ARM kprobes: don't let a single-stepped stmdb corrupt the exception stack

If kprobes installs a breakpoint on a "stmdb sp!, {...}" instruction,
and then single-step it by simulation from the exception context, it will
corrupt the saved regs on the stack from the previous context.

To avoid this, let's add an optional parameter to the svc_entry macro
allowing for a hole to be created on the stack before saving the
interrupted context, and use it in the undef_svc handler when kprobes
is enabled.
Signed-off-by: NNicolas Pitre <nico@marvell.com>

arch/arm/kernel/entry-armv.S

@@ -112,8 +112,8 @@ common_invalid:
 #define SPFIX(code...)
 #endif
 
-	.macro	svc_entry
-	sub	sp, sp, #S_FRAME_SIZE
+	.macro	svc_entry, stack_hole=0
+	sub	sp, sp, #(S_FRAME_SIZE + \stack_hole)
  SPFIX(	tst	sp, #4		)
  SPFIX(	bicne	sp, sp, #4	)
 	stmib	sp, {r1 - r12}
@@ -121,7 +121,7 @@ common_invalid:
 	ldmia	r0, {r1 - r3}
 	add	r5, sp, #S_SP		@ here for interlock avoidance
 	mov	r4, #-1			@  ""  ""      ""       ""
-	add	r0, sp, #S_FRAME_SIZE   @  ""  ""      ""       ""
+	add	r0, sp, #(S_FRAME_SIZE + \stack_hole)
  SPFIX(	addne	r0, r0, #4	)
 	str	r1, [sp]		@ save the "real" r0 copied
 					@ from the exception stack
@@ -242,7 +242,14 @@ svc_preempt:
 
 	.align	5
 __und_svc:
+#ifdef CONFIG_KPROBES
+	@ If a kprobe is about to simulate a "stmdb sp..." instruction,
+	@ it obviously needs free stack space which then will belong to
+	@ the saved context.
+	svc_entry 64
+#else
 	svc_entry
+#endif
 
 	@
 	@ call emulation code, which returns using r9 if it has emulated