Re: [PATCH 10/11] scs: generic scs code updated to leverage hw assisted shadow stack

[Deepak Gupta <debug@rivosinc.com>]

Sorry forgot to respond to rest of the comments.

On Fri, Jul 25, 2025 at 09:42:39AM -0700, Deepak Gupta wrote:
>On Fri, Jul 25, 2025 at 04:13:27PM +0000, Sami Tolvanen wrote:
>>On Thu, Jul 24, 2025 at 04:37:03PM -0700, Deepak Gupta wrote:
>>>If shadow stack have memory protections from underlying cpu, use those
>>>protections. arches can define PAGE_KERNEL_SHADOWSTACK to vmalloc such shadow
>>>stack pages. Hw assisted shadow stack pages grow downwards like regular
>>>stack. Clang based software shadow call stack grows low to high address.
>>
>>Is this the case for all the current hardware shadow stack
>>implementations? If not, we might want a separate config for the
>>shadow stack direction instead.
>
>Is there something like this for regular stack as well?
>I could copy same mechanism.
>
>>
>>>Thus this patch addresses some of those needs due to opposite direction
>>>of shadow stack. Furthermore, hw shadow stack can't be memset because memset
>>>uses normal stores. Lastly to store magic word at base of shadow stack, arch
>>>specific shadow stack store has to be performed.
>>>
>>>Signed-off-by: Deepak Gupta <debug@rivosinc.com>
>>>---
>>> include/linux/scs.h | 26 +++++++++++++++++++++++++-
>>> kernel/scs.c        | 38 +++++++++++++++++++++++++++++++++++---
>>> 2 files changed, 60 insertions(+), 4 deletions(-)
>>>
>>>diff --git a/include/linux/scs.h b/include/linux/scs.h
>>>index 4ab5bdc898cf..6ceee07c2d1a 100644
>>>--- a/include/linux/scs.h
>>>+++ b/include/linux/scs.h
>>>@@ -12,6 +12,7 @@
>>> #include <linux/poison.h>
>>> #include <linux/sched.h>
>>> #include <linux/sizes.h>
>>>+#include <asm/scs.h>
>>>
>>> #ifdef CONFIG_SHADOW_CALL_STACK
>>>
>>>@@ -37,22 +38,45 @@ static inline void scs_task_reset(struct task_struct *tsk)
>>> 	 * Reset the shadow stack to the base address in case the task
>>> 	 * is reused.
>>> 	 */
>>>+#ifdef CONFIG_ARCH_HAS_KERNEL_SHADOW_STACK
>>>+	task_scs_sp(tsk) = task_scs(tsk) + SCS_SIZE;
>>>+#else
>>> 	task_scs_sp(tsk) = task_scs(tsk);
>>>+#endif
>>> }
>>>
>>> static inline unsigned long *__scs_magic(void *s)
>>> {
>>>+#ifdef CONFIG_ARCH_HAS_KERNEL_SHADOW_STACK
>>>+	return (unsigned long *)(s);
>>>+#else
>>> 	return (unsigned long *)(s + SCS_SIZE) - 1;
>>>+#endif
>>> }
>>>
>>> static inline bool task_scs_end_corrupted(struct task_struct *tsk)
>>> {
>>> 	unsigned long *magic = __scs_magic(task_scs(tsk));
>>>-	unsigned long sz = task_scs_sp(tsk) - task_scs(tsk);
>>>+	unsigned long sz;
>>>+
>>>+#ifdef CONFIG_ARCH_HAS_KERNEL_SHADOW_STACK
>>>+	sz = (task_scs(tsk) + SCS_SIZE) - task_scs_sp(tsk);
>>>+#else
>>>+	sz = task_scs_sp(tsk) - task_scs(tsk);
>>>+#endif
>>>
>>> 	return sz >= SCS_SIZE - 1 || READ_ONCE_NOCHECK(*magic) != SCS_END_MAGIC;
>>> }
>>>
>>>+static inline void __scs_store_magic(unsigned long *s, unsigned long magic_val)
>>>+{
>>>+#ifdef CONFIG_ARCH_HAS_KERNEL_SHADOW_STACK
>>>+	arch_scs_store(s, magic_val);
>>>+#else
>>>+	*__scs_magic(s) = magic_val;
>>>+#endif
>>>+}
>>>+
>>
>>I'm not a huge fan of all the ifdefs. We could clean this up by
>>allowing architectures to simply override some these functions, or at
>>least use if (IS_ENABLED(CONFIG...)) instead. Will, any thoughts about
>>this?

