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, 1 insertions, 2 deletions

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 5bcdede71ba4..f760a22f95dc 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -209,7 +209,7 @@ config X86_TRAMPOLINE
 
 config X86_32_LAZY_GS
         def_bool y
-        depends on X86_32
+        depends on X86_32 && !CC_STACKPROTECTOR
 
 config KTIME_SCALAR
         def_bool X86_32
@@ -1356,7 +1356,6 @@ config CC_STACKPROTECTOR_ALL
 
 config CC_STACKPROTECTOR
         bool "Enable -fstack-protector buffer overflow detection (EXPERIMENTAL)"
-        depends on X86_64
         select CC_STACKPROTECTOR_ALL
         help
           This option turns on the -fstack-protector GCC feature. This