Older versions of GCC fixed the location of the stack protector canary
at %gs:40. This constraint forced the percpu section to be linked at
absolute address 0 so that the canary could be the first data object in
the percpu section. Supporting the zero-based percpu section requires
additional code to handle relocations for RIP-relative references to
percpu data, extra complexity to kallsyms, and workarounds for linker
bugs due to the use of absolute symbols.
GCC 8.1 supports redefining where the canary is located, allowng it to
become a normal percpu variable instead of at a fixed location. This
removes the contraint that the percpu section must be zero-based.
Signed-off-by: Brian Gerst <brgerst@gmail.com>
---
arch/x86/Makefile | 20 +++++++++------
arch/x86/entry/entry.S | 2 --
arch/x86/entry/entry_64.S | 2 +-
arch/x86/include/asm/processor.h | 16 ++----------
arch/x86/include/asm/stackprotector.h | 36 ++++-----------------------
arch/x86/kernel/asm-offsets_64.c | 6 -----
arch/x86/kernel/cpu/common.c | 5 +---
arch/x86/kernel/head_64.S | 3 +--
arch/x86/xen/xen-head.S | 3 +--
9 files changed, 23 insertions(+), 70 deletions(-)
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 5b773b34768d..88a1705366f9 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -140,14 +140,7 @@ ifeq ($(CONFIG_X86_32),y)
# temporary until string.h is fixed
KBUILD_CFLAGS += -ffreestanding
- ifeq ($(CONFIG_STACKPROTECTOR),y)
- ifeq ($(CONFIG_SMP),y)
- KBUILD_CFLAGS += -mstack-protector-guard-reg=fs \
- -mstack-protector-guard-symbol=__ref_stack_chk_guard
- else
- KBUILD_CFLAGS += -mstack-protector-guard=global
- endif
- endif
+ percpu_seg := fs
else
BITS := 64
UTS_MACHINE := x86_64
@@ -197,6 +190,17 @@ else
KBUILD_CFLAGS += -mcmodel=kernel
KBUILD_RUSTFLAGS += -Cno-redzone=y
KBUILD_RUSTFLAGS += -Ccode-model=kernel
+
+ percpu_seg := gs
+endif
+
+ifeq ($(CONFIG_STACKPROTECTOR),y)
+ ifeq ($(CONFIG_SMP),y)
+ KBUILD_CFLAGS += -mstack-protector-guard-reg=$(percpu_seg)
+ KBUILD_CFLAGS += -mstack-protector-guard-symbol=__ref_stack_chk_guard
+ else
+ KBUILD_CFLAGS += -mstack-protector-guard=global
+ endif
endif
#
diff --git a/arch/x86/entry/entry.S b/arch/x86/entry/entry.S
index b7ea3e8e9ecc..fe5344a249a1 100644
--- a/arch/x86/entry/entry.S
+++ b/arch/x86/entry/entry.S
@@ -52,7 +52,6 @@ EXPORT_SYMBOL_GPL(mds_verw_sel);
THUNK warn_thunk_thunk, __warn_thunk
-#ifndef CONFIG_X86_64
/*
* Clang's implementation of TLS stack cookies requires the variable in
* question to be a TLS variable. If the variable happens to be defined as an
@@ -66,4 +65,3 @@ THUNK warn_thunk_thunk, __warn_thunk
#ifdef CONFIG_STACKPROTECTOR
EXPORT_SYMBOL(__ref_stack_chk_guard);
#endif
-#endif
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 1b5be07f8669..f78ef9667c39 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -192,7 +192,7 @@ SYM_FUNC_START(__switch_to_asm)
#ifdef CONFIG_STACKPROTECTOR
movq TASK_stack_canary(%rsi), %rbx
- movq %rbx, PER_CPU_VAR(fixed_percpu_data + FIXED_stack_canary)
+ movq %rbx, PER_CPU_VAR(__stack_chk_guard)
#endif
/*
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index c0975815980c..a113c3f4f558 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -419,16 +419,8 @@ 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.
