[PATCH v9 42/42] x86/shstk: Add ARCH_SHSTK_STATUS

From: Rick Edgecombe <rick.p.edgecombe@intel.com>
Date: 2023-06-13 00:20:49
Also in: linux-arch, linux-doc, linux-mm, lkml
Subsystem: documentation, the rest, x86 architecture (32-bit and 64-bit) · Maintainers: Jonathan Corbet, Linus Torvalds, Thomas Gleixner, Ingo Molnar, Borislav Petkov, Dave Hansen

CRIU and GDB need to get the current shadow stack and WRSS enablement
status. This information is already available via /proc/pid/status, but
this is inconvenient for CRIU because it involves parsing the text output
in an area of the code where this is difficult. Provide a status
arch_prctl(), ARCH_SHSTK_STATUS for retrieving the status. Have arg2 be a
userspace address, and make the new arch_prctl simply copy the features
out to userspace.

Suggested-by: Mike Rapoport <rppt@kernel.org>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Kees Cook <redacted>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Tested-by: Pengfei Xu <redacted>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Kees Cook <redacted>
---
 Documentation/arch/x86/shstk.rst  | 6 ++++++
 arch/x86/include/asm/shstk.h      | 2 +-
 arch/x86/include/uapi/asm/prctl.h | 1 +
 arch/x86/kernel/process_64.c      | 1 +
 arch/x86/kernel/shstk.c           | 8 +++++++-
 5 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/Documentation/arch/x86/shstk.rst b/Documentation/arch/x86/shstk.rst
index f3553cc8c758..60260e809baf 100644
--- a/Documentation/arch/x86/shstk.rst
+++ b/Documentation/arch/x86/shstk.rst

@@ -79,6 +79,11 @@ 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.
 
+arch_prctl(ARCH_SHSTK_STATUS, unsigned long addr)
+    Copy the currently enabled features to the address passed in addr. The
+    features are described using the bits passed into the others in
+    'features'.
+
 The return values are as follows. On success, return 0. On error, errno can
 be::

@@ -86,6 +91,7 @@ be::
         -ENOTSUPP if the feature is not supported by the hardware or
          kernel.
         -EINVAL arguments (non existing feature, etc)
+        -EFAULT if could not copy information back to userspace
 
 The feature's bits supported are::

diff --git a/arch/x86/include/asm/shstk.h b/arch/x86/include/asm/shstk.h
index ecb23a8ca47d..42fee8959df7 100644
--- a/arch/x86/include/asm/shstk.h
+++ b/arch/x86/include/asm/shstk.h

@@ -14,7 +14,7 @@ struct thread_shstk {
 	u64	size;
 };
 
-long shstk_prctl(struct task_struct *task, int option, unsigned long features);
+long shstk_prctl(struct task_struct *task, int option, unsigned long arg2);
 void reset_thread_features(void);
 unsigned long shstk_alloc_thread_stack(struct task_struct *p, unsigned long clone_flags,
 				       unsigned long stack_size);

diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
index 3189c4a96468..384e2cc6ac19 100644
--- a/arch/x86/include/uapi/asm/prctl.h
+++ b/arch/x86/include/uapi/asm/prctl.h

@@ -34,6 +34,7 @@
 #define ARCH_SHSTK_DISABLE		0x5002
 #define ARCH_SHSTK_LOCK			0x5003
 #define ARCH_SHSTK_UNLOCK		0x5004
+#define ARCH_SHSTK_STATUS		0x5005
 
 /* 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 e6db21c470aa..33b268747bb7 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c

@@ -900,6 +900,7 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
 	case ARCH_SHSTK_DISABLE:
 	case ARCH_SHSTK_LOCK:
 	case ARCH_SHSTK_UNLOCK:
+	case ARCH_SHSTK_STATUS:
 		return shstk_prctl(task, option, arg2);
 	default:
 		ret = -EINVAL;

diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
index d43b7a9c57ce..b26810c7cd1c 100644
--- a/arch/x86/kernel/shstk.c
+++ b/arch/x86/kernel/shstk.c

@@ -482,8 +482,14 @@ SYSCALL_DEFINE3(map_shadow_stack, unsigned long, addr, unsigned long, size, unsi
 	return alloc_shstk(addr, aligned_size, size, set_tok);
 }
 
-long shstk_prctl(struct task_struct *task, int option, unsigned long features)
+long shstk_prctl(struct task_struct *task, int option, unsigned long arg2)
 {
+	unsigned long features = arg2;
+
+	if (option == ARCH_SHSTK_STATUS) {
+		return put_user(task->thread.features, (unsigned long __user *)arg2);
+	}
+
 	if (option == ARCH_SHSTK_LOCK) {
 		task->thread.features_locked |= features;
 		return 0;

-- 
2.34.1

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