[PATCH 5.10.y 1/2] arm64: Always load shadow stack pointer directly from the task struct

[Ard Biesheuvel <ardb@kernel.org>]

All occurrences of the scs_load macro load the value of the shadow call
stack pointer from the task which is current at that point. So instead
of taking a task struct register argument in the scs_load macro to
specify the task struct to load from, let's always reference the current
task directly. This should make it much harder to exploit any
instruction sequences reloading the shadow call stack pointer register
from memory.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230109174800.3286265-2-ardb@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/arm64/include/asm/scs.h | 7 ++++---
 arch/arm64/kernel/entry.S    | 4 ++--
 arch/arm64/kernel/head.S     | 2 +-
 3 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/include/asm/scs.h b/arch/arm64/include/asm/scs.h
index eaa2cd92e4c10122..7155055a5bebc17d 100644
--- a/arch/arm64/include/asm/scs.h
+++ b/arch/arm64/include/asm/scs.h
@@ -9,15 +9,16 @@
 #ifdef CONFIG_SHADOW_CALL_STACK
 	scs_sp	.req	x18
 
-	.macro scs_load tsk, tmp
-	ldr	scs_sp, [\tsk, #TSK_TI_SCS_SP]
+	.macro scs_load_current
+	get_current_task scs_sp
+	ldr	scs_sp, [scs_sp, #TSK_TI_SCS_SP]
 	.endm
 
 	.macro scs_save tsk, tmp
 	str	scs_sp, [\tsk, #TSK_TI_SCS_SP]
 	.endm
 #else
-	.macro scs_load tsk, tmp
+	.macro scs_load_current
 	.endm
 
 	.macro scs_save tsk, tmp
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index d5bc1dbdd2fda84c..28d4cdeee5ae6083 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -221,7 +221,7 @@ alternative_else_nop_endif
 
 	ptrauth_keys_install_kernel tsk, x20, x22, x23
 
-	scs_load tsk, x20
+	scs_load_current
 	.else
 	add	x21, sp, #S_FRAME_SIZE
 	get_current_task tsk
@@ -1025,7 +1025,7 @@ SYM_FUNC_START(cpu_switch_to)
 	msr	sp_el0, x1
 	ptrauth_keys_install_kernel x1, x8, x9, x10
 	scs_save x0, x8
-	scs_load x1, x8
+	scs_load_current
 	ret
 SYM_FUNC_END(cpu_switch_to)
 NOKPROBE(cpu_switch_to)
diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index e1c25fa3b8e6ca65..351ee64c7deb4c96 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -747,7 +747,7 @@ SYM_FUNC_START_LOCAL(__secondary_switched)
 	ldr	x2, [x0, #CPU_BOOT_TASK]
 	cbz	x2, __secondary_too_slow
 	msr	sp_el0, x2
-	scs_load x2, x3
+	scs_load_current
 	mov	x29, #0
 	mov	x30, #0
 
-- 
2.39.2