[PATCH v4 38/39] x86/shstk: Add ARCH_SHSTK_UNLOCK

[Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>]

From: Mike Rapoport <rppt@xxxxxxxxxxxxx>

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 shadow stack operations, but restrict it being
called by the ptrace arch_pctl() interface.

Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
Tested-by: John Allen <john.allen@xxxxxxx>
Signed-off-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>
[Merged into recent API changes, added commit log and docs]
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
---

v4:
 - Add to docs that it is ptrace only.
 - Remove "CET" references

v3:
 - Depend on CONFIG_CHECKPOINT_RESTORE (Kees)

 Documentation/x86/shstk.rst       | 4 ++++
 arch/x86/include/uapi/asm/prctl.h | 1 +
 arch/x86/kernel/process_64.c      | 1 +
 arch/x86/kernel/shstk.c           | 9 +++++++--
 4 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/Documentation/x86/shstk.rst b/Documentation/x86/shstk.rst
index 8e0b2fe83ef8..0d7d1ccfff06 100644
--- a/Documentation/x86/shstk.rst
+++ b/Documentation/x86/shstk.rst
@@ -73,6 +73,10 @@ arch_prctl(ARCH_SHSTK_LOCK, unsigned long features)
     are ignored. The mask is ORed with the existing value. So any feature bits
     set here cannot be enabled or disabled afterwards.
 
+arch_prctl(ARCH_SHSTK_UNLOCK, unsigned long features)
+    Unlock features. 'features' is a mask of all features to unlock. All
+    bits set are processed, unset bits are ignored. Only works via ptrace.
+
 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 f13751c6bae4..0c95688cf58e 100644
--- a/arch/x86/include/uapi/asm/prctl.h
+++ b/arch/x86/include/uapi/asm/prctl.h
@@ -30,6 +30,7 @@
 #define ARCH_SHSTK_ENABLE		0x5001
 #define ARCH_SHSTK_DISABLE		0x5002
 #define ARCH_SHSTK_LOCK			0x5003
+#define ARCH_SHSTK_UNLOCK		0x5004
 
 /* ARCH_SHSTK_ features bits */
 #define ARCH_SHSTK_SHSTK		(1ULL <<  0)
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 4ddd7d9209e1..2be6e01fb144 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -921,6 +921,7 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
 	case ARCH_SHSTK_ENABLE:
 	case ARCH_SHSTK_DISABLE:
 	case ARCH_SHSTK_LOCK:
+	case ARCH_SHSTK_UNLOCK:
 		return shstk_prctl(task, option, arg2);
 	default:
 		ret = -EINVAL;
diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
index 5d91e653f77a..95579f7bace3 100644
--- a/arch/x86/kernel/shstk.c
+++ b/arch/x86/kernel/shstk.c
@@ -455,9 +455,14 @@ long shstk_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_SHSTK_UNLOCK && IS_ENABLED(CONFIG_CHECKPOINT_RESTORE)) {
+			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