- */
char gs_base[40];
- unsigned long stack_canary;
+ unsigned long reserved;
};
DECLARE_PER_CPU_FIRST(struct fixed_percpu_data, fixed_percpu_data) __visible;
@@ -443,11 +435,7 @@ extern asmlinkage void entry_SYSCALL32_ignore(void);
/* Save actual FS/GS selectors and bases to current->thread */
void current_save_fsgs(void);
-#else /* X86_64 */
-#ifdef CONFIG_STACKPROTECTOR
-DECLARE_PER_CPU(unsigned long, __stack_chk_guard);
-#endif
-#endif /* !X86_64 */
+#endif /* X86_64 */
struct perf_event;
diff --git a/arch/x86/include/asm/stackprotector.h b/arch/x86/include/asm/stackprotector.h
index 00473a650f51..d43fb589fcf6 100644
--- a/arch/x86/include/asm/stackprotector.h
+++ b/arch/x86/include/asm/stackprotector.h
@@ -2,26 +2,10 @@
/*
* GCC stack protector support.
*
- * Stack protector works by putting predefined pattern at the start of
+ * Stack protector works by putting a predefined pattern at the start of
* the stack frame and verifying that it hasn't been overwritten when
- * returning from the function. The pattern is called stack canary
- * and unfortunately gcc historically required it to be at a fixed offset
- * from the percpu segment base. On x86_64, the offset is 40 bytes.
- *
- * The same segment is shared by percpu area and stack canary. On
- * x86_64, percpu symbols are zero based and %gs (64-bit) points to the
- * base of percpu area. The first occupant of the percpu area is always
- * fixed_percpu_data which contains stack_canary at the appropriate
- * offset. On x86_32, the stack canary is just a regular percpu
- * variable.
- *
- * Putting percpu data in %fs on 32-bit is a minor optimization compared to
- * using %gs. Since 32-bit userspace normally has %fs == 0, we are likely
- * to load 0 into %fs on exit to usermode, whereas with percpu data in
- * %gs, we are likely to load a non-null %gs on return to user mode.
- *
- * Once we are willing to require GCC 8.1 or better for 64-bit stackprotector
- * support, we can remove some of this complexity.
+ * returning from the function. The pattern is called the stack canary
+ * and is a unique value for each task.
*/
#ifndef _ASM_STACKPROTECTOR_H
@@ -36,6 +20,8 @@
#include <linux/sched.h>
+DECLARE_PER_CPU(unsigned long, __stack_chk_guard);
+
/*
* Initialize the stackprotector canary value.
*
@@ -51,25 +37,13 @@ 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);
-#else
this_cpu_write(__stack_chk_guard, canary);
-#endif
}
static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
{
-#ifdef CONFIG_X86_64
- per_cpu(fixed_percpu_data.stack_canary, cpu) = idle->stack_canary;
-#else
per_cpu(__stack_chk_guard, cpu) = idle->stack_canary;
-#endif
}
#else /* STACKPROTECTOR */
diff --git a/arch/x86/kernel/asm-offsets_64.c b/arch/x86/kernel/asm-offsets_64.c
index bb65371ea9df..590b6cd0eac0 100644
--- a/arch/x86/kernel/asm-offsets_64.c
+++ b/arch/x86/kernel/asm-offsets_64.c
@@ -54,11 +54,5 @@ int main(void)
BLANK();
#undef ENTRY
- BLANK();
-
-#ifdef CONFIG_STACKPROTECTOR
- OFFSET(FIXED_stack_canary, fixed_percpu_data, stack_canary);
- BLANK();
-#endif
return 0;
}
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 9d42bd15e06c..1f33d5feb050 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -2086,8 +2086,7 @@ void syscall_init(void)
if (!cpu_feature_enabled(X86_FEATURE_FRED))
idt_syscall_init();
}
-
-#else /* CONFIG_X86_64 */
+#endif /* CONFIG_X86_64 */
#ifdef CONFIG_STACKPROTECTOR
DEFINE_PER_CPU(unsigned long, __stack_chk_guard);
@@ -2096,8 +2095,6 @@ EXPORT_PER_CPU_SYMBOL(__stack_chk_guard);
#endif
#endif
-#endif /* CONFIG_X86_64 */
-
/*
* Clear all 6 debug registers:
*/
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 56163e2124cf..c3028b4df85f 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -355,8 +355,7 @@ SYM_INNER_LABEL(common_startup_64, SYM_L_LOCAL)
/* Set up %gs.
