Re: [PATCH RFT v4 2/5] fork: Add shadow stack support to clone3()

[Deepak Gupta <debug@rivosinc.com>]

On Tue, Nov 28, 2023 at 06:22:40PM +0000, Mark Brown wrote:
>Unlike with the normal stack there is no API for configuring the 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 the size of a shadow stack for
>the newly created process.  If no shadow stack is specified then the
>existing implicit allocation behaviour is maintained.
>
>If the architecture does not support shadow stacks the shadow stack size
>parameter must be zero, architectures that do support the feature are
>expected to enforce the same requirement on individual systems that lack
>shadow stack support.
>
>Update the existing x86 implementation to pay attention to the newly added
>arguments, in order to maintain compatibility we use the existing behaviour
>if no shadow stack is specified. Minimal validation is done of the supplied
>parameters, detailed enforcement is left to when the thread is executed.
>Since we are now using more fields from the kernel_clone_args we pass that
>into the shadow stack code rather than individual fields.
>
>Signed-off-by: Mark Brown <broonie@kernel.org>
>---
> arch/x86/include/asm/shstk.h | 11 +++++----
> arch/x86/kernel/process.c    |  2 +-
> arch/x86/kernel/shstk.c      | 56 ++++++++++++++++++++++++++++++--------------
> include/linux/sched/task.h   |  1 +
> include/uapi/linux/sched.h   |  4 ++++
> kernel/fork.c                | 53 +++++++++++++++++++++++++++++++----------
> 6 files changed, 92 insertions(+), 35 deletions(-)
>
>diff --git a/arch/x86/include/asm/shstk.h b/arch/x86/include/asm/shstk.h
>index 42fee8959df7..8be7b0a909c3 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);
>@@ -26,8 +27,10 @@ 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 b6f4e8399fca..a9ca80ea5056 100644
>--- a/arch/x86/kernel/process.c
>+++ b/arch/x86/kernel/process.c
>@@ -207,7 +207,7 @@ 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 59e15dd8d0f8..0d1325d2d94a 100644
>--- a/arch/x86/kernel/shstk.c
>+++ b/arch/x86/kernel/shstk.c
>@@ -191,38 +191,58 @@ 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)
>+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
>+	 * explciitly specify one.
> 	 */
>-	if (!features_enabled(ARCH_SHSTK_SHSTK))
>-		return 0;
>+	if (!features_enabled(ARCH_SHSTK_SHSTK)) {
>+		if (args->shadow_stack_size)
>+			return (unsigned long)ERR_PTR(-EINVAL);
>
>-	/*
>-	 * For CLONE_VFORK the child will share the parents shadow stack.
>-	 * Make sure to clear the internal tracking of the thread shadow
>-	 * stack so the freeing logic run for child knows to leave it alone.
>-	 */
>-	if (clone_flags & CLONE_VFORK) {
>-		shstk->base = 0;
>-		shstk->size = 0;
> 		return 0;
> 	}
>
> 	/*
>-	 * For !CLONE_VM the child will use a copy of the parents shadow
>-	 * stack.
>+	 * If the user specified a shadow stack then do some basic
>+	 * validation and use it, otherwise fall back to a default
>+	 * shadow stack size if the clone_flags don't indicate an
>+	 * allocation is unneeded.
> 	 */
>-	if (!(clone_flags & CLONE_VM))
>-		return 0;
>+	if (args->shadow_stack_size) {
>+		size = args->shadow_stack_size;
>+	} else {
>+		/*
>+		 * For CLONE_VFORK the child will share the parents
>+		 * shadow stack.  Make sure to clear the internal
>+		 * tracking of the thread shadow stack so the freeing
>+		 * logic run for child knows to leave it alone.
>+		 */
>+		if (clone_flags & CLONE_VFORK) {
>+			shstk->base = 0;
>+			shstk->size = 0;
>+			return 0;
>+		}
>+
>+		/*
>+		 * For !CLONE_VM the child will use a copy of the
>+		 * parents shadow stack.
>+		 */
>+		if (!(clone_flags & CLONE_VM))
>+			return 0;
>+
>+		size = args->stack_size;
>+
>+	}
>
>-	size = adjust_shstk_size(stack_size);
>+	size = adjust_shstk_size(size);
> 	addr = alloc_shstk(0, size, 0, false);
> 	if (IS_ERR_VALUE(addr))
> 		return addr;
>diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
>index a23af225c898..e86a09cfccd8 100644
>--- a/include/linux/sched/task.h
>+++ b/include/linux/sched/task.h
>@@ -41,6 +41,7 @@ struct kernel_clone_args {
> 	void *fn_arg;
> 	struct cgroup *cgrp;
> 	struct css_set *cset;
>+	unsigned long shadow_stack_size;
> };
>
> /*
>diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
>index 3bac0a8ceab2..a998b6d0c897 100644
>--- a/include/uapi/linux/sched.h
>+++ b/include/uapi/linux/sched.h
>@@ -84,6 +84,8 @@
>  *                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_size: Specify the size of the shadow stack to allocate
>+ *                     for the child process.
>  *
>  * The structure is versioned by size and thus extensible.
>  * New struct members must go at the end of the struct and
>@@ -101,12 +103,14 @@ struct clone_args {
> 	__aligned_u64 set_tid;
> 	__aligned_u64 set_tid_size;
> 	__aligned_u64 cgroup;
>+	__aligned_u64 shadow_stack_size;
> };
> #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_VER3 96 /* sizeof fourth published struct */
>
> /*
>  * Scheduling policies
>diff --git a/kernel/fork.c b/kernel/fork.c
>index 10917c3e1f03..35131acd43d2 100644
>--- a/kernel/fork.c
>+++ b/kernel/fork.c
>@@ -121,6 +121,11 @@
>  */
> #define MAX_THREADS FUTEX_TID_MASK
>
>+/*
>+ * Require that shadow stacks can store at least one element
>+ */
>+#define SHADOW_STACK_SIZE_MIN 8

nit:
Sorry, should've mentioned it earlier.
Can this be "#define SHADOW_STACK_SIZE_MIN sizeof(unsigned long)"


>+
> /*
>  * Protected counters by write_lock_irq(&tasklist_lock)
>  */
>@@ -3067,7 +3072,9 @@ noinline static 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);