arm64: scs: Store absolute SCS stack pointer value in thread_info

Storing the SCS information in thread_info as a {base,offset} pair
introduces an additional load instruction on the ret-to-user path,
since the SCS stack pointer in x18 has to be converted back to an offset
by subtracting the base.

Replace the offset with the absolute SCS stack pointer value instead
and avoid the redundant load.
Tested-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/scs.h

@@ -4,16 +4,15 @@
 
 #ifdef __ASSEMBLY__
 
+#include <asm/asm-offsets.h>
+
 #ifdef CONFIG_SHADOW_CALL_STACK
 	.macro scs_load tsk, tmp
-	ldp	x18, \tmp, [\tsk, #TSK_TI_SCS_BASE]
-	add	x18, x18, \tmp
+	ldr	x18, [\tsk, #TSK_TI_SCS_SP]
 	.endm
 
 	.macro scs_save tsk, tmp
-	ldr	\tmp, [\tsk, #TSK_TI_SCS_BASE]
-	sub	\tmp, x18, \tmp
-	str	\tmp, [\tsk, #TSK_TI_SCS_OFFSET]
+	str	x18, [\tsk, #TSK_TI_SCS_SP]
 	.endm
 #else
 	.macro scs_load tsk, tmp

arch/arm64/include/asm/thread_info.h

@@ -43,7 +43,7 @@ struct thread_info {
 	};
 #ifdef CONFIG_SHADOW_CALL_STACK
 	void			*scs_base;
-	unsigned long		scs_offset;
+	void			*scs_sp;
 #endif
 };
 
@@ -107,7 +107,7 @@ void arch_release_task_struct(struct task_struct *tsk);
 #ifdef CONFIG_SHADOW_CALL_STACK
 #define INIT_SCS							\
 	.scs_base	= init_shadow_call_stack,			\
-	.scs_offset	= 0,
+	.scs_sp		= init_shadow_call_stack,
 #else
 #define INIT_SCS
 #endif

arch/arm64/kernel/asm-offsets.c

@@ -36,7 +36,7 @@ int main(void)
 #endif
 #ifdef CONFIG_SHADOW_CALL_STACK
   DEFINE(TSK_TI_SCS_BASE,	offsetof(struct task_struct, thread_info.scs_base));
-  DEFINE(TSK_TI_SCS_OFFSET,	offsetof(struct task_struct, thread_info.scs_offset));
+  DEFINE(TSK_TI_SCS_SP,		offsetof(struct task_struct, thread_info.scs_sp));
 #endif
   DEFINE(TSK_STACK,		offsetof(struct task_struct, stack));
 #ifdef CONFIG_STACKPROTECTOR

include/linux/scs.h

@@ -27,7 +27,7 @@
 #define SCS_END_MAGIC		(0x5f6UL + POISON_POINTER_DELTA)
 
 #define task_scs(tsk)		(task_thread_info(tsk)->scs_base)
-#define task_scs_offset(tsk)	(task_thread_info(tsk)->scs_offset)
+#define task_scs_sp(tsk)	(task_thread_info(tsk)->scs_sp)
 
 void scs_init(void);
 int scs_prepare(struct task_struct *tsk, int node);
@@ -39,7 +39,7 @@ static inline void scs_task_reset(struct task_struct *tsk)
 	 * Reset the shadow stack to the base address in case the task
 	 * is reused.
 	 */
-	task_scs_offset(tsk) = 0;
+	task_scs_sp(tsk) = task_scs(tsk);
 }
 
 static inline unsigned long *__scs_magic(void *s)
@@ -50,9 +50,9 @@ static inline unsigned long *__scs_magic(void *s)
 static inline bool scs_corrupted(struct task_struct *tsk)
 {
 	unsigned long *magic = __scs_magic(task_scs(tsk));
+	unsigned long sz = task_scs_sp(tsk) - task_scs(tsk);
 
-	return (task_scs_offset(tsk) >= SCS_SIZE - 1 ||
-		READ_ONCE_NOCHECK(*magic) != SCS_END_MAGIC);
+	return sz >= SCS_SIZE - 1 || READ_ONCE_NOCHECK(*magic) != SCS_END_MAGIC;
 }
 
 #else /* CONFIG_SHADOW_CALL_STACK */

kernel/scs.c

@@ -60,8 +60,7 @@ int scs_prepare(struct task_struct *tsk, int node)
 	if (!s)
 		return -ENOMEM;
 
-	task_scs(tsk) = s;
-	task_scs_offset(tsk) = 0;
+	task_scs(tsk) = task_scs_sp(tsk) = s;
 	scs_account(tsk, 1);
 	return 0;
 }