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pc64: ensure user pc is never set to a non-canonical address through setregisters

on intel processors, a general protection exception is fired if a non-canonical address is loaded into PC during SYSRET. this will cause the kernel to panic.

see http://www.kb.cert.org/vuls/id/649219 and the intel software developer manual for more information.

--- a/sys/src/9/pc64/mem.h

+++ b/sys/src/9/pc64/mem.h

@@ -44,9 +44,10 @@

  *  Address spaces. User:

  */

 #define UTZERO		(0x0000000000200000ull)		/* first address in user text */

+#define UADDRMASK	(0x00007fffffffffffull)		/* canonical address mask */

 #define TSTKTOP		(0x00007ffffffff000ull)

-#define USTKSIZE	(16*MiB)		/* size of user stack */

-#define USTKTOP		(TSTKTOP-USTKSIZE)	/* end of new stack in sysexec */

+#define USTKSIZE	(16*MiB)			/* size of user stack */

+#define USTKTOP		(TSTKTOP-USTKSIZE)		/* end of new stack in sysexec */

 /*

  *  Address spaces. Kernel, sorted by address.

--- a/sys/src/9/pc64/trap.c

+++ b/sys/src/9/pc64/trap.c

@@ -992,6 +992,7 @@

 	if(ureg->gs != UDSEL)

 		ureg->gs = 0;

 	ureg->flags = (ureg->flags & 0x00ff) | (flags & 0xff00);

+	ureg->pc &= UADDRMASK;

 }

 static void