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Subject[PATCH v4 38/39] x86/shstk: Add ARCH_SHSTK_UNLOCK
Date
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 shadow stack 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>
---

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
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