*
- * The base of %gs always points to fixed_percpu_data. If the
- * stack protector canary is enabled, it is located at %gs:40.
+ * The base of %gs always points to fixed_percpu_data.
* Note that, on SMP, the boot cpu uses init data section until
* the per cpu areas are set up.
*/
diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
index 758bcd47b72d..ae4672ea00bb 100644
--- a/arch/x86/xen/xen-head.S
+++ b/arch/x86/xen/xen-head.S
@@ -53,8 +53,7 @@ SYM_CODE_START(startup_xen)
/* Set up %gs.
*
- * The base of %gs always points to fixed_percpu_data. If the
- * stack protector canary is enabled, it is located at %gs:40.
+ * The base of %gs always points to fixed_percpu_data.
* Note that, on SMP, the boot cpu uses init data section until
* the per cpu areas are set up.
*/
--
2.47.0On Tue, Nov 5, 2024 at 4:58 PM Brian Gerst <brgerst@gmail.com> wrote:
>
> Older versions of GCC fixed the location of the stack protector canary
> at %gs:40. This constraint forced the percpu section to be linked at
> absolute address 0 so that the canary could be the first data object in
> the percpu section. Supporting the zero-based percpu section requires
> additional code to handle relocations for RIP-relative references to
> percpu data, extra complexity to kallsyms, and workarounds for linker
> bugs due to the use of absolute symbols.
>
> GCC 8.1 supports redefining where the canary is located, allowng it to
> become a normal percpu variable instead of at a fixed location. This
> removes the contraint that the percpu section must be zero-based.
>
> Signed-off-by: Brian Gerst <brgerst@gmail.com>
Reviewed-by: Uros Bizjak <ubizjak@gmail.com>
> ---
> arch/x86/Makefile | 20 +++++++++------
> arch/x86/entry/entry.S | 2 --
> arch/x86/entry/entry_64.S | 2 +-
> arch/x86/include/asm/processor.h | 16 ++----------
> arch/x86/include/asm/stackprotector.h | 36 ++++-----------------------
> arch/x86/kernel/asm-offsets_64.c | 6 -----
> arch/x86/kernel/cpu/common.c | 5 +---
> arch/x86/kernel/head_64.S | 3 +--
> arch/x86/xen/xen-head.S | 3 +--
> 9 files changed, 23 insertions(+), 70 deletions(-)
>
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 5b773b34768d..88a1705366f9 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -140,14 +140,7 @@ ifeq ($(CONFIG_X86_32),y)
> # temporary until string.h is fixed
> KBUILD_CFLAGS += -ffreestanding
>
> - ifeq ($(CONFIG_STACKPROTECTOR),y)
> - ifeq ($(CONFIG_SMP),y)
> - KBUILD_CFLAGS += -mstack-protector-guard-reg=fs \
> - -mstack-protector-guard-symbol=__ref_stack_chk_guard
> - else
> - KBUILD_CFLAGS += -mstack-protector-guard=global
> - endif
> - endif
> + percpu_seg := fs
> else
> BITS := 64
> UTS_MACHINE := x86_64
> @@ -197,6 +190,17 @@ else
> KBUILD_CFLAGS += -mcmodel=kernel
> KBUILD_RUSTFLAGS += -Cno-redzone=y
> KBUILD_RUSTFLAGS += -Ccode-model=kernel
> +
> + percpu_seg := gs
> +endif
> +
> +ifeq ($(CONFIG_STACKPROTECTOR),y)
> + ifeq ($(CONFIG_SMP),y)
> + KBUILD_CFLAGS += -mstack-protector-guard-reg=$(percpu_seg)
