ARM: entry: rework stack realignment code in svc_entry

The original Thumb-2 enablement patches updated the stack realignment
code in svc_entry to work around the lack of a STMIB instruction in
Thumb-2, by subtracting 4 from the frame size, inverting the sense of
the misaligment check, and changing to a STMIA instruction and a final
stack push of a 4 byte quantity that results in the stack becoming
aligned at the end of the sequence. It also pushes and pops R0 to the
stack in order to have a temp register that Thumb-2 allows in general
purpose ALU instructions, as TST using SP is not permitted.

Both are a bit problematic for vmap'ed stacks, as using the stack is
only permitted after we decide that we did not overflow the stack, or
have already switched to the overflow stack.

As for the alignment check: the current approach creates a corner case
where, if the initial SUB of SP ends up right at the start of the stack,
we will end up subtracting another 8 bytes and overflowing it.  This
means we would need to add the overflow check *after* the SUB that
deliberately misaligns the stack. However, this would require us to keep
local state (i.e., whether we performed the subtract or not) across the
overflow check, but without any GPRs or stack available.

So let's switch to an approach where we don't use the stack, and where
the alignment check of the stack pointer occurs in the usual way, as
this is guaranteed not to result in overflow. This means we will be able
to do the overflow check first.

While at it, switch to R1 so the mode stack pointer in R0 remains
accessible.
Acked-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Tested-by: Marc Zyngier <maz@kernel.org>
Tested-by: Vladimir Murzin <vladimir.murzin@arm.com> # ARMv7M

arch/arm/kernel/entry-armv.S

@@ -191,24 +191,27 @@ ENDPROC(__und_invalid)
 	.macro	svc_entry, stack_hole=0, trace=1, uaccess=1
  UNWIND(.fnstart		)
  UNWIND(.save {r0 - pc}		)
-	sub	sp, sp, #(SVC_REGS_SIZE + \stack_hole - 4)
+	sub	sp, sp, #(SVC_REGS_SIZE + \stack_hole)
 #ifdef CONFIG_THUMB2_KERNEL
- SPFIX(	str	r0, [sp]	)	@ temporarily saved
- SPFIX(	mov	r0, sp		)
- SPFIX(	tst	r0, #4		)	@ test original stack alignment
- SPFIX(	ldr	r0, [sp]	)	@ restored
+	add	sp, r1			@ get SP in a GPR without
+	sub	r1, sp, r1		@ using a temp register
+	tst	r1, #4			@ test stack pointer alignment
+	sub	r1, sp, r1		@ restore original R1
+	sub	sp, r1			@ restore original SP
 #else
  SPFIX(	tst	sp, #4		)
 #endif
- SPFIX(	subeq	sp, sp, #4	)
-	stmia	sp, {r1 - r12}
+ SPFIX(	subne	sp, sp, #4	)
+
+ ARM(	stmib	sp, {r1 - r12}	)
+ THUMB(	stmia	sp, {r0 - r12}	)	@ No STMIB in Thumb-2
 
 	ldmia	r0, {r3 - r5}
-	add	r7, sp, #S_SP - 4	@ here for interlock avoidance
+	add	r7, sp, #S_SP		@ here for interlock avoidance
 	mov	r6, #-1			@  ""  ""      ""       ""
-	add	r2, sp, #(SVC_REGS_SIZE + \stack_hole - 4)
- SPFIX(	addeq	r2, r2, #4	)
-	str	r3, [sp, #-4]!		@ save the "real" r0 copied
+	add	r2, sp, #(SVC_REGS_SIZE + \stack_hole)
+ SPFIX(	addne	r2, r2, #4	)
+	str	r3, [sp]		@ save the "real" r0 copied
 					@ from the exception stack
 
 	mov	r3, lr