[PATCH v3 36/37] x86/cet/shstk: Add ARCH_CET_UNLOCK

Rick Edgecombe
2022-11-04T22:46:29Z

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.

Tested-by: Pengfei Xu <pengfei.xu@intel.com>
Tested-by: John Allen <john.allen@amd.com>
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>
---

v3:
- Depend on CONFIG_CHECKPOINT_RESTORE (Kees)

Documentation/x86/cet.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/cet.rst b/Documentation/x86/cet.rst
index b56811566531..f69cafb1feff 100644
--- a/Documentation/x86/cet.rst
+++ b/Documentation/x86/cet.rst
@@ -66,6 +66,10 @@ arch_prctl(ARCH_CET_LOCK, unsigned int 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_CET_UNLOCK, unsigned int features)
+ Unlock features. 'features' is a mask of all features to unlock. All
+ bits set are processed, unset bits are ignored.
+
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 5f1d3181e4a1..0c37fd0ad8d9 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 0x5001
#define ARCH_CET_DISABLE 0x5002
#define ARCH_CET_LOCK 0x5003
+#define ARCH_CET_UNLOCK 0x5004

/* ARCH_CET_ features bits */
#define CET_SHSTK (1ULL << 0)
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 17fec059317c..03bc16c9cc19 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -835,6 +835,7 @@ 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 71620b77a654..bed7032d35f2 100644
--- a/arch/x86/kernel/shstk.c
+++ b/arch/x86/kernel/shstk.c
@@ -450,9 +450,14 @@ 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 && 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