Re: [PATCH v20 4/8] fork: Add shadow stack support to clone3()

On Tue, Sep 02, 2025 at 11:21:48AM +0100, Mark Brown wrote:

Unlike with the normal stack there is no API for configuring the shadow stack for a new thread, instead the kernel will dynamically allocate a new shadow stack with the same size as the normal stack. This appears to be due to the shadow stack series having been in development since before the more extensible clone3() was added rather than anything more deliberate.

Add a parameter to clone3() specifying a shadow stack pointer to use for the new thread, this is inconsistent with the way we specify the normal stack but during review concerns were expressed about having to identify where the shadow stack pointer should be placed especially in cases where the shadow stack has been previously active. If no shadow stack is specified then the existing implicit allocation behaviour is maintained.

If a shadow stack pointer is specified then it is required to have an architecture defined token placed on the stack, this will be consumed by the new task, the shadow stack is specified by pointing to this token. If no valid token is present then this will be reported with -EINVAL. This token prevents new threads being created pointing at the shadow stack of an existing running thread. On architectures with support for userspace pivoting of shadow stacks it is expected that the same format and placement of tokens will be used, this is the case for arm64 and x86.

If the architecture does not support shadow stacks the shadow stack pointer must be not be specified, architectures that do support the feature are expected to enforce the same requirement on individual systems that lack shadow stack support.

Update the existing arm64 and x86 implementations to pay attention to the newly added arguments, in order to maintain compatibility we use the existing behaviour if no shadow stack is specified. Since we are now using more fields from the kernel_clone_args we pass that into the shadow stack code rather than individual fields.

Portions of the x86 architecture code were written by Rick Edgecombe.

Acked-by: Yury Khrustalev yury.khrustalev@arm.com Tested-by: Yury Khrustalev yury.khrustalev@arm.com Tested-by: Rick Edgecombe rick.p.edgecombe@intel.com Reviewed-by: Rick Edgecombe rick.p.edgecombe@intel.com Signed-off-by: Mark Brown broonie@kernel.org

---

arch/arm64/mm/gcs.c | 47 +++++++++++++++++++- arch/x86/include/asm/shstk.h | 11 +++-- arch/x86/kernel/process.c | 2 +- arch/x86/kernel/shstk.c | 53 ++++++++++++++++++++--- include/asm-generic/cacheflush.h | 11 +++++ include/linux/sched/task.h | 17 ++++++++ include/uapi/linux/sched.h | 9 ++-- kernel/fork.c | 93 ++++++++++++++++++++++++++++++++++------ 8 files changed, 217 insertions(+), 26 deletions(-)

diff --git a/arch/arm64/mm/gcs.c b/arch/arm64/mm/gcs.c index 3abcbf9adb5c..249ff05bca45 100644 --- a/arch/arm64/mm/gcs.c +++ b/arch/arm64/mm/gcs.c @@ -43,8 +43,23 @@ int gcs_alloc_thread_stack(struct task_struct *tsk, { unsigned long addr, size;

• if (!system_supports_gcs())

• if (!system_supports_gcs()) {

• if (args->shadow_stack_token)
  

• 	return -EINVAL;
  

• return 0;
• }
• /*

• * If the user specified a GCS then use it, otherwise fall
  

• * back to a default allocation strategy. Validation is done
  

• * in arch_shstk_validate_clone().
  

• */
  

• if (args->shadow_stack_token) {

• tsk->thread.gcs_base = 0;
  

• tsk->thread.gcs_size = 0;
  

• return 0;
  

• }

if (!task_gcs_el0_enabled(tsk)) return 0; @@ -68,6 +83,36 @@ int gcs_alloc_thread_stack(struct task_struct *tsk, return 0; } +static bool gcs_consume_token(struct vm_area_struct *vma, struct page *page,

• 	      unsigned long user_addr)
  

+{

• u64 expected = GCS_CAP(user_addr);
• u64 *token = page_address(page) + offset_in_page(user_addr);
• if (!cmpxchg_to_user_page(vma, page, user_addr, token, expected, 0))

• return false;
  

• set_page_dirty_lock(page);
• return true;

+}

+int arch_shstk_validate_clone(struct task_struct *tsk,

• 	      struct vm_area_struct *vma,
  

• 	      struct page *page,
  

• 	      struct kernel_clone_args *args)
  

+{

• unsigned long gcspr_el0;
• int ret = 0;
• gcspr_el0 = args->shadow_stack_token;
• if (!gcs_consume_token(vma, page, gcspr_el0))

• return -EINVAL;
  

• tsk->thread.gcspr_el0 = gcspr_el0 + sizeof(u64);
• return ret;

