[OPTIONAL/RFC v2 38/39] x86/cet/shstk: Add ARCH_CET_UNLOCK

[Rick Edgecombe <rick.p.edgecombe@intel.com>]

From: Mike Rapoport <rppt@linux.ibm.com>

Userspace loaders may lock features before a CRIU restore operation has
the chance to set them to whatever state is required by the process
being restored. Allow a way for CRIU to unlock features. Add it as an
arch_prctl() like the other CET operations, but restrict it being called
by the ptrace arch_pctl() interface.

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
[Merged into recent API changes, added commit log and docs]
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
---

v2:
 - New patch

 Documentation/x86/cet.rst         | 3 +++
 arch/x86/include/uapi/asm/prctl.h | 1 +
 arch/x86/kernel/process_64.c      | 1 +
 arch/x86/kernel/shstk.c           | 9 +++++++--
 4 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/Documentation/x86/cet.rst b/Documentation/x86/cet.rst
index 4a0dfb6830f9..6b270a24ebc3 100644
--- a/Documentation/x86/cet.rst
+++ b/Documentation/x86/cet.rst
@@ -81,6 +81,9 @@ arch_prctl(ARCH_CET_DISABLE, unsigned int feature)
 arch_prctl(ARCH_CET_LOCK, unsigned int features)
     Lock in features at their current enabled or disabled status.
 
+arch_prctl(ARCH_CET_UNLOCK, unsigned int features)
+    Unlock features.
+
 The return values are as following:
     On success, return 0. On error, errno can be::
 
diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
index d811f0c5fc4f..2f4d81ab4849 100644
--- a/arch/x86/include/uapi/asm/prctl.h
+++ b/arch/x86/include/uapi/asm/prctl.h
@@ -25,6 +25,7 @@
 #define ARCH_CET_ENABLE			0x4001
 #define ARCH_CET_DISABLE		0x4002
 #define ARCH_CET_LOCK			0x4003
+#define ARCH_CET_UNLOCK			0x4004
 
 #define CET_SHSTK			0x1
 #define CET_WRSS			0x2
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index be544b4b4c8b..fbb2062dd0d2 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -834,6 +834,7 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
 	case ARCH_CET_ENABLE:
 	case ARCH_CET_DISABLE:
 	case ARCH_CET_LOCK:
+	case ARCH_CET_UNLOCK:
 		return cet_prctl(task, option, arg2);
 	default:
 		ret = -EINVAL;
diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
index 0efec02dbe6b..af1255164f0c 100644
--- a/arch/x86/kernel/shstk.c
+++ b/arch/x86/kernel/shstk.c
@@ -464,9 +464,14 @@ long cet_prctl(struct task_struct *task, int option, unsigned long features)
 		return 0;
 	}
 
-	/* Don't allow via ptrace */
-	if (task != current)
+	/* Only allow via ptrace */
+	if (task != current) {
+		if (option == ARCH_CET_UNLOCK) {
+			task->thread.features_locked &= ~features;
+			return 0;
+		}
 		return -EINVAL;
+	}
 
 	/* Do not allow to change locked features */
 	if (features & task->thread.features_locked)
-- 
2.17.1