powerpc/64/syscall: Zero volatile registers when returning

Kernel addresses and potentially other sensitive data could be leaked
in volatile registers after a syscall.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200225173541.1549955-27-npiggin@gmail.com

arch/powerpc/kernel/entry_64.S

@@ -141,6 +141,19 @@ END_FTR_SECTION_IFCLR(CPU_FTR_STCX_CHECKS_ADDRESS)
 
 	cmpdi	r3,0
 	bne	.Lsyscall_restore_regs
+	/* Zero volatile regs that may contain sensitive kernel data */
+	li	r0,0
+	li	r4,0
+	li	r5,0
+	li	r6,0
+	li	r7,0
+	li	r8,0
+	li	r9,0
+	li	r10,0
+	li	r11,0
+	li	r12,0
+	mtctr	r0
+	mtspr	SPRN_XER,r0
 .Lsyscall_restore_regs_cont:
 
 BEGIN_FTR_SECTION