+}

SYSCALL_DEFINE3(map_shadow_stack, unsigned long, addr, unsigned long, size, unsigned int, flags) { unsigned long alloc_size; diff --git a/arch/x86/include/asm/shstk.h b/arch/x86/include/asm/shstk.h index ba6f2fe43848..827e983430aa 100644 --- a/arch/x86/include/asm/shstk.h +++ b/arch/x86/include/asm/shstk.h @@ -6,6 +6,7 @@ #include <linux/types.h> struct task_struct; +struct kernel_clone_args; struct ksignal; #ifdef CONFIG_X86_USER_SHADOW_STACK @@ -16,8 +17,8 @@ struct thread_shstk { long shstk_prctl(struct task_struct *task, int option, unsigned long arg2); void reset_thread_features(void); -unsigned long shstk_alloc_thread_stack(struct task_struct *p, unsigned long clone_flags,

• 		       unsigned long stack_size);
  

+unsigned long shstk_alloc_thread_stack(struct task_struct *p,

• 		       const struct kernel_clone_args *args);
  

void shstk_free(struct task_struct *p); int setup_signal_shadow_stack(struct ksignal *ksig); int restore_signal_shadow_stack(void); @@ -28,8 +29,10 @@ static inline long shstk_prctl(struct task_struct *task, int option, unsigned long arg2) { return -EINVAL; } static inline void reset_thread_features(void) {} static inline unsigned long shstk_alloc_thread_stack(struct task_struct *p,

• 				     unsigned long clone_flags,
  

• 				     unsigned long stack_size) { return 0; }
  

• 				     const struct kernel_clone_args *args)
  

+{

• return 0;

+} static inline void shstk_free(struct task_struct *p) {} static inline int setup_signal_shadow_stack(struct ksignal *ksig) { return 0; } static inline int restore_signal_shadow_stack(void) { return 0; } diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c index 1b7960cf6eb0..0a54af6c60df 100644 --- a/arch/x86/kernel/process.c +++ b/arch/x86/kernel/process.c @@ -209,7 +209,7 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args) * is disabled, new_ssp will remain 0, and fpu_clone() will know not to * update it. */

• new_ssp = shstk_alloc_thread_stack(p, clone_flags, args->stack_size);

• new_ssp = shstk_alloc_thread_stack(p, args); if (IS_ERR_VALUE(new_ssp)) return PTR_ERR((void *)new_ssp);

diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c index 2ddf23387c7e..9926d58e5d41 100644 --- a/arch/x86/kernel/shstk.c +++ b/arch/x86/kernel/shstk.c @@ -191,18 +191,61 @@ void reset_thread_features(void) current->thread.features_locked = 0; } -unsigned long shstk_alloc_thread_stack(struct task_struct *tsk, unsigned long clone_flags,

• 		       unsigned long stack_size)
  

+int arch_shstk_validate_clone(struct task_struct *t,

• 	      struct vm_area_struct *vma,
  

• 	      struct page *page,
  

• 	      struct kernel_clone_args *args)
  

+{

• void *maddr = page_address(page);
• unsigned long token;
• int offset;
• u64 expected;
• /*

• * kernel_clone_args() verification assures token address is 8
  

• * byte aligned.
  

• */
  

• token = args->shadow_stack_token;
• expected = (token + SS_FRAME_SIZE) | BIT(0);
• offset = offset_in_page(token);
• if (!cmpxchg_to_user_page(vma, page, token, (unsigned long *)(maddr + offset),

• 		  expected, 0))
  

• return -EINVAL;
  

• set_page_dirty_lock(page);
• return 0;

+}

+unsigned long shstk_alloc_thread_stack(struct task_struct *tsk,

• 		       const struct kernel_clone_args *args)
  

{ struct thread_shstk *shstk = &tsk->thread.shstk;

• unsigned long clone_flags = args->flags; unsigned long addr, size;

/* * If shadow stack is not enabled on the new thread, skip any

• * switch to a new shadow stack.
  

• * implicit switch to a new shadow stack and reject attempts to
  

• * explicitly specify one.
  

  */

• if (!features_enabled(ARCH_SHSTK_SHSTK))

• if (!features_enabled(ARCH_SHSTK_SHSTK)) {

• if (args->shadow_stack_token)
  

• 	return (unsigned long)ERR_PTR(-EINVAL);
  

• return 0;
• }
• /*

• * If the user specified a shadow stack then use it, otherwise
  

• * fall back to a default allocation strategy. Validation is
  

• * done in arch_shstk_validate_clone().
  

