[PATCH 11/11] x86: implement x86_32 stack protector

Tejun Heo
2009-02-09T13:43:23Z

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>
---
arch/x86/Kconfig | 3 +-
arch/x86/include/asm/processor.h | 4 +
arch/x86/include/asm/segment.h | 9 +++-
arch/x86/include/asm/stackprotector.h | 91 +++++++++++++++++++++++++++--
arch/x86/include/asm/system.h | 21 +++++++
arch/x86/kernel/Makefile | 18 ++++++
arch/x86/kernel/cpu/common.c | 17 ++++--
arch/x86/kernel/entry_32.S | 2 +-
arch/x86/kernel/head_32.S | 20 ++++++-
arch/x86/kernel/process_32.c | 1 +
arch/x86/kernel/setup_percpu.c | 2 +
scripts/gcc-x86_32-has-stack-protector.sh | 8 +++
12 files changed, 180 insertions(+), 16 deletions(-)
create mode 100644 scripts/gcc-x86_32-has-stack-protector.sh

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index e91093a..31c555f 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -198,7 +198,7 @@ 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
@@ -1342,7 +1342,6 @@ 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
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 80b8be2..a013383 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -396,7 +396,11 @@ DECLARE_PER_CPU(union irq_stack_union, irq_stack_union);
DECLARE_INIT_PER_CPU(irq_stack_union);

DECLARE_PER_CPU(char *, irq_stack_ptr);
+#else /* X86_64 */
+#ifdef CONFIG_CC_STACKPROTECTOR
+DECLARE_PER_CPU(unsigned long, stack_canary);
#endif
+#endif /* X86_64 */

extern void print_cpu_info(struct cpuinfo_x86 *);
extern unsigned int xstate_size;
diff --git a/arch/x86/include/asm/segment.h b/arch/x86/include/asm/segment.h
index 1dc1b51..14e0ed8 100644
--- a/arch/x86/include/asm/segment.h
+++ b/arch/x86/include/asm/segment.h
@@ -61,7 +61,7 @@
*
* 26 - ESPFIX small SS
* 27 - per-cpu [ offset to per-cpu data area ]
- * 28 - unused
+ * 28 - stack_canary-20 [ for stack protector ]
* 29 - unused
* 30 - unused
* 31 - TSS for double fault handler
@@ -95,6 +95,13 @@
#define __KERNEL_PERCPU 0
#endif

+#define GDT_ENTRY_STACK_CANARY (GDT_ENTRY_KERNEL_BASE + 16)
+#ifdef CONFIG_CC_STACKPROTECTOR
+#define __KERNEL_STACK_CANARY (GDT_ENTRY_STACK_CANARY * 8)
+#else
+#define __KERNEL_STACK_CANARY 0
+#endif
+
#define GDT_ENTRY_DOUBLEFAULT_TSS 31

/*
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 */
diff --git a/arch/x86/include/asm/system.h b/arch/x86/include/asm/system.h
index e658bbe..4ded307 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)
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index a11e5ac..0da4e4d 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -24,6 +24,24 @@ CFLAGS_vsyscall_64.o := $(PROFILING) -g0 $(nostackp)
CFLAGS_hpet.o := $(nostackp)
CFLAGS_tsc.o := $(nostackp)
CFLAGS_paravirt.o := $(nostackp)
+#
+# On x86_32, register frame is passed verbatim on stack as struct
+# pt_regs. gcc considers the parameter to belong to the callee and
+# with -fstack-protector it copies pt_regs to the callee's stack frame
+# to put the structure after the stack canary causing changes made by
+# the exception handlers to be lost. Turn off stack protector for all
+# files containing functions which take struct pt_regs from register
+# frame.
+#
+# The proper way to fix this is to teach gcc that the argument belongs
+# to the caller for these functions, oh well...
+#
+ifdef CONFIG_X86_32
+CFLAGS_process_32.o := $(nostackp)
+CFLAGS_vm86_32.o := $(nostackp)
+CFLAGS_signal.o := $(nostackp)
+CFLAGS_traps.o := $(nostackp)
+endif

obj-y := process_$(BITS).o signal.o entry_$(BITS).o
obj-y += traps.o irq.o irq_$(BITS).o dumpstack_$(BITS).o
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 2a3c9a5..3a9d45a 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -40,6 +40,7 @@
#include <asm/sections.h>
#include <asm/setup.h>
#include <asm/hypervisor.h>
+#include <asm/stackprotector.h>

#include "cpu.h"

@@ -123,6 +124,7 @@ DEFINE_PER_CPU_PAGE_ALIGNED(struct gdt_page, gdt_page) = { .gdt = {

[GDT_ENTRY_ESPFIX_SS] = { { { 0x00000000, 0x00c09200 } } },
[GDT_ENTRY_PERCPU] = { { { 0x0000ffff, 0x00cf9200 } } },
+ GDT_STACK_CANARY_INIT
#endif
} };
EXPORT_PER_CPU_SYMBOL_GPL(gdt_page);
@@ -305,6 +307,7 @@ void load_percpu_segment(int cpu)
loadsegment(gs, 0);
wrmsrl(MSR_GS_BASE, (unsigned long)per_cpu(irq_stack_union.gs_base, cpu));
#endif
+ load_stack_canary_segment();
}

