5c79d2a517
Impact: fix x86_32 stack protector Brian Gerst found out that %gs was being initialized to stack_canary instead of stack_canary - 20, which basically gave the same canary value for all threads. Fixing this also exposed the following bugs. * cpu_idle() didn't call boot_init_stack_canary() * stack canary switching in switch_to() was being done too late making the initial run of a new thread use the old stack canary value. Fix all of them and while at it update comment in cpu_idle() about calling boot_init_stack_canary(). Reported-by: Brian Gerst <brgerst@gmail.com> Signed-off-by: Tejun Heo <tj@kernel.org> Signed-off-by: Ingo Molnar <mingo@elte.hu>
125 lines
3.9 KiB
C
125 lines
3.9 KiB
C
/*
|
|
* 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
|
|
|
|
#ifdef CONFIG_CC_STACKPROTECTOR
|
|
|
|
#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,
|
|
* and it must always be inlined.
|
|
*/
|
|
static __always_inline void boot_init_stack_canary(void)
|
|
{
|
|
u64 canary;
|
|
u64 tsc;
|
|
|
|
#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,
|
|
* there it already has some randomness on most systems. Later
|
|
* on during the bootup the random pool has true entropy too.
|
|
*/
|
|
get_random_bytes(&canary, sizeof(canary));
|
|
tsc = __native_read_tsc();
|
|
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) - 20;
|
|
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 */
|
|
#endif /* _ASM_STACKPROTECTOR_H */
|