[PATCH x86#core/percpu] x86: fix x86_32 stack protector bugs

Tejun Heo
2009-02-11T07:31:52Z

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().

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Brian Gerst <brgerst@gmail.com>
---
arch/x86/include/asm/stackprotector.h | 2 +-
arch/x86/include/asm/system.h | 8 +++-----
arch/x86/kernel/head_32.S | 1 +
arch/x86/kernel/process_32.c | 10 ++++++++++
arch/x86/kernel/process_64.c | 11 +++++------
5 files changed, 20 insertions(+), 12 deletions(-)

diff --git a/arch/x86/include/asm/stackprotector.h b/arch/x86/include/asm/stackprotector.h
index fa7e5bd..c2d742c 100644
--- a/arch/x86/include/asm/stackprotector.h
+++ b/arch/x86/include/asm/stackprotector.h
@@ -85,7 +85,7 @@ static __always_inline void boot_init_stack_canary(void)
static inline void setup_stack_canary_segment(int cpu)
{
#ifdef CONFIG_X86_32
- unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu);
+ 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;

diff --git a/arch/x86/include/asm/system.h b/arch/x86/include/asm/system.h
index 2692ee8..7a80f72 100644
--- a/arch/x86/include/asm/system.h
+++ b/arch/x86/include/asm/system.h
@@ -25,13 +25,11 @@ struct task_struct *__switch_to(struct task_struct *prev,

#ifdef CONFIG_CC_STACKPROTECTOR
#define __switch_canary \
- "movl "__percpu_arg([current_task])",%%ebx\n\t" \
- "movl %P[task_canary](%%ebx),%%ebx\n\t" \
- "movl %%ebx,"__percpu_arg([stack_canary])"\n\t"
+ "movl %P[task_canary](%[next]), %%ebx\n\t" \
+ "movl %%ebx, "__percpu_arg([stack_canary])"\n\t"
#define __switch_canary_oparam \
, [stack_canary] "=m" (per_cpu_var(stack_canary))
#define __switch_canary_iparam \
- , [current_task] "m" (per_cpu_var(current_task)) \
, [task_canary] "i" (offsetof(struct task_struct, stack_canary))
#else /* CC_STACKPROTECTOR */
#define __switch_canary
@@ -60,9 +58,9 @@ do { \
"movl %[next_sp],%%esp\n\t" /* restore ESP */ \
"movl $1f,%[prev_ip]\n\t" /* save EIP */ \
"pushl %[next_ip]\n\t" /* restore EIP */ \
+ __switch_canary \
"jmp __switch_to\n" /* regparm call */ \
"1:\t" \
- __switch_canary \
"popl %%ebp\n\t" /* restore EBP */ \
"popfl\n" /* restore flags */ \
\
diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index 924e316..cf21fd0 100644
--- a/arch/x86/kernel/head_32.S
+++ b/arch/x86/kernel/head_32.S
@@ -447,6 +447,7 @@ is386: movl $2,%ecx # set MP
jne 1f
movl $per_cpu__gdt_page,%eax
movl $per_cpu__stack_canary,%ecx
+ subl $20, %ecx
movw %cx, 8 * GDT_ENTRY_STACK_CANARY + 2(%eax)
shrl $16, %ecx
movb %cl, 8 * GDT_ENTRY_STACK_CANARY + 4(%eax)
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 9a62383..b50604b 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -11,6 +11,7 @@

#include <stdarg.h>

+#include <linux/stackprotector.h>
#include <linux/cpu.h>
#include <linux/errno.h>
#include <linux/sched.h>
@@ -91,6 +92,15 @@ void cpu_idle(void)
{
int cpu = smp_processor_id();

+ /*
+ * If we're the non-boot CPU, nothing set the stack canary up
+ * for us. CPU0 already has it initialized but no harm in
+ * doing it again. This is a good place for updating it, as
+ * we wont ever return from this function (so the invalid
+ * canaries already on the stack wont ever trigger).
+ */
+ boot_init_stack_canary();
+
current_thread_info()->status |= TS_POLLING;

/* endless idle loop with no priority at all */
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 8eb169e..836ef65 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -120,12 +120,11 @@ void cpu_idle(void)
current_thread_info()->status |= TS_POLLING;

/*
- * If we're the non-boot CPU, nothing set the PDA stack
- * canary up for us - and if we are the boot CPU we have
- * a 0 stack canary. This is a good place for updating
- * it, as we wont ever return from this function (so the
- * invalid canaries already on the stack wont ever
- * trigger):
+ * If we're the non-boot CPU, nothing set the stack canary up
+ * for us. CPU0 already has it initialized but no harm in
+ * doing it again. This is a good place for updating it, as
+ * we wont ever return from this function (so the invalid
+ * canaries already on the stack wont ever trigger).
*/
boot_init_stack_canary();

--
1.6.0.2