[RFC PATCH 05/28] x86: Define the stack protector guard symbol explicitly

Ard Biesheuvel
2024-09-25T15:02:03Z

From: Ard Biesheuvel <ardb@kernel.org>

Specify the guard symbol for the stack cookie explicitly, rather than
positioning it exactly 40 bytes into the per-CPU area. Doing so removes
the need for the per-CPU region to be absolute rather than relative to
the placement of the per-CPU template region in the kernel image, and
this allows the special handling for absolute per-CPU symbols to be
removed entirely.

This is a worthwhile cleanup in itself, but it is also a prerequisite
for PIE codegen and PIE linking, which can replace our bespoke and
rather clunky runtime relocation handling.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
arch/x86/Makefile | 4 ++++
arch/x86/include/asm/init.h | 2 +-
arch/x86/include/asm/processor.h | 11 +++--------
arch/x86/include/asm/stackprotector.h | 4 ----
tools/perf/util/annotate.c | 4 ++--
5 files changed, 10 insertions(+), 15 deletions(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 6b3fe6e2aadd..b78b7623a4a9 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -193,6 +193,10 @@ else
KBUILD_RUSTFLAGS += -Cno-redzone=y
KBUILD_RUSTFLAGS += -Ccode-model=kernel

+ ifeq ($(CONFIG_STACKPROTECTOR),y)
+ KBUILD_CFLAGS += -mstack-protector-guard-symbol=fixed_percpu_data
+ endif
+
# Don't emit relaxable GOTPCREL relocations
KBUILD_AFLAGS_KERNEL += -Wa,-mrelax-relocations=no
KBUILD_CFLAGS_KERNEL += -Wa,-mrelax-relocations=no
diff --git a/arch/x86/include/asm/init.h b/arch/x86/include/asm/init.h
index 14d72727d7ee..3ed0e8ec973f 100644
--- a/arch/x86/include/asm/init.h
+++ b/arch/x86/include/asm/init.h
@@ -2,7 +2,7 @@
#ifndef _ASM_X86_INIT_H
#define _ASM_X86_INIT_H

-#define __head __section(".head.text")
+#define __head __section(".head.text") __no_stack_protector

struct x86_mapping_info {
void *(*alloc_pgt_page)(void *); /* allocate buf for page table */
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 4a686f0e5dbf..56bc36116814 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -402,14 +402,9 @@ struct irq_stack {
#ifdef CONFIG_X86_64
struct fixed_percpu_data {
/*
- * GCC hardcodes the stack canary as %gs:40. Since the
- * irq_stack is the object at %gs:0, we reserve the bottom
- * 48 bytes of the irq stack for the canary.
- *
- * Once we are willing to require -mstack-protector-guard-symbol=
- * support for x86_64 stackprotector, we can get rid of this.
+ * Since the irq_stack is the object at %gs:0, the bottom 8 bytes of
+ * the irq stack are reserved for the canary.
*/
- char gs_base[40];
unsigned long stack_canary;
};

@@ -418,7 +413,7 @@ DECLARE_INIT_PER_CPU(fixed_percpu_data);

static inline unsigned long cpu_kernelmode_gs_base(int cpu)
{
- return (unsigned long)per_cpu(fixed_percpu_data.gs_base, cpu);
+ return (unsigned long)&per_cpu(fixed_percpu_data, cpu);
}

extern asmlinkage void entry_SYSCALL32_ignore(void);
diff --git a/arch/x86/include/asm/stackprotector.h b/arch/x86/include/asm/stackprotector.h
index 00473a650f51..d1dcd22a0a4c 100644
--- a/arch/x86/include/asm/stackprotector.h
+++ b/arch/x86/include/asm/stackprotector.h
@@ -51,10 +51,6 @@ static __always_inline void boot_init_stack_canary(void)
{
unsigned long canary = get_random_canary();

-#ifdef CONFIG_X86_64
- BUILD_BUG_ON(offsetof(struct fixed_percpu_data, stack_canary) != 40);
-#endif
-
current->stack_canary = canary;
#ifdef CONFIG_X86_64
this_cpu_write(fixed_percpu_data.stack_canary, canary);
diff --git a/tools/perf/util/annotate.c b/tools/perf/util/annotate.c
index 37ce43c4eb8f..7ecfedf5edb9 100644
--- a/tools/perf/util/annotate.c
+++ b/tools/perf/util/annotate.c
@@ -2485,10 +2485,10 @@ static bool is_stack_operation(struct arch *arch, struct disasm_line *dl)

static bool is_stack_canary(struct arch *arch, struct annotated_op_loc *loc)
{
- /* On x86_64, %gs:40 is used for stack canary */
+ /* On x86_64, %gs:0 is used for stack canary */
if (arch__is(arch, "x86")) {
if (loc->segment == INSN_SEG_X86_GS && loc->imm &&
- loc->offset == 40)
+ loc->offset == 0)
return true;
}

--
2.46.0.792.g87dc391469-goog