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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>
This commit is contained in:
Will Deacon 2020-05-15 14:11:05 +01:00
parent cc49c71d2a
commit 51189c7a7e
5 changed files with 12 additions and 14 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

9
arch/arm64/include/asm/scs.h
View File

@ -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

4
arch/arm64/include/asm/thread_info.h
View File

@ -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

2
arch/arm64/kernel/asm-offsets.c
View File

@ -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

8
include/linux/scs.h
View File

@ -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 */

3
kernel/scs.c
View File

@ -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;
}