kaslr_early_init(void)
if (arch_get_random_seed_long_early(&raw))
seed ^= raw;
- if (!seed) {
+ if (!seed || !kimage_voffset) {
kaslr_status = KASLR_DISABLED_NO_SEED;
- return 0;
+ return;
}
- /*
- * 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;
- if (!IS_ENABLED(CONFIG_KASAN_VMALLOC) &&
- (IS_ENABLED(CONFIG_KASAN_GENERIC) ||
- IS_ENABLED(CONFIG_KASAN_SW_TAGS)))
- /*
- * KASAN without KASAN_VMALLOC does not expect the module region
- * to intersect the vmalloc region, since shadow memory is
- * allocated for each module at load time, whereas the vmalloc
- * region is shadowed by KASAN zero pages. So keep modules
- * out of the vmalloc region if KASAN is enabled without
- * KASAN_VMALLOC, and put the kernel well within 4 GB of the
- * module region.
- */
- return offset % SZ_2G;
-
if (IS_ENABLED(CONFIG_RANDOMIZE_MODULE_REGION_FULL)) {
u64 end = (u64)_end - (u64)_text + KIMAGE_VADDR;
@@ -152,7 +104,7 @@ u64 __init kaslr_early_init(void)
* resolved normally.)
*/
module_range = SZ_2G - (u64)(_end - _stext);
- module_alloc_base = max(end + offset - SZ_2G,
+ module_alloc_base = max(end + kimage_voffset - SZ_2G,
(u64)MODULES_VADDR);
} else {
u64 end = (u64)_etext - (u64)_text + KIMAGE_VADDR;
@@ -167,17 +119,15 @@ u64 __init kaslr_early_init(void)
* when ARM64_MODULE_PLTS is enabled.
*/
module_range = MODULES_VSIZE - (u64)(_etext - _stext);
- module_alloc_base = end + offset - MODULES_VSIZE;
+ module_alloc_base = end + kimage_voffset - MODULES_VSIZE;
}
/* use the lower 21 bits to randomize the base of the module region */
module_alloc_base += (module_range * (seed & ((1 << 21) - 1))) >> 21;
module_alloc_base &= PAGE_MASK;
-
- return offset;
}
-static int __init kaslr_init(void)
+static int __init kaslr_report_status(void)
{
switch (kaslr_status) {
case KASLR_ENABLED:
@@ -189,11 +139,8 @@ static int __init kaslr_init(void)
case KASLR_DISABLED_NO_SEED:
pr_warn("KASLR disabled due to lack of seed\n");
break;
- case KASLR_DISABLED_FDT_REMAP:
- pr_warn("KASLR disabled due to FDT remapping failure\n");
- break;
}
return 0;
}
-core_initcall(kaslr_init)
+core_initcall(kaslr_report_status)
diff --git a/arch/arm64/kernel/pi/Makefile b/arch/arm64/kernel/pi/Makefile
new file mode 100644
index 000000000000..3c503312b5dc
--- /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 \
+ $(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..bbba01807ab6
--- /dev/null
+++ b/arch/arm64/kernel/pi/kaslr_early.c
@@ -0,0 +1,128 @@
+// SPDX-License-Identifier: GPL-2.0-only
+// Copyright 2022 Google LLC
+// Author: Ard Biesheuvel <ardb at 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;
+ const fdt64_t *prop;
+ u64 ret;
+
+ node = fdt_path_offset(fdt, "/chosen");
+ if (node < 0)
+ return 0;
+
+ prop = fdt_getprop(fdt, node, "kaslr-seed", &len);
+ if (!prop || len != sizeof(u64))
+ return 0;
+
+ ret = fdt64_to_cpu(*prop);
+ return ret;
+}
+
+asmlinkage u64 kaslr_early_init(void *fdt)
+{
+ u64 seed, mask, offset;
+
+ if (is_kaslr_disabled_cmdline(fdt))
+ return 0;
+
+ seed = get_kaslr_seed(fdt);
+ if (!seed && (!__early_cpu_has_rndr() ||
+ !__arm64_rndr((unsigned long *)&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);
+
+ if (!IS_ENABLED(CONFIG_KASAN_VMALLOC) &&
+ (IS_ENABLED(CONFIG_KASAN_GENERIC) ||
+ IS_ENABLED(CONFIG_KASAN_SW_TAGS)))
+ /*
+ * KASAN without KASAN_VMALLOC does not expect the module region
+ * to intersect the vmalloc region, since shadow memory is
+ * allocated for each module at load time, whereas the vmalloc
+ * region is shadowed by KASAN zero pages. So keep modules
+ * out of the vmalloc region if KASAN is enabled without
+ * KASAN_VMALLOC, and put the kernel well within 4 GB of the
+ * module region.
+ */
+ return offset % SZ_2G;
+
+ return offset;
+}
+
diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
index ebf69312eabf..26fb4bce5f8b 100644
--- a/arch/arm64/kernel/setup.c
+++ b/arch/arm64/kernel/setup.c
@@ -313,6 +313,8 @@ void __init __no_sanitize_address setup_arch(char **cmdline_p)
setup_machine_fdt(__fdt_pointer);
+ kaslr_init(initial_boot_params);
+
/* Early fixups are done, map the FDT as read-only now */
fixmap_remap_fdt(__fdt_pointer, NULL, PAGE_KERNEL_RO);
--
2.30.2
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