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>
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 @@
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 @@
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 */