Yes I don't like it either.
I'll do something about it in next iteration.

>>
>>> DECLARE_STATIC_KEY_FALSE(dynamic_scs_enabled);
>>>
>>> static inline bool scs_is_dynamic(void)
>>>diff --git a/kernel/scs.c b/kernel/scs.c
>>>index d7809affe740..5910c0a8eabd 100644
>>>--- a/kernel/scs.c
>>>+++ b/kernel/scs.c
>>>@@ -11,6 +11,7 @@
>>> #include <linux/scs.h>
>>> #include <linux/vmalloc.h>
>>> #include <linux/vmstat.h>
>>>+#include <asm-generic/set_memory.h>
>>>
>>> #ifdef CONFIG_DYNAMIC_SCS
>>> DEFINE_STATIC_KEY_FALSE(dynamic_scs_enabled);
>>>@@ -32,19 +33,31 @@ static void *__scs_alloc(int node)
>>> {
>>> 	int i;
>>> 	void *s;
>>>+	pgprot_t prot = PAGE_KERNEL;
>>>+
>>>+#ifdef CONFIG_ARCH_HAS_KERNEL_SHADOW_STACK
>>>+	prot = PAGE_KERNEL_SHADOWSTACK;
>>>+#endif
>>
>>I would rather define the shadow stack protection flags in the header
>>file and allow them to be overridden in asm/scs.h.

Yes that's good idea. I'll do that.

>>
>>> 	for (i = 0; i < NR_CACHED_SCS; i++) {
>>> 		s = this_cpu_xchg(scs_cache[i], NULL);
>>> 		if (s) {
>>> 			s = kasan_unpoison_vmalloc(s, SCS_SIZE,
>>> 						   KASAN_VMALLOC_PROT_NORMAL);
>>>+/*
>>>+ * If software shadow stack, its safe to memset. Else memset is not
>>>+ * possible on hw protected shadow stack. memset constitutes stores and
>>>+ * stores to shadow stack memory are disallowed and will fault.
>>>+ */
>>>+#ifndef CONFIG_ARCH_HAS_KERNEL_SHADOW_STACK
>>> 			memset(s, 0, SCS_SIZE);
>>>+#endif
>>
>>This could also be moved to a static inline function that
>>architectures can override if they have hardware shadow stacks that
>>cannot be cleared at this point.

Make sense.

>>
>>> 			goto out;
>>> 		}
>>> 	}
>>>
>>> 	s = __vmalloc_node_range(SCS_SIZE, 1, VMALLOC_START, VMALLOC_END,
>>>-				    GFP_SCS, PAGE_KERNEL, 0, node,
>>>+				    GFP_SCS, prot, 0, node,
>>> 				    __builtin_return_address(0));
>>>
>>> out:
>>>@@ -59,7 +72,7 @@ void *scs_alloc(int node)
>>> 	if (!s)
>>> 		return NULL;
>>>
>>>-	*__scs_magic(s) = SCS_END_MAGIC;
>>>+	__scs_store_magic(__scs_magic(s), SCS_END_MAGIC);
>>>
>>> 	/*
>>> 	 * Poison the allocation to catch unintentional accesses to
>>>@@ -87,6 +100,16 @@ void scs_free(void *s)
>>> 			return;
>>>
>>> 	kasan_unpoison_vmalloc(s, SCS_SIZE, KASAN_VMALLOC_PROT_NORMAL);
>>>+	/*
>>>+	 * Hardware protected shadow stack is not writeable by regular stores
>>>+	 * Thus adding this back to free list will raise faults by vmalloc
>>>+	 * It needs to be writeable again. It's good sanity as well because
>>>+	 * then it can't be inadvertently accesses and if done, it will fault.
>>>+	 */
>>>+#ifdef CONFIG_ARCH_HAS_KERNEL_SHADOW_STACK
>>>+	set_memory_rw((unsigned long)s, (SCS_SIZE/PAGE_SIZE));
>>>+#endif
>>
>>Another candidate for an arch-specific function to reduce the number
>>of ifdefs in the generic code.

Yes I'll do these changes in next iteration.
>>
>>Sami