[PATCH v3 4/5] x86/shstk: support via prctl

[Bill Roberts]

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 at arm.com>
---
 arch/x86/kernel/shstk.c | 56 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 56 insertions(+)

diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
index 0ca64900192f..2c2a96b2d936 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 +631,58 @@ bool shstk_is_enabled(void)
 {
 	return features_enabled(ARCH_SHSTK_SHSTK);
 }
+
+#define PR_SHADOW_STACK_SUPPORTED_STATUS_MASK \
+		(PR_SHADOW_STACK_ENABLE | PR_SHADOW_STACK_WRITE)
+
+/* 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;
+	int tmp_rc;
+
+	if (status & ~PR_SHADOW_STACK_SUPPORTED_STATUS_MASK)
+		return -EINVAL;
+
+	/*
+	 * x86 arch_prctl is single bit at a time, so handle these one at time
+	 * If anything fails, rollback state.
+	 */
+	if (!(status & PR_SHADOW_STACK_ENABLE))
+		return shstk_prctl(t, ARCH_SHSTK_DISABLE, ARCH_SHSTK_SHSTK);
+
+	rc = shstk_prctl(t, ARCH_SHSTK_ENABLE, ARCH_SHSTK_SHSTK);
+	if (rc)
+		return rc;
+
+	if (status & PR_SHADOW_STACK_WRITE) {
+		rc = shstk_prctl(t, ARCH_SHSTK_ENABLE, ARCH_SHSTK_WRSS);
+		/* rollback, best attempt, if we actually enabled shadow stack in feature bits */
+		if (rc && (status & PR_SHADOW_STACK_ENABLE)) {
+			tmp_rc = shstk_prctl(t, ARCH_SHSTK_DISABLE, ARCH_SHSTK_SHSTK);
+			WARN(tmp_rc, "Could not rollback shadow stack enabled: %d\n", tmp_rc);
+		}
+
+		return rc;
+
+	}
+
+	return shstk_prctl(t, ARCH_SHSTK_DISABLE, ARCH_SHSTK_WRSS);
+}
+
+/* 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