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scs: add support for dynamic shadow call stacks

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In order to allow arches to use code patching to conditionally emit the
shadow stack pushes and pops, rather than always taking the performance
hit even on CPUs that implement alternatives such as stack pointer
authentication on arm64, add a Kconfig symbol that can be set by the
arch to omit the SCS codegen itself, without otherwise affecting how
support code for SCS and compiler options (for register reservation, for
instance) are emitted.

Also, add a static key and some plumbing to omit the allocation of
shadow call stack for dynamic SCS configurations if SCS is disabled at
runtime.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Tested-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://lore.kernel.org/r/20221027155908.1940624-3-ardb@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>
This commit is contained in:
Ard Biesheuvel 2022-10-27 17:59:07 +02:00 committed by Will Deacon
parent 68c76ad4a9
commit 9beccca098
4 changed files with 39 additions and 2 deletions
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2
Makefile
View File

@ -966,8 +966,10 @@ LDFLAGS_vmlinux += --gc-sections
endif
ifdef CONFIG_SHADOW_CALL_STACK
ifndef CONFIG_DYNAMIC_SCS
CC_FLAGS_SCS := -fsanitize=shadow-call-stack
KBUILD_CFLAGS += $(CC_FLAGS_SCS)
endif
export CC_FLAGS_SCS
endif

7
arch/Kconfig
View File

@ -651,6 +651,13 @@ config SHADOW_CALL_STACK
reading and writing arbitrary memory may be able to locate them
and hijack control flow by modifying the stacks.
config DYNAMIC_SCS
bool
help
Set by the arch code if it relies on code patching to insert the
shadow call stack push and pop instructions rather than on the
compiler.
config LTO
bool
help

18
include/linux/scs.h
View File

@ -53,6 +53,22 @@ static inline bool task_scs_end_corrupted(struct task_struct *tsk)
return sz >= SCS_SIZE - 1 || READ_ONCE_NOCHECK(*magic) != SCS_END_MAGIC;
}
DECLARE_STATIC_KEY_FALSE(dynamic_scs_enabled);
static inline bool scs_is_dynamic(void)
{
if (!IS_ENABLED(CONFIG_DYNAMIC_SCS))
return false;
return static_branch_likely(&dynamic_scs_enabled);
}
static inline bool scs_is_enabled(void)
{
if (!IS_ENABLED(CONFIG_DYNAMIC_SCS))
return true;
return scs_is_dynamic();
}
#else /* CONFIG_SHADOW_CALL_STACK */
static inline void *scs_alloc(int node) { return NULL; }
@ -62,6 +78,8 @@ static inline void scs_task_reset(struct task_struct *tsk) {}
static inline int scs_prepare(struct task_struct *tsk, int node) { return 0; }
static inline void scs_release(struct task_struct *tsk) {}
static inline bool task_scs_end_corrupted(struct task_struct *tsk) { return false; }
static inline bool scs_is_enabled(void) { return false; }
static inline bool scs_is_dynamic(void) { return false; }
#endif /* CONFIG_SHADOW_CALL_STACK */

14
kernel/scs.c
View File

@ -12,6 +12,10 @@
#include <linux/vmalloc.h>
#include <linux/vmstat.h>
#ifdef CONFIG_DYNAMIC_SCS
DEFINE_STATIC_KEY_FALSE(dynamic_scs_enabled);
#endif
static void __scs_account(void *s, int account)
{
struct page *scs_page = vmalloc_to_page(s);
@ -101,14 +105,20 @@ static int scs_cleanup(unsigned int cpu)
void __init scs_init(void)
{
if (!scs_is_enabled())
return;
cpuhp_setup_state(CPUHP_BP_PREPARE_DYN, "scs:scs_cache", NULL,
scs_cleanup);
}
int scs_prepare(struct task_struct *tsk, int node)
{
void *s = scs_alloc(node);
void *s;
if (!scs_is_enabled())
return 0;
s = scs_alloc(node);
if (!s)
return -ENOMEM;
@ -148,7 +158,7 @@ void scs_release(struct task_struct *tsk)
{
void *s = task_scs(tsk);
if (!s)
if (!scs_is_enabled() || !s)
return;
WARN(task_scs_end_corrupted(tsk),