/* Current gdt points %fs at the "master" per-cpu area: after this,
@@ -984,16 +987,21 @@ unsigned long kernel_eflags;
*/
DEFINE_PER_CPU(struct orig_ist, orig_ist);

-#else
+#else /* x86_64 */
+
+#ifdef CONFIG_CC_STACKPROTECTOR
+DEFINE_PER_CPU(unsigned long, stack_canary);
+#endif

-/* Make sure %fs is initialized properly in idle threads */
+/* Make sure %fs and %gs are initialized properly in idle threads */
struct pt_regs * __cpuinit idle_regs(struct pt_regs *regs)
{
memset(regs, 0, sizeof(struct pt_regs));
regs->fs = __KERNEL_PERCPU;
+ regs->gs = __KERNEL_STACK_CANARY;
return regs;
}
-#endif
+#endif /* x86_64 */

/*
* cpu_init() initializes state that is per-CPU. Some data is already
@@ -1155,9 +1163,6 @@ void __cpuinit cpu_init(void)
__set_tss_desc(cpu, GDT_ENTRY_DOUBLEFAULT_TSS, &doublefault_tss);
#endif

- /* Clear %gs. */
- asm volatile ("mov %0, %%gs" : : "r" (0));
-
/* Clear all 6 debug registers: */
set_debugreg(0, 0);
set_debugreg(0, 1);
diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
index d3be889..f7d0837 100644
--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -186,7 +186,7 @@
/*CFI_REL_OFFSET gs, PT_GS*/
.endm
.macro SET_KERNEL_GS reg
- xorl \reg, \reg
+ movl $(__KERNEL_STACK_CANARY), \reg
movl \reg, %gs
.endm

diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index 0cd61bb..410c9bb 100644
--- a/arch/x86/kernel/head_32.S
+++ b/arch/x86/kernel/head_32.S
@@ -19,6 +19,7 @@
#include <asm/asm-offsets.h>
#include <asm/setup.h>
#include <asm/processor-flags.h>
+#include <asm/percpu.h>

/* Physical address */
#define pa(X) ((X) - __PAGE_OFFSET)
@@ -437,8 +438,25 @@ is386: movl $2,%ecx # set MP
movl $(__KERNEL_PERCPU), %eax
movl %eax,%fs # set this cpu's percpu

- xorl %eax,%eax # Clear GS and LDT
+#ifdef CONFIG_CC_STACKPROTECTOR
+ /*
+ * The linker can't handle this by relocation. Manually set
+ * base address in stack canary segment descriptor.
+ */
+ cmpb $0,ready
+ jne 1f
+ movl $per_cpu__gdt_page,%eax
+ movl $per_cpu__stack_canary,%ecx
+ movw %cx, 8 * GDT_ENTRY_STACK_CANARY + 2(%eax)
+ shrl $16, %ecx
+ movb %cl, 8 * GDT_ENTRY_STACK_CANARY + 4(%eax)
+ movb %ch, 8 * GDT_ENTRY_STACK_CANARY + 7(%eax)
+1:
+#endif
+ movl $(__KERNEL_STACK_CANARY),%eax
movl %eax,%gs
+
+ xorl %eax,%eax # Clear LDT
lldt %ax

cld # gcc2 wants the direction flag cleared at all times
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 461d0a7..13d224b 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -210,6 +210,7 @@ int kernel_thread(int (*fn)(void *), void *arg, unsigned long flags)
regs.ds = __USER_DS;
regs.es = __USER_DS;
regs.fs = __KERNEL_PERCPU;
+ regs.gs = __KERNEL_STACK_CANARY;
regs.orig_ax = -1;
regs.ip = (unsigned long) kernel_thread_helper;
regs.cs = __KERNEL_CS | get_kernel_rpl();
diff --git a/arch/x86/kernel/setup_percpu.c b/arch/x86/kernel/setup_percpu.c
index ef91747..d992e6c 100644
--- a/arch/x86/kernel/setup_percpu.c
+++ b/arch/x86/kernel/setup_percpu.c
@@ -16,6 +16,7 @@
#include <asm/proto.h>
#include <asm/cpumask.h>
#include <asm/cpu.h>
+#include <asm/stackprotector.h>

#ifdef CONFIG_DEBUG_PER_CPU_MAPS
# define DBG(x...) printk(KERN_DEBUG x)
@@ -95,6 +96,7 @@ void __init setup_per_cpu_areas(void)
per_cpu(this_cpu_off, cpu) = per_cpu_offset(cpu);
per_cpu(cpu_number, cpu) = cpu;
setup_percpu_segment(cpu);
+ setup_stack_canary_segment(cpu);
/*
* Copy data used in early init routines from the
* initial arrays to the per cpu data areas. These
diff --git a/scripts/gcc-x86_32-has-stack-protector.sh b/scripts/gcc-x86_32-has-stack-protector.sh
new file mode 100644
index 0000000..4fdf6ce
--- /dev/null
+++ b/scripts/gcc-x86_32-has-stack-protector.sh
@@ -0,0 +1,8 @@
+#!/bin/sh
+
+echo "int foo(void) { char X[200]; return 3; }" | $1 -S -xc -c -O0 -fstack-protector - -o - 2> /dev/null | grep -q "%gs"
+if [ "$?" -eq "0" ] ; then
+ echo y
+else
+ echo n
+fi
--
1.6.0.2