> + KBUILD_CFLAGS += -mstack-protector-guard-symbol=__ref_stack_chk_guard
> + else
> + KBUILD_CFLAGS += -mstack-protector-guard=global
> + endif
> endif
>
> #
> diff --git a/arch/x86/entry/entry.S b/arch/x86/entry/entry.S
> index b7ea3e8e9ecc..fe5344a249a1 100644
> --- a/arch/x86/entry/entry.S
> +++ b/arch/x86/entry/entry.S
> @@ -52,7 +52,6 @@ EXPORT_SYMBOL_GPL(mds_verw_sel);
>
> THUNK warn_thunk_thunk, __warn_thunk
>
> -#ifndef CONFIG_X86_64
> /*
> * Clang's implementation of TLS stack cookies requires the variable in
> * question to be a TLS variable. If the variable happens to be defined as an
> @@ -66,4 +65,3 @@ THUNK warn_thunk_thunk, __warn_thunk
> #ifdef CONFIG_STACKPROTECTOR
> EXPORT_SYMBOL(__ref_stack_chk_guard);
> #endif
> -#endif
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 1b5be07f8669..f78ef9667c39 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -192,7 +192,7 @@ SYM_FUNC_START(__switch_to_asm)
>
> #ifdef CONFIG_STACKPROTECTOR
> movq TASK_stack_canary(%rsi), %rbx
> - movq %rbx, PER_CPU_VAR(fixed_percpu_data + FIXED_stack_canary)
> + movq %rbx, PER_CPU_VAR(__stack_chk_guard)
> #endif
>
> /*
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index c0975815980c..a113c3f4f558 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -419,16 +419,8 @@ 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.
> - */
> char gs_base[40];
> - unsigned long stack_canary;
> + unsigned long reserved;
> };
>
> DECLARE_PER_CPU_FIRST(struct fixed_percpu_data, fixed_percpu_data) __visible;
> @@ -443,11 +435,7 @@ extern asmlinkage void entry_SYSCALL32_ignore(void);
>
> /* Save actual FS/GS selectors and bases to current->thread */
> void current_save_fsgs(void);
> -#else /* X86_64 */
> -#ifdef CONFIG_STACKPROTECTOR
> -DECLARE_PER_CPU(unsigned long, __stack_chk_guard);
> -#endif
> -#endif /* !X86_64 */
> +#endif /* X86_64 */
>
> struct perf_event;
>
> diff --git a/arch/x86/include/asm/stackprotector.h b/arch/x86/include/asm/stackprotector.h
> index 00473a650f51..d43fb589fcf6 100644
> --- a/arch/x86/include/asm/stackprotector.h
> +++ b/arch/x86/include/asm/stackprotector.h
> @@ -2,26 +2,10 @@
> /*
> * GCC stack protector support.
> *
> - * Stack protector works by putting predefined pattern at the start of
> + * Stack protector works by putting a predefined pattern at the start of
> * the stack frame and verifying that it hasn't been overwritten when
> - * returning from the function. The pattern is called stack canary
> - * and unfortunately gcc historically required it to be at a fixed offset
> - * from the percpu segment base. On x86_64, the offset is 40 bytes.
> - *
> - * The same segment is shared by percpu area and stack canary. On
> - * x86_64, percpu symbols are zero based and %gs (64-bit) points to the
> - * base of percpu area. The first occupant of the percpu area is always
> - * fixed_percpu_data which contains stack_canary at the appropriate
> - * offset. On x86_32, the stack canary is just a regular percpu
> - * variable.
