Prevent speculative execution past ERET

Even though ERET always causes a jump to another address, aarch64 CPUs
speculatively execute following instructions as if the ERET
instruction was not a jump instruction.
The speculative execution does not cross privilege-levels (to the jump
target as one would expect), but it continues on the kernel privilege
level as if the ERET instruction did not change the control flow -
thus execution anything that is accidentally linked after the ERET
instruction. Later, the results of this speculative execution are
always architecturally discarded, however they can leak data using
microarchitectural side channels. This speculative execution is very
reliable (seems to be unconditional) and it manages to complete even
relatively performance-heavy operations (e.g. multiple dependent
fetches from uncached memory).

This was fixed in Linux, FreeBSD, OpenBSD and Optee OS:
https://github.com/torvalds/linux/commit/679db70801da9fda91d26caf13bf5b5ccc74e8e8
https://github.com/freebsd/freebsd/commit/29fb48ace4186a41c409fde52bcf4216e9e50b61
https://github.com/openbsd/src/commit/3a08873ece1cb28ace89fd65e8f3c1375cc98de2
https://github.com/OP-TEE/optee_os/commit/abfd092aa19f9c0251e3d5551e2d68a9ebcfec8a

It is demonstrated in a SafeSide example:
https://github.com/google/safeside/blob/master/demos/eret_hvc_smc_wrapper.cc
https://github.com/google/safeside/blob/master/kernel_modules/kmod_eret_hvc_smc/eret_hvc_smc_module.c

Signed-off-by: Anthony Steinhauser <asteinhauser@google.com>
Change-Id: Iead39b0b9fb4b8d8b5609daaa8be81497ba63a0f

bl1/aarch64/bl1_exceptions.S

 /*
- * Copyright (c) 2013-2019, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2013-2020, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -202,7 +202,7 @@ debug_loop:
 	ldp	x4, x5, [x20, #(ENTRY_POINT_INFO_ARGS_OFFSET + 0x20)]
 	ldp	x2, x3, [x20, #(ENTRY_POINT_INFO_ARGS_OFFSET + 0x10)]
 	ldp	x0, x1, [x20, #(ENTRY_POINT_INFO_ARGS_OFFSET + 0x0)]
-	eret
+	exception_return
 endfunc smc_handler64
 
 unexpected_sync_exception:

bl2/aarch64/bl2_el3_entrypoint.S

 /*
- * Copyright (c) 2017-2019, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2017-2020, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -95,5 +95,5 @@ func bl2_run_next_image
 	ldp	x4, x5, [x20, #(ENTRY_POINT_INFO_ARGS_OFFSET + 0x20)]
 	ldp	x2, x3, [x20, #(ENTRY_POINT_INFO_ARGS_OFFSET + 0x10)]
 	ldp	x0, x1, [x20, #(ENTRY_POINT_INFO_ARGS_OFFSET + 0x0)]
-	eret
+	exception_return
 endfunc bl2_run_next_image

bl31/aarch64/runtime_exceptions.S

@@ -456,7 +456,7 @@ smc_unknown:
 smc_prohibited:
 	ldr	x30, [sp, #CTX_GPREGS_OFFSET + CTX_GPREG_LR]
 	mov	x0, #SMC_UNK
-	eret
+	exception_return
 
 #if DEBUG
 rt_svc_fw_critical_error:

bl32/tsp/aarch64/tsp_exceptions.S

 /*
- * Copyright (c) 2013-2016, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2013-2020, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -64,7 +64,7 @@
 	smc	#0
 interrupt_exit_\label:
 	restore_caller_regs_and_lr
-	eret
+	exception_return
 	.endm
 
 	.globl	tsp_exceptions

include/arch/aarch64/asm_macros.S

 /*
- * Copyright (c) 2013-2019, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2013-2020, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -218,4 +218,13 @@
 	ret
 	.endm
 
+	/*
+	 * Macro for mitigating against speculative execution beyond ERET.
+	 */
+	.macro exception_return
+	eret
+	dsb nsh
+	isb
+	.endm
+
 #endif /* ASM_MACROS_S */

lib/cpus/aarch64/cortex_a76.S

 /*
- * Copyright (c) 2017-2019, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2017-2020, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -85,7 +85,7 @@
 		bic	x3, x2, #CORTEX_A76_CPUACTLR2_EL1_DISABLE_LOAD_PASS_STORE
 		csel	x3, x3, x1, eq
 		msr	CORTEX_A76_CPUACTLR2_EL1, x3
-		eret	/* ERET implies ISB */
+		exception_return /* exception_return contains ISB */
 	.endif
 1:
 	/*

lib/cpus/aarch64/neoverse_n1.S

 /*
- * Copyright (c) 2017-2019, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2017-2020, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -553,7 +553,7 @@ func neoverse_n1_errata_ic_trap_handler
 	 */
 	esb
 #endif
-	eret
+	exception_return
 1:
 	ret
 endfunc neoverse_n1_errata_ic_trap_handler

lib/cpus/aarch64/wa_cve_2017_5715_mmu.S

 /*
- * Copyright (c) 2017-2018, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2017-2020, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -48,7 +48,7 @@ vector_base wa_cve_2017_5715_mmu_vbar
 		ccmp	w0, w1, #0, eq
 		/* Static predictor will predict a fall through */
 		bne	1f
-		eret
+		exception_return
 1:
 	.endif
 

lib/el3_runtime/aarch64/context.S

@@ -534,6 +534,6 @@ func el3_exit
 	 */
 	esb
 #endif
-	eret
+	exception_return
 
 endfunc el3_exit

plat/renesas/rcar/aarch64/plat_helpers.S

 /*
- * Copyright (c) 2013-2014, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2013-2020, ARM Limited and Contributors. All rights reserved.
  * Copyright (c) 2015-2019, Renesas Electronics Corporation. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
@@ -189,7 +189,7 @@ func bl2_enter_bl31
 	ldp	x0, x1, [x20, #ENTRY_POINT_INFO_PC_OFFSET]
 	msr	elr_el3, x0
 	msr	spsr_el3, x1
-	eret
+	exception_return
 endfunc bl2_enter_bl31
 
 	/* -----------------------------------------------------

services/std_svc/spm_mm/aarch64/spm_mm_shim_exceptions.S

 /*
- * Copyright (c) 2017, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2017-2020, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -87,7 +87,7 @@ vector_entry SynchronousExceptionA64, .spm_shim_exceptions
 do_smc:
 	mrs	x30, tpidr_el1
 	smc	#0
-	eret
+	exception_return
 
 	/* AArch64 system instructions trap are handled as a panic for now */
 handle_sys_trap: