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>

1 files changed, 21 insertions, 0 deletions

diff --git a/arch/x86/include/asm/system.h b/arch/x86/include/asm/system.h
index 79b98e5b96f4..2692ee8ef031 100644
--- a/arch/x86/include/asm/system.h
+++ b/arch/x86/include/asm/system.h
@@ -23,6 +23,22 @@ struct task_struct *__switch_to(struct task_struct *prev,
 
 #ifdef CONFIG_X86_32
 
+#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"
+#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
+#define __switch_canary_oparam
+#define __switch_canary_iparam
+#endif  /* CC_STACKPROTECTOR */
+
 /*
  * Saving eflags is important. It switches not only IOPL between tasks,
  * it also protects other tasks from NT leaking through sysenter etc.
@@ -46,6 +62,7 @@ do {									\
                      "pushl %[next_ip]\n\t"     /* restore EIP   */     \
                      "jmp __switch_to\n"        /* regparm call  */     \
                      "1:\t"                                             \
+                     __switch_canary                                    \
                      "popl %%ebp\n\t"           /* restore EBP   */     \
                      "popfl\n"                  /* restore flags */     \
                                                                         \
@@ -58,6 +75,8 @@ do {									\
                        "=b" (ebx), "=c" (ecx), "=d" (edx),              \
                        "=S" (esi), "=D" (edi)                           \
                                                                         \
+                       __switch_canary_oparam                           \
+                                                                        \
                        /* input parameters: */                          \
                      : [next_sp]  "m" (next->thread.sp),                \
                        [next_ip]  "m" (next->thread.ip),                \
@@ -66,6 +85,8 @@ do {									\
                        [prev]     "a" (prev),                           \
                        [next]     "d" (next)                            \
                                                                         \
+                       __switch_canary_iparam                           \
+                                                                        \
                      : /* reloaded segment registers */                 \
                         "memory");                                      \
 } while (0)