[PATCH v4 02/10] arm64: smccc: clear the Xn registers after SMC calls

Abdellatif El Khlifi
2022-09-26T10:18:49Z

set to zero the x0-x17 registers

As per the SMCCC v1.2 spec, unused result and scratch registers
can leak information after an SMC call. We can mitigate against
this risk by returning zero in each register.

The leakage we are referring to is data leakage across exception
levels. The intent is to prevent lower exception levels (EL1/EL0)
from reading the SMC data exchanged at EL2.

Changelog:
===============

v4:

* move the clearing code into a new macro: clear_gp_regs

v3:

* clear the Xn registers after SMC calls

Signed-off-by: Abdellatif El Khlifi <abdellatif.elkhlifi@arm.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Cc: Jens Wiklander <jens.wiklander@linaro.org>
---
arch/arm/cpu/armv8/smccc-call.S | 9 +++++++++
1 file changed, 9 insertions(+)

diff --git a/arch/arm/cpu/armv8/smccc-call.S b/arch/arm/cpu/armv8/smccc-call.S
index ec6f299bc9..32f3eb8eeb 100644
--- a/arch/arm/cpu/armv8/smccc-call.S
+++ b/arch/arm/cpu/armv8/smccc-call.S
@@ -50,6 +50,12 @@ ENDPROC(__arm_smccc_hvc)

#ifdef CONFIG_ARM64

+ .macro clear_gp_regs
+ .irp n,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17
+ mov x\n, xzr
+ .endr
+ .endm
+
.macro SMCCC_1_2 instr
/* Save `res` and free a GPR that won't be clobbered */
stp x1, x19, [sp, #-16]!
@@ -84,6 +90,9 @@ ENDPROC(__arm_smccc_hvc)
stp x14, x15, [x19, #ARM_SMCCC_1_2_REGS_X14_OFFS]
stp x16, x17, [x19, #ARM_SMCCC_1_2_REGS_X16_OFFS]

+ /* x0-x17 registers can leak information after an SMC or HVC call. Let's clear them */
+ clear_gp_regs
+
/* Restore original x19 */
ldp xzr, x19, [sp], #16
ret
--
2.17.1