x86/i386: Make sure stack-protector segment base is cache aligned

The Intel Optimization Reference Guide says:

	In Intel Atom microarchitecture, the address generation unit
	assumes that the segment base will be 0 by default. Non-zero
	segment base will cause load and store operations to experience
	a delay.
		- If the segment base isn't aligned to a cache line
		  boundary, the max throughput of memory operations is
		  reduced to one [e]very 9 cycles.
	[...]
	Assembly/Compiler Coding Rule 15. (H impact, ML generality)
	For Intel Atom processors, use segments with base set to 0
	whenever possible; avoid non-zero segment base address that is
	not aligned to cache line boundary at all cost.

We can't avoid having a non-zero base for the stack-protector
segment, but we can make it cache-aligned.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: <stable@kernel.org>
LKML-Reference: <4AA01893.6000507@goop.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/include/asm/processor.h

@@ -403,7 +403,17 @@ extern unsigned long kernel_eflags;
 extern asmlinkage void ignore_sysret(void);
 #else	/* X86_64 */
 #ifdef CONFIG_CC_STACKPROTECTOR
-DECLARE_PER_CPU(unsigned long, stack_canary);
+/*
+ * Make sure stack canary segment base is cached-aligned:
+ *   "For Intel Atom processors, avoid non zero segment base address
+ *    that is not aligned to cache line boundary at all cost."
+ * (Optim Ref Manual Assembly/Compiler Coding Rule 15.)
+ */
+struct stack_canary {
+	char __pad[20];		/* canary at %gs:20 */
+	unsigned long canary;
+};
+DECLARE_PER_CPU(struct stack_canary, stack_canary) ____cacheline_aligned;
 #endif
 #endif	/* X86_64 */
 

arch/x86/include/asm/stackprotector.h

@@ -78,14 +78,14 @@ static __always_inline void boot_init_stack_canary(void)
 #ifdef CONFIG_X86_64
 	percpu_write(irq_stack_union.stack_canary, canary);
 #else
-	percpu_write(stack_canary, canary);
+	percpu_write(stack_canary.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) - 20;
+	unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu);
 	struct desc_struct *gdt_table = get_cpu_gdt_table(cpu);
 	struct desc_struct desc;
 

arch/x86/include/asm/system.h

@@ -31,7 +31,7 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
 	"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))
+	, [stack_canary] "=m" (per_cpu_var(stack_canary.canary))
 #define __switch_canary_iparam						\
 	, [task_canary] "i" (offsetof(struct task_struct, stack_canary))
 #else	/* CC_STACKPROTECTOR */

arch/x86/kernel/cpu/common.c

@@ -1043,7 +1043,7 @@ DEFINE_PER_CPU(struct orig_ist, orig_ist);
 #else	/* CONFIG_X86_64 */
 
 #ifdef CONFIG_CC_STACKPROTECTOR
-DEFINE_PER_CPU(unsigned long, stack_canary);
+DEFINE_PER_CPU(struct stack_canary, stack_canary) ____cacheline_aligned;
 #endif
 
 /* Make sure %fs and %gs are initialized properly in idle threads */

arch/x86/kernel/head_32.S

@@ -439,7 +439,6 @@ 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)