x86: fix stackprotector canary updates during context switches

fix a bug noticed and fixed by pageexec@freemail.hu.

if built with -fstack-protector-all then we'll have canary checks built
into the __switch_to() function. That does not work well with the
canary-switching code there: while we already use the %rsp of the
new task, we still call __switch_to() whith the previous task's canary
value in the PDA, hence the __switch_to() ssp prologue instructions
will store the previous canary. Then we update the PDA and upon return
from __switch_to() the canary check triggers and we panic.

so update the canary after we have called __switch_to(), where we are
at the same stackframe level as the last stackframe of the next
(and now freshly current) task.

Note: this means that we call __switch_to() [and its sub-functions]
still with the old canary, but that is not a problem, both the previous
and the next task has a high-quality canary. The only (mostly academic)
disadvantage is that the canary of one task may leak onto the stack of
another task, increasing the risk of information leaks, were an attacker
able to read the stack of specific tasks (but not that of others).

To solve this we'll have to reorganize the way we switch tasks, and move
the PDA setting into the switch_to() assembly code. That will happen in
another patch.
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/process_64.c

@@ -640,7 +640,6 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 	write_pda(kernelstack,
 	(unsigned long)task_stack_page(next_p) + THREAD_SIZE - PDA_STACKOFFSET);
 #ifdef CONFIG_CC_STACKPROTECTOR
-	write_pda(stack_canary, next_p->stack_canary);
 	/*
 	 * Build time only check to make sure the stack_canary is at
 	 * offset 40 in the pda; this is a gcc ABI requirement

include/asm-x86/pda.h

@@ -16,11 +16,9 @@ struct x8664_pda {
 	unsigned long oldrsp;		/* 24 user rsp for system call */
 	int irqcount;			/* 32 Irq nesting counter. Starts -1 */
 	unsigned int cpunumber;		/* 36 Logical CPU number */
-#ifdef CONFIG_CC_STACKPROTECTOR
 	unsigned long stack_canary;	/* 40 stack canary value */
 					/* gcc-ABI: this canary MUST be at
 					   offset 40!!! */
-#endif
 	char *irqstackptr;
 	unsigned int __softirq_pending;
 	unsigned int __nmi_count;	/* number of NMI on this CPUs */

include/asm-x86/system.h

@@ -92,6 +92,8 @@ do {									\
 	     ".globl thread_return\n"					  \
 	     "thread_return:\n\t"					  \
 	     "movq %%gs:%P[pda_pcurrent],%%rsi\n\t"			  \
+	     "movq %P[task_canary](%%rsi),%%r8\n\t"			  \
+	     "movq %%r8,%%gs:%P[pda_canary]\n\t"			  \
 	     "movq %P[thread_info](%%rsi),%%r8\n\t"			  \
 	     LOCK_PREFIX "btr  %[tif_fork],%P[ti_flags](%%r8)\n\t"	  \
 	     "movq %%rax,%%rdi\n\t" 					  \
@@ -103,7 +105,9 @@ do {									\
 	       [ti_flags] "i" (offsetof(struct thread_info, flags)),	  \
 	       [tif_fork] "i" (TIF_FORK),			  	  \
 	       [thread_info] "i" (offsetof(struct task_struct, stack)),   \
-	       [pda_pcurrent] "i" (offsetof(struct x8664_pda, pcurrent))  \
+	       [task_canary] "i" (offsetof(struct task_struct, stack_canary)),\
+	       [pda_pcurrent] "i" (offsetof(struct x8664_pda, pcurrent)), \
+	       [pda_canary] "i" (offsetof(struct x8664_pda, stack_canary))\
 	     : "memory", "cc" __EXTRA_CLOBBER)
 #endif
 

include/linux/sched.h

@@ -1096,10 +1096,9 @@ struct task_struct {
 	pid_t pid;
 	pid_t tgid;
 
-#ifdef CONFIG_CC_STACKPROTECTOR
 	/* Canary value for the -fstack-protector gcc feature */
 	unsigned long stack_canary;
-#endif
+
 	/* 
 	 * pointers to (original) parent process, youngest child, younger sibling,
 	 * older sibling, respectively.  (p->father can be replaced with