• */
  

• if (args->shadow_stack_token) {

• shstk->base = 0;
  

• shstk->size = 0;
  

• return args->shadow_stack_token + 8;
  

• }

/* * For CLONE_VFORK the child will share the parents shadow stack. @@ -222,7 +265,7 @@ unsigned long shstk_alloc_thread_stack(struct task_struct *tsk, unsigned long cl if (!(clone_flags & CLONE_VM)) return 0;

• size = adjust_shstk_size(stack_size);

• size = adjust_shstk_size(args->stack_size); addr = alloc_shstk(0, size, 0, false); if (IS_ERR_VALUE(addr)) return addr;

diff --git a/include/asm-generic/cacheflush.h b/include/asm-generic/cacheflush.h index 7ee8a179d103..96cc0c7a5c90 100644 --- a/include/asm-generic/cacheflush.h +++ b/include/asm-generic/cacheflush.h @@ -124,4 +124,15 @@ static inline void flush_cache_vunmap(unsigned long start, unsigned long end) } while (0) #endif +#ifndef cmpxchg_to_user_page +#define cmpxchg_to_user_page(vma, page, vaddr, ptr, old, new) \ +({ \

• bool ret; \

• 						  \
  

• ret = try_cmpxchg(ptr, &old, new); \
• flush_icache_user_page(vma, page, vaddr, sizeof(*ptr)); \
• ret; \

+}) +#endif

#endif /* _ASM_GENERIC_CACHEFLUSH_H */ diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h index ea41795a352b..b501f752fc9a 100644 --- a/include/linux/sched/task.h +++ b/include/linux/sched/task.h @@ -16,6 +16,7 @@ struct task_struct; struct rusage; union thread_union; struct css_set; +struct vm_area_struct; /* All the bits taken by the old clone syscall. */ #define CLONE_LEGACY_FLAGS 0xffffffffULL @@ -44,6 +45,7 @@ struct kernel_clone_args { struct cgroup *cgrp; struct css_set *cset; unsigned int kill_seq;

• unsigned long shadow_stack_token;

}; /* @@ -226,4 +228,19 @@ static inline void task_unlock(struct task_struct *p) DEFINE_GUARD(task_lock, struct task_struct *, task_lock(_T), task_unlock(_T)) +#ifdef CONFIG_ARCH_HAS_USER_SHADOW_STACK +int arch_shstk_validate_clone(struct task_struct *p,

• 	      struct vm_area_struct *vma,
  

• 	      struct page *page,
  

• 	      struct kernel_clone_args *args);
  

+#else +static inline int arch_shstk_validate_clone(struct task_struct *p,

• 			    struct vm_area_struct *vma,
  

• 			    struct page *page,
  

• 			    struct kernel_clone_args *args)
  

+{

• return 0;

+} +#endif

#endif /* _LINUX_SCHED_TASK_H */ diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h index 359a14cc76a4..9cf5c419e109 100644 --- a/include/uapi/linux/sched.h +++ b/include/uapi/linux/sched.h @@ -84,6 +84,7 @@

•            kernel's limit of nested PID namespaces.
  

• @cgroup: If CLONE_INTO_CGROUP is specified set this to

•            a file descriptor for the cgroup.
  

• • @shadow_stack_token: Pointer to shadow stack token at top of stack.
  • The structure is versioned by size and thus extensible.
  • New struct members must go at the end of the struct and

@@ -101,12 +102,14 @@ struct clone_args { __aligned_u64 set_tid; __aligned_u64 set_tid_size; __aligned_u64 cgroup;

• __aligned_u64 shadow_stack_token;

}; #endif -#define CLONE_ARGS_SIZE_VER0 64 /* sizeof first published struct */ -#define CLONE_ARGS_SIZE_VER1 80 /* sizeof second published struct */ -#define CLONE_ARGS_SIZE_VER2 88 /* sizeof third published struct */ +#define CLONE_ARGS_SIZE_VER0 64 /* sizeof first published struct */ +#define CLONE_ARGS_SIZE_VER1 80 /* sizeof second published struct */ +#define CLONE_ARGS_SIZE_VER2 88 /* sizeof third published struct */ +#define CLONE_ARGS_SIZE_VER3 96 /* sizeof fourth published struct */ /*

• Scheduling policies

diff --git a/kernel/fork.c b/kernel/fork.c index af673856499d..d484ebeded33 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -1907,6 +1907,51 @@ static bool need_futex_hash_allocate_default(u64 clone_flags) return true; } +static int shstk_validate_clone(struct task_struct *p,

• 		struct kernel_clone_args *args)
  

+{

• struct mm_struct *mm;
• struct vm_area_struct *vma;
• struct page *page;
• unsigned long addr;
• int ret;
• if (!IS_ENABLED(CONFIG_ARCH_HAS_USER_SHADOW_STACK))

• return 0;
  

• if (!args->shadow_stack_token)

• return 0;
  

• mm = get_task_mm(p);
• if (!mm)

• return -EFAULT;
  

• mmap_read_lock(mm);
• addr = untagged_addr_remote(mm, args->shadow_stack_token);
• page = get_user_page_vma_remote(mm, addr, FOLL_FORCE | FOLL_WRITE,

• 			&vma);
  

• if (IS_ERR(page)) {

• ret = -EFAULT;
  

• goto out;
  

• }
• if (!(vma->vm_flags & VM_SHADOW_STACK) ||

•    !(vma->vm_flags & VM_WRITE)) {
  

• ret = -EFAULT;
  

• goto out_page;
  

• }
• ret = arch_shstk_validate_clone(p, vma, page, args);

+out_page:

• put_page(page);

+out:

• mmap_read_unlock(mm);
• mmput(mm);
• return ret;

+}

/*

• This creates a new process as a copy of the old one,
• but does not actually start it yet.

