[PATCH 21/35] arm64/gcs: Implement shadow stack prctl() interface

[Mark Brown]

Implement the architecture neutral prtctl() interface for setting the
shadow stack status, this supports setting and reading the current GCS
configuration for the current thread.

Userspace can enable basic GCS functionality and additionally also
support for GCS pushes and arbatrary GCS stores.  It is expected that
this prctl() will be called very early in application startup, for
example by the dynamic linker, and not subsequently adjusted during
normal operation.  Users should carefully note that after enabling GCS
for a thread GCS will become active with no call stack so it is not
normally possible to return from the function that invoked the prctl().

State is stored per thread, enabling GCS for a thread causes a GCS to be
allocated for that thread.

Userspace may lock the current GCS configuration by specifying
PR_SHADOW_STACK_ENABLE_LOCK, this prevents any further changes to the
GCS configuration via any means.

If GCS is not being enabled then all flags other than _LOCK are ignored,
it is not possible to enable stores or pops without enabling GCS.

When disabling the GCS we do not free the allocated stack, this allows
for inspection of the GCS after disabling as part of fault reporting.
Since it is not an expected use case and since it presents some
complications in determining what to do with previously initialsed data
on the GCS attempts to reenable GCS after this are rejected.  This can
be revisted if a use case arises.

Signed-off-by: Mark Brown <broonie at kernel.org>
---
 arch/arm64/include/asm/gcs.h |  4 +++
 arch/arm64/mm/gcs.c          | 64 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 68 insertions(+)

diff --git a/arch/arm64/include/asm/gcs.h b/arch/arm64/include/asm/gcs.h
index 4371a2f99b4a..8655ba8054c7 100644
--- a/arch/arm64/include/asm/gcs.h
+++ b/arch/arm64/include/asm/gcs.h
@@ -48,6 +48,10 @@ static inline u64 gcsss2(void)
 	return Xt;
 }
 
+#define PR_SHADOW_STACK_SUPPORTED_STATUS_MASK \
+	(PR_SHADOW_STACK_LOCK | PR_SHADOW_STACK_ENABLE | \
+	 PR_SHADOW_STACK_WRITE | PR_SHADOW_STACK_PUSH)
+
 #ifdef CONFIG_ARM64_GCS
 
 static inline bool task_gcs_el0_enabled(struct task_struct *task)
diff --git a/arch/arm64/mm/gcs.c b/arch/arm64/mm/gcs.c
index 1e059c37088d..b137493c594d 100644
--- a/arch/arm64/mm/gcs.c
+++ b/arch/arm64/mm/gcs.c
@@ -93,3 +93,67 @@ void gcs_free(struct task_struct *task)
 	task->thread.gcs_base = 0;
 	task->thread.gcs_size = 0;
 }
+
+int arch_set_shadow_stack_status(struct task_struct *task, unsigned long arg)
+{
+	unsigned long gcs, size;
+
+	if (!system_supports_gcs())
+		return -EINVAL;
+
+	if (is_compat_thread(task_thread_info(task)))
+		return -EINVAL;
+
+	/* Reject unknown flags */
+	if (arg & ~PR_SHADOW_STACK_SUPPORTED_STATUS_MASK)
+		return -EINVAL;
+
+	/* If the task has been locked block any attempted changes */
+	if (task->thread.gcs_el0_mode & PR_SHADOW_STACK_LOCK)
+		return -EBUSY;
+
+	/* Drop flags other than lock if disabling */
+	if (!(arg & PR_SHADOW_STACK_ENABLE))
+		arg &= ~PR_SHADOW_STACK_LOCK;
+
+	/* If we are enabling GCS then make sure we have a stack */
+	if (arg & PR_SHADOW_STACK_ENABLE) {
+		if (!task_gcs_el0_enabled(task)) {
+			/* Do not allow GCS to be reenabled */
+			if (task->thread.gcs_base)
+				return -EINVAL;
+
+			size = gcs_size(0);
+			gcs = alloc_gcs(task->thread.gcspr_el0, size,
+					0, 0);
+			if (!gcs)
+				return -ENOMEM;
+
+			task->thread.gcspr_el0 = gcs + size - sizeof(u64);
+			task->thread.gcs_base = gcs;
+			task->thread.gcs_size = size;
+			if (task == current)
+				write_sysreg_s(task->thread.gcspr_el0,
+					       SYS_GCSPR_EL0);
+
+		}
+	}
+
+	task->thread.gcs_el0_mode = arg;
+	if (task == current)
+		gcs_set_el0_mode(task);
+
+	return 0;
+}
+
+int arch_get_shadow_stack_status(struct task_struct *task,
+				 unsigned long __user *arg)
+{
+	if (!system_supports_gcs())
+		return -EINVAL;
+
+	if (is_compat_thread(task_thread_info(task)))
+		return -EINVAL;
+
+	return put_user(task->thread.gcs_el0_mode, arg);
+}

-- 
2.30.2