s390: add no-execute support

Bit 0x100 of a page table, segment table of region table entry
can be used to disallow code execution for the virtual addresses
associated with the entry.

There is one tricky bit, the system call to return from a signal
is part of the signal frame written to the user stack. With a
non-executable stack this would stop working. To avoid breaking
things the protection fault handler checks the opcode that caused
the fault for 0x0a77 (sys_sigreturn) and 0x0aad (sys_rt_sigreturn)
and injects a system call. This is preferable to the alternative
solution with a stub function in the vdso because it works for
vdso=off and statically linked binaries as well.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/entry.S

@@ -380,13 +380,11 @@ ENTRY(system_call)
 	brasl	%r14,do_signal
 	TSTMSK	__PT_FLAGS(%r11),_PIF_SYSCALL
 	jno	.Lsysc_return
+.Lsysc_do_syscall:
+	lghi	%r13,__TASK_thread
 	lmg	%r2,%r7,__PT_R2(%r11)	# load svc arguments
-	lghi	%r8,0			# svc 0 returns -ENOSYS
-	llgh	%r1,__PT_INT_CODE+2(%r11)	# load new svc number
-	cghi	%r1,NR_syscalls
-	jnl	.Lsysc_nr_ok		# invalid svc number -> do svc 0
-	slag	%r8,%r1,2
-	j	.Lsysc_nr_ok		# restart svc
+	lghi	%r1,0			# svc 0 returns -ENOSYS
+	j	.Lsysc_do_svc
 
 #
 # _TIF_NOTIFY_RESUME is set, call do_notify_resume
@@ -528,6 +526,8 @@ ENTRY(pgm_check_handler)
 	LOCKDEP_SYS_EXIT
 	tm	__PT_PSW+1(%r11),0x01	# returning to user ?
 	jno	.Lsysc_restore
+	TSTMSK	__PT_FLAGS(%r11),_PIF_SYSCALL
+	jo	.Lsysc_do_syscall
 	j	.Lsysc_tif
 
 #