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author | Tejun Heo <tj@kernel.org> | 2009-02-09 22:17:40 +0900 |
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committer | Ingo Molnar <mingo@elte.hu> | 2009-02-10 00:42:01 +0100 |
commit | 60a5317ff0f42dd313094b88f809f63041568b08 (patch) | |
tree | 307dfd9715fbc9ff83c3c3ae3b0e8f03888083f2 /arch/x86/include/asm/stackprotector.h | |
parent | ccbeed3a05908d201b47b6c3dd1a373138bba566 (diff) | |
download | op-kernel-dev-60a5317ff0f42dd313094b88f809f63041568b08.zip op-kernel-dev-60a5317ff0f42dd313094b88f809f63041568b08.tar.gz |
x86: implement x86_32 stack protector
Impact: stack protector for x86_32
Implement stack protector for x86_32. GDT entry 28 is used for it.
It's set to point to stack_canary-20 and have the length of 24 bytes.
CONFIG_CC_STACKPROTECTOR turns off CONFIG_X86_32_LAZY_GS and sets %gs
to the stack canary segment on entry. As %gs is otherwise unused by
the kernel, the canary can be anywhere. It's defined as a percpu
variable.
x86_32 exception handlers take register frame on stack directly as
struct pt_regs. With -fstack-protector turned on, gcc copies the
whole structure after the stack canary and (of course) doesn't copy
back on return thus losing all changed. For now, -fno-stack-protector
is added to all files which contain those functions. We definitely
need something better.
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Diffstat (limited to 'arch/x86/include/asm/stackprotector.h')
-rw-r--r-- | arch/x86/include/asm/stackprotector.h | 91 |
1 files changed, 86 insertions, 5 deletions
diff --git a/arch/x86/include/asm/stackprotector.h b/arch/x86/include/asm/stackprotector.h index ee275e9..fa7e5bd 100644 --- a/arch/x86/include/asm/stackprotector.h +++ b/arch/x86/include/asm/stackprotector.h @@ -1,3 +1,35 @@ +/* + * GCC stack protector support. + * + * Stack protector works by putting 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 requires it to be at a fixed offset from %gs. + * On x86_64, the offset is 40 bytes and on x86_32 20 bytes. x86_64 + * and x86_32 use segment registers differently and thus handles this + * requirement differently. + * + * On x86_64, %gs is shared by percpu area and stack canary. All + * percpu symbols are zero based and %gs points to the base of percpu + * area. The first occupant of the percpu area is always + * irq_stack_union which contains stack_canary at offset 40. Userland + * %gs is always saved and restored on kernel entry and exit using + * swapgs, so stack protector doesn't add any complexity there. + * + * On x86_32, it's slightly more complicated. As in x86_64, %gs is + * used for userland TLS. Unfortunately, some processors are much + * slower at loading segment registers with different value when + * entering and leaving the kernel, so the kernel uses %fs for percpu + * area and manages %gs lazily so that %gs is switched only when + * necessary, usually during task switch. + * + * As gcc requires the stack canary at %gs:20, %gs can't be managed + * lazily if stack protector is enabled, so the kernel saves and + * restores userland %gs on kernel entry and exit. This behavior is + * controlled by CONFIG_X86_32_LAZY_GS and accessors are defined in + * system.h to hide the details. + */ + #ifndef _ASM_STACKPROTECTOR_H #define _ASM_STACKPROTECTOR_H 1 @@ -6,9 +38,19 @@ #include <asm/tsc.h> #include <asm/processor.h> #include <asm/percpu.h> +#include <asm/system.h> +#include <asm/desc.h> #include <linux/random.h> /* + * 24 byte read-only segment initializer for stack canary. Linker + * can't handle the address bit shifting. Address will be set in + * head_32 for boot CPU and setup_per_cpu_areas() for others. + */ +#define GDT_STACK_CANARY_INIT \ + [GDT_ENTRY_STACK_CANARY] = { { { 0x00000018, 0x00409000 } } }, + +/* * Initialize the stackprotector canary value. * * NOTE: this must only be called from functions that never return, @@ -19,12 +61,9 @@ static __always_inline void boot_init_stack_canary(void) u64 canary; u64 tsc; - /* - * Build time only check to make sure the stack_canary is at - * offset 40 in the pda; this is a gcc ABI requirement - */ +#ifdef CONFIG_X86_64 BUILD_BUG_ON(offsetof(union irq_stack_union, stack_canary) != 40); - +#endif /* * We both use the random pool and the current TSC as a source * of randomness. The TSC only matters for very early init, @@ -36,7 +75,49 @@ static __always_inline void boot_init_stack_canary(void) canary += tsc + (tsc << 32UL); current->stack_canary = canary; +#ifdef CONFIG_X86_64 percpu_write(irq_stack_union.stack_canary, canary); +#else + percpu_write(stack_canary, canary); +#endif +} + +static inline void setup_stack_canary_segment(int cpu) +{ +#ifdef CONFIG_X86_32 + unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu); + struct desc_struct *gdt_table = get_cpu_gdt_table(cpu); + struct desc_struct desc; + + desc = gdt_table[GDT_ENTRY_STACK_CANARY]; + desc.base0 = canary & 0xffff; + desc.base1 = (canary >> 16) & 0xff; + desc.base2 = (canary >> 24) & 0xff; + write_gdt_entry(gdt_table, GDT_ENTRY_STACK_CANARY, &desc, DESCTYPE_S); +#endif +} + +static inline void load_stack_canary_segment(void) +{ +#ifdef CONFIG_X86_32 + asm("mov %0, %%gs" : : "r" (__KERNEL_STACK_CANARY) : "memory"); +#endif +} + +#else /* CC_STACKPROTECTOR */ + +#define GDT_STACK_CANARY_INIT + +/* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */ + +static inline void setup_stack_canary_segment(int cpu) +{ } + +static inline void load_stack_canary_segment(void) +{ +#ifdef CONFIG_X86_32 + asm volatile ("mov %0, %%gs" : : "r" (0)); +#endif } #endif /* CC_STACKPROTECTOR */ |