[PATCH v5 38/39] x86/shstk: Add ARCH_SHSTK_UNLOCK

From: Rick Edgecombe <rick.p.edgecombe@intel.com>
To: x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
	linux-mm@kvack.org, linux-arch@vger.kernel.org,
	linux-api@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
	Andy Lutomirski <luto@kernel.org>,
	Balbir Singh <bsingharora@gmail.com>,
	Borislav Petkov <bp@alien8.de>,
	Cyrill Gorcunov <gorcunov@gmail.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Eugene Syromiatnikov <esyr@redhat.com>,
	Florian Weimer <fweimer@redhat.com>,
	"H . J . Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Kees Cook <keescook@chromium.org>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Nadav Amit <nadav.amit@gmail.com>,
	Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
	Peter Zijlstra <peterz@infradead.org>,
	Randy Dunlap <rdunlap@infradead.org>,
	Weijiang Yang <weijiang.yang@intel.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	John Allen <john.allen@amd.com>,
	kcc@google.com, eranian@google.com, rppt@kernel.org,
	jamorris@linux.microsoft.com, dethoma@microsoft.com,
	akpm@linux-foundation.org, Andrew.Cooper3@citrix.com,
	christina.schimpe@intel.com
Cc: rick.p.edgecombe@intel.com, Mike Rapoport <rppt@linux.ibm.com>
Subject: [PATCH v5 38/39] x86/shstk: Add ARCH_SHSTK_UNLOCK
Date: Thu, 19 Jan 2023 13:23:16 -0800	[thread overview]
Message-ID: <20230119212317.8324-39-rick.p.edgecombe@intel.com> (raw)
In-Reply-To: <20230119212317.8324-1-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 shadow stack operations, but restrict it being
called by the ptrace arch_pctl() interface.

Reviewed-by: Kees Cook <keescook@chromium.org>
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 f2e6f323cf68..e8ed5fc0f7ae 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 follows. 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 e31495668056..200efbbe5809 100644
--- a/arch/x86/include/uapi/asm/prctl.h
+++ b/arch/x86/include/uapi/asm/prctl.h
@@ -25,6 +25,7 @@
 #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 71094c8a305f..d368854fa9c4 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_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 07142e6f05f6..a639119a21c5 100644
--- a/arch/x86/kernel/shstk.c
+++ b/arch/x86/kernel/shstk.c
@@ -452,9 +452,14 @@ 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

