ARM: spectre-v1: fix syscall entry

Commit 10573ae5 upstream.

Prevent speculation at the syscall table decoding by clamping the index
used to zero on invalid system call numbers, and using the csdb
speculative barrier.
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Boot-tested-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: David A. Long <dave.long@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/arm/kernel/entry-common.S

@@ -241,9 +241,7 @@ local_restart:
 	tst	r10, #_TIF_SYSCALL_WORK		@ are we tracing syscalls?
 	bne	__sys_trace
 
-	cmp	scno, #NR_syscalls		@ check upper syscall limit
-	badr	lr, ret_fast_syscall		@ return address
-	ldrcc	pc, [tbl, scno, lsl #2]		@ call sys_* routine
+	invoke_syscall tbl, scno, r10, ret_fast_syscall
 
 	add	r1, sp, #S_OFF
 2:	cmp	scno, #(__ARM_NR_BASE - __NR_SYSCALL_BASE)
@@ -277,14 +275,8 @@ __sys_trace:
 	mov	r1, scno
 	add	r0, sp, #S_OFF
 	bl	syscall_trace_enter
-
-	badr	lr, __sys_trace_return		@ return address
-	mov	scno, r0			@ syscall number (possibly new)
-	add	r1, sp, #S_R0 + S_OFF		@ pointer to regs
-	cmp	scno, #NR_syscalls		@ check upper syscall limit
-	ldmccia	r1, {r0 - r6}			@ have to reload r0 - r6
-	stmccia	sp, {r4, r5}			@ and update the stack args
-	ldrcc	pc, [tbl, scno, lsl #2]		@ call sys_* routine
+	mov	scno, r0
+	invoke_syscall tbl, scno, r10, __sys_trace_return, reload=1
 	cmp	scno, #-1			@ skip the syscall?
 	bne	2b
 	add	sp, sp, #S_OFF			@ restore stack
@@ -362,6 +354,10 @@ sys_syscall:
 		bic	scno, r0, #__NR_OABI_SYSCALL_BASE
 		cmp	scno, #__NR_syscall - __NR_SYSCALL_BASE
 		cmpne	scno, #NR_syscalls	@ check range
+#ifdef CONFIG_CPU_SPECTRE
+		movhs	scno, #0
+		csdb
+#endif
 		stmloia	sp, {r5, r6}		@ shuffle args
 		movlo	r0, r1
 		movlo	r1, r2

arch/arm/kernel/entry-header.S

@@ -378,6 +378,31 @@
 #endif
 	.endm
 
+	.macro	invoke_syscall, table, nr, tmp, ret, reload=0
+#ifdef CONFIG_CPU_SPECTRE
+	mov	\tmp, \nr
+	cmp	\tmp, #NR_syscalls		@ check upper syscall limit
+	movcs	\tmp, #0
+	csdb
+	badr	lr, \ret			@ return address
+	.if	\reload
+	add	r1, sp, #S_R0 + S_OFF		@ pointer to regs
+	ldmccia	r1, {r0 - r6}			@ reload r0-r6
+	stmccia	sp, {r4, r5}			@ update stack arguments
+	.endif
+	ldrcc	pc, [\table, \tmp, lsl #2]	@ call sys_* routine
+#else
+	cmp	\nr, #NR_syscalls		@ check upper syscall limit
+	badr	lr, \ret			@ return address
+	.if	\reload
+	add	r1, sp, #S_R0 + S_OFF		@ pointer to regs
+	ldmccia	r1, {r0 - r6}			@ reload r0-r6
+	stmccia	sp, {r4, r5}			@ update stack arguments
+	.endif
+	ldrcc	pc, [\table, \nr, lsl #2]	@ call sys_* routine
+#endif
+	.endm
+
 /*
  * These are the registers used in the syscall handler, and allow us to
  * have in theory up to 7 arguments to a function - r0 to r6.