@@ -2182,6 +2227,9 @@ __latent_entropy struct task_struct *copy_process( if (retval) goto bad_fork_cleanup_namespaces; retval = copy_thread(p, args);

• if (retval)

• goto bad_fork_cleanup_io;
  

• retval = shstk_validate_clone(p, args); if (retval) goto bad_fork_cleanup_io;

@@ -2763,7 +2811,9 @@ static noinline int copy_clone_args_from_user(struct kernel_clone_args *kargs, CLONE_ARGS_SIZE_VER1); BUILD_BUG_ON(offsetofend(struct clone_args, cgroup) != CLONE_ARGS_SIZE_VER2);

• BUILD_BUG_ON(sizeof(struct clone_args) != CLONE_ARGS_SIZE_VER2);

• BUILD_BUG_ON(offsetofend(struct clone_args, shadow_stack_token) !=

•      CLONE_ARGS_SIZE_VER3);
  

• BUILD_BUG_ON(sizeof(struct clone_args) != CLONE_ARGS_SIZE_VER3);

if (unlikely(usize > PAGE_SIZE)) return -E2BIG; @@ -2796,16 +2846,17 @@ static noinline int copy_clone_args_from_user(struct kernel_clone_args *kargs, return -EINVAL; *kargs = (struct kernel_clone_args){

• .flags		= args.flags,
  

• .pidfd		= u64_to_user_ptr(args.pidfd),
  

• .child_tid	= u64_to_user_ptr(args.child_tid),
  

• .parent_tid	= u64_to_user_ptr(args.parent_tid),
  

• .exit_signal	= args.exit_signal,
  

• .stack		= args.stack,
  

• .stack_size	= args.stack_size,
  

• .tls		= args.tls,
  

• .set_tid_size	= args.set_tid_size,
  

• .cgroup		= args.cgroup,
  

• .flags			= args.flags,
  

• .pidfd			= u64_to_user_ptr(args.pidfd),
  

• .child_tid		= u64_to_user_ptr(args.child_tid),
  

• .parent_tid		= u64_to_user_ptr(args.parent_tid),
  

• .exit_signal		= args.exit_signal,
  

• .stack			= args.stack,
  

• .stack_size		= args.stack_size,
  

• .tls			= args.tls,
  

• .set_tid_size		= args.set_tid_size,
  

• .cgroup			= args.cgroup,
  

• .shadow_stack_token	= args.shadow_stack_token,
  

I'm not sure why this has to be named "shadow_stack_token" I think that's just confusing and we should just call it "shadow_stack" and be done with it. It's also a bit long of a field name imho.

I have a kernel-6.18.clone3 branch https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git/log/?h=kernel-6.... because there's another cross-arch cleanup that cleans up copy_thread(), copy_sighand(), and copy_process() and - surprisingly - also adds clone3() support for nios2...

Anyway, if you just want me to slap it on top of that branch then I can simply rename while applying so no need to resend in that case.

}; if (args.set_tid && @@ -2846,6 +2897,24 @@ static inline bool clone3_stack_valid(struct kernel_clone_args *kargs) return true; } +/**

• • clone3_shadow_stack_valid - check and prepare shadow stack
• • @kargs: kernel clone args
• • Verify that shadow stacks are only enabled if supported.
• */

+static inline bool clone3_shadow_stack_valid(struct kernel_clone_args *kargs) +{

• if (!kargs->shadow_stack_token)

• return true;
  

• if (!IS_ALIGNED(kargs->shadow_stack_token, sizeof(void *)))

• return false;
  

• /* Fail if the kernel wasn't built with shadow stacks */
• return IS_ENABLED(CONFIG_ARCH_HAS_USER_SHADOW_STACK);

+}

static bool clone3_args_valid(struct kernel_clone_args *kargs) { /* Verify that no unknown flags are passed along. */ @@ -2868,7 +2937,7 @@ static bool clone3_args_valid(struct kernel_clone_args *kargs) kargs->exit_signal) return false;

• if (!clone3_stack_valid(kargs))

• if (!clone3_stack_valid(kargs) || !clone3_shadow_stack_valid(kargs)) return false;

return true;

-- 2.39.5