[PATCH v4 4/6] x86/shstk: support via prctl

[Bill Roberts <bill.roberts@arm.com>]

Historically, managing the user-space shadow stack state on x86 has
been handled exclusively through the arch_prctl() interface via the
ARCH_SHSTK_* operations. However, other architectures (such as arm64 and
riscv) do not implement arch_prctl() and instead utilize the newer,
arch-agnostic, prctl() interface (i.e. PR_GET_SHADOW_STACK_STATUS and
PR_SET_SHADOW_STACK_STATUS).

To provide language runtimes, toolchains, and libc implementations with a
consistent, cross-architecture interface for managing control-flow
integrity, wire up the generic shadow stack prctl handlers for x86.

Map the generic PR_SHADOW_STACK_ENABLE, PR_SHADOW_STACK_DISABLE, and
PR_SHADOW_STACK_LOCK operations onto the underlying x86 internal CET helper
routines. This allows portable userspace applications to toggle or query
shadow stack states without relying on architecture-specific system calls,
while maintaining backward compatibility with existing arch_prctl() calls.

Signed-off-by: Bill Roberts <bill.roberts@arm.com>
---
 arch/x86/kernel/shstk.c | 83 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 83 insertions(+)

diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
index 0ca64900192f..249c4998732a 100644
--- a/arch/x86/kernel/shstk.c
+++ b/arch/x86/kernel/shstk.c
@@ -18,6 +18,7 @@
 #include <linux/sizes.h>
 #include <linux/user.h>
 #include <linux/syscalls.h>
+#include <linux/prctl.h>
 #include <asm/msr.h>
 #include <asm/fpu/xstate.h>
 #include <asm/fpu/types.h>
@@ -630,3 +631,85 @@ bool shstk_is_enabled(void)
 {
 	return features_enabled(ARCH_SHSTK_SHSTK);
 }
+
+/* Handles the generic prctl interface for PR_SET_SHADOW_STACK_STATUS and its feature bits */
+int arch_set_shadow_stack_status(struct task_struct *t, unsigned long status)
+{
+	int rc;
+	bool shstk_was_enabled = features_enabled(ARCH_SHSTK_SHSTK);
+
+	/*
+	 * One must explicitly perform enable/disable calls for the whole feature set so
+	 * that the locked bits are checked. It is also important to capture entry state
+	 * for roll-back if needed. However, we must NOT try to enable or disable a feature
+	 * unless there is a change in feature status from old to new state. This is
+	 * because shstk_prctl(), correctly, checks locked bits before feature enabled/disabled
+	 * short circuit.
+	 */
+	switch (status) {
+	case 0:
+		/*
+		 * We do NOT explicitly disable WRSS here, disabling shadow stack will disable WRSS,
+		 * and we don't want a locked WRSS to prevent disable.
+		 */
+		return shstk_was_enabled ?
+			shstk_prctl(t, ARCH_SHSTK_DISABLE, ARCH_SHSTK_SHSTK) : 0;
+
+	case PR_SHADOW_STACK_ENABLE:
+		if (!shstk_was_enabled) {
+			rc = shstk_prctl(t, ARCH_SHSTK_ENABLE, ARCH_SHSTK_SHSTK);
+			if (rc)
+				return rc;
+		}
+
+		if (features_enabled(ARCH_SHSTK_WRSS)) {
+			rc = shstk_prctl(t, ARCH_SHSTK_DISABLE, ARCH_SHSTK_WRSS);
+			if (rc && !shstk_was_enabled)
+				shstk_prctl(t, ARCH_SHSTK_DISABLE, ARCH_SHSTK_SHSTK);
+			return rc;
+		}
+
+		return 0;
+
+	case PR_SHADOW_STACK_ENABLE|PR_SHADOW_STACK_WRITE:
+		if (!shstk_was_enabled) {
+			rc = shstk_prctl(t, ARCH_SHSTK_ENABLE, ARCH_SHSTK_SHSTK);
+			if (rc)
+				return rc;
+		}
+
+		if (!features_enabled(ARCH_SHSTK_WRSS)) {
+			rc = shstk_prctl(t, ARCH_SHSTK_ENABLE, ARCH_SHSTK_WRSS);
+			if (rc && !shstk_was_enabled)
+				shstk_prctl(t, ARCH_SHSTK_DISABLE, ARCH_SHSTK_SHSTK);
+
+			return rc;
+		}
+
+		return 0;
+
+	/*
+	 * Unknown bit mapping, unsupported PR_SHADOW_STACK_PUSH or *only*
+	 * PR_SHADOW_STACK_WRITE.
+	 */
+	default:
+		return -EINVAL;
+	}
+}
+
+/* Handles the generic prctl interface for PR_LOCK_SHADOW_STACK_STATUS and its feature bits */
+int arch_lock_shadow_stack_status(struct task_struct *t, unsigned long status)
+{
+	return shstk_prctl(t, ARCH_SHSTK_LOCK, status);
+}
+
+/*
+ * We assume the prctl() feature bits line up with the arch_prctl() specific ones. If not,
+ * the flags returned via arch_get_shadow_stack_status will be mapped wrong.
+ */
+static_assert(PR_SHADOW_STACK_ENABLE == ARCH_SHSTK_SHSTK);
+static_assert(PR_SHADOW_STACK_WRITE  == ARCH_SHSTK_WRSS);
+int arch_get_shadow_stack_status(struct task_struct *t, unsigned long __user *status)
+{
+	return shstk_prctl(t, ARCH_SHSTK_STATUS, (unsigned long)status);
+}
-- 
2.55.0