> - *
> - * Putting percpu data in %fs on 32-bit is a minor optimization compared to
> - * using %gs. Since 32-bit userspace normally has %fs == 0, we are likely
> - * to load 0 into %fs on exit to usermode, whereas with percpu data in
> - * %gs, we are likely to load a non-null %gs on return to user mode.
> - *
> - * Once we are willing to require GCC 8.1 or better for 64-bit stackprotector
> - * support, we can remove some of this complexity.
> + * returning from the function. The pattern is called the stack canary
> + * and is a unique value for each task.
> */
>
> #ifndef _ASM_STACKPROTECTOR_H
> @@ -36,6 +20,8 @@
>
> #include <linux/sched.h>
>
> +DECLARE_PER_CPU(unsigned long, __stack_chk_guard);
> +
> /*
> * Initialize the stackprotector canary value.
> *
> @@ -51,25 +37,13 @@ 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);
> -#else
> this_cpu_write(__stack_chk_guard, canary);
> -#endif
> }
>
> static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
> {
> -#ifdef CONFIG_X86_64
> - per_cpu(fixed_percpu_data.stack_canary, cpu) = idle->stack_canary;
> -#else
> per_cpu(__stack_chk_guard, cpu) = idle->stack_canary;
> -#endif
> }
>
> #else /* STACKPROTECTOR */
> diff --git a/arch/x86/kernel/asm-offsets_64.c b/arch/x86/kernel/asm-offsets_64.c
> index bb65371ea9df..590b6cd0eac0 100644
> --- a/arch/x86/kernel/asm-offsets_64.c
> +++ b/arch/x86/kernel/asm-offsets_64.c
> @@ -54,11 +54,5 @@ int main(void)
> BLANK();
> #undef ENTRY
>
> - BLANK();
> -
> -#ifdef CONFIG_STACKPROTECTOR
> - OFFSET(FIXED_stack_canary, fixed_percpu_data, stack_canary);
> - BLANK();
> -#endif
> return 0;
> }
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 9d42bd15e06c..1f33d5feb050 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -2086,8 +2086,7 @@ void syscall_init(void)
> if (!cpu_feature_enabled(X86_FEATURE_FRED))
> idt_syscall_init();
> }
> -
> -#else /* CONFIG_X86_64 */
> +#endif /* CONFIG_X86_64 */
>
> #ifdef CONFIG_STACKPROTECTOR
> DEFINE_PER_CPU(unsigned long, __stack_chk_guard);
> @@ -2096,8 +2095,6 @@ EXPORT_PER_CPU_SYMBOL(__stack_chk_guard);
> #endif
> #endif
>
> -#endif /* CONFIG_X86_64 */
> -
> /*
> * Clear all 6 debug registers:
> */
> diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> index 56163e2124cf..c3028b4df85f 100644
> --- a/arch/x86/kernel/head_64.S
> +++ b/arch/x86/kernel/head_64.S
> @@ -355,8 +355,7 @@ SYM_INNER_LABEL(common_startup_64, SYM_L_LOCAL)
>
> /* Set up %gs.
> *
> - * The base of %gs always points to fixed_percpu_data. If the
> - * stack protector canary is enabled, it is located at %gs:40.
> + * The base of %gs always points to fixed_percpu_data.
> * Note that, on SMP, the boot cpu uses init data section until
> * the per cpu areas are set up.
> */
> diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
> index 758bcd47b72d..ae4672ea00bb 100644
> --- a/arch/x86/xen/xen-head.S
> +++ b/arch/x86/xen/xen-head.S
> @@ -53,8 +53,7 @@ SYM_CODE_START(startup_xen)
>
> /* Set up %gs.
> *
> - * The base of %gs always points to fixed_percpu_data. If the
> - * stack protector canary is enabled, it is located at %gs:40.
> + * The base of %gs always points to fixed_percpu_data.
> * Note that, on SMP, the boot cpu uses init data section until
> * the per cpu areas are set up.
> */
> --
> 2.47.0
>
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