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authorBrian Gerst <brgerst@gmail.com>2025-01-23 14:07:39 -0500
committerIngo Molnar <mingo@kernel.org>2025-02-18 10:15:09 +0100
commit80d47defddc000271502057ebd7efa4fd6481542 (patch)
tree8b352a713b9f87567eb3d3a4ac1bcc55bc9311ed
parent78c4374ef8b842c6abf195d6f963853c7ec464d2 (diff)
downloadlinux-80d47defddc0.tar.gz
x86/stackprotector/64: Convert to normal per-CPU variable
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, allowing it to become a normal percpu variable instead of at a fixed location. This removes the constraint that the percpu section must be zero-based. Signed-off-by: Brian Gerst <brgerst@gmail.com> Signed-off-by: Ingo Molnar <mingo@kernel.org> Reviewed-by: Ard Biesheuvel <ardb@kernel.org> Reviewed-by: Uros Bizjak <ubizjak@gmail.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Link: https://lore.kernel.org/r/20250123190747.745588-8-brgerst@gmail.com
Diffstat
-rw-r--r--arch/x86/Makefile20
-rw-r--r--arch/x86/entry/entry.S2
-rw-r--r--arch/x86/entry/entry_64.S2
-rw-r--r--arch/x86/include/asm/processor.h16
-rw-r--r--arch/x86/include/asm/stackprotector.h36
-rw-r--r--arch/x86/kernel/asm-offsets_64.c6
-rw-r--r--arch/x86/kernel/cpu/common.c5
-rw-r--r--arch/x86/kernel/head_64.S3
-rw-r--r--arch/x86/xen/xen-head.S3
9 files changed, 23 insertions, 70 deletions
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 5b773b34768d12..88a1705366f918 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 b7ea3e8e9eccd5..fe5344a249a1ba 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 f52dbe0ad93cd1..33a955aa01d895 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 c0cd10182e90c1..a4687122951f75 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -422,16 +422,8 @@ 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;
@@ -446,11 +438,7 @@ 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 00473a650f512b..d43fb589fcf663 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 bb65371ea9df52..590b6cd0eac0e4 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 7cce91b19fb2c5..b71178f0ed6c65 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -2089,8 +2089,7 @@ 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);
@@ -2099,8 +2098,6 @@ 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 31345e0ba0064e..c3d73c04603f41 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -361,8 +361,7 @@ 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 894edf8d6d62f8..a31b057e641e44 100644
--- a/arch/x86/xen/xen-head.S
+++ b/arch/x86/xen/xen-head.S
@@ -33,8 +33,7 @@ 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.
*/
generated by cgit 1.2.3-korg (git 2.43.0) at 2025-11-24 18:08:55 +0000