[PATCH v10 28/28] x86/arch_prctl: ARCH_GET_FEATURES_WITH_KERNEL_ASSISTANCE

[Chang S. Bae]

arch_prctl(ARCH_GET_FEATURES_WITH_KERNEL_ASSISTANCE, u64 *bitmask)
    Return the bitmask of the kernel-supported features. If XSAVE is
    disabled, the bitmask indicates only legacy states.

Suggested-by: Borislav Petkov <bp@xxxxxxx>
Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
---
Changes from v9:
* Add as a new patch. (Thiago Macieira and Borislav Petkov)
---
 arch/x86/include/asm/fpu/xstate.h |  1 +
 arch/x86/include/uapi/asm/prctl.h | 23 ++++++++---------
 arch/x86/kernel/fpu/xstate.c      | 41 +++++++++++++++++++------------
 arch/x86/kernel/process.c         |  2 ++
 4 files changed, 40 insertions(+), 27 deletions(-)

diff --git a/arch/x86/include/asm/fpu/xstate.h b/arch/x86/include/asm/fpu/xstate.h
index 013478c1aa42..4aa7049cc079 100644
--- a/arch/x86/include/asm/fpu/xstate.h
+++ b/arch/x86/include/asm/fpu/xstate.h
@@ -194,6 +194,7 @@ static inline bool dynamic_state_permitted(struct task_struct *tsk, u64 state_ma
 }
 
 void reset_dynamic_state_perm(struct task_struct *tsk);
+u64 get_features_mask_uabi(void);
 long set_group_state_perm(struct task_struct *tsk, u64 state_perm);
 u64 get_group_state_perm(struct task_struct *tsk);
 
diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
index c73e141ce90a..6912d5fe85f3 100644
--- a/arch/x86/include/uapi/asm/prctl.h
+++ b/arch/x86/include/uapi/asm/prctl.h
@@ -2,19 +2,20 @@
 #ifndef _ASM_X86_PRCTL_H
 #define _ASM_X86_PRCTL_H
 
-#define ARCH_SET_GS		0x1001
-#define ARCH_SET_FS		0x1002
-#define ARCH_GET_FS		0x1003
-#define ARCH_GET_GS		0x1004
+#define ARCH_SET_GS					0x1001
+#define ARCH_SET_FS					0x1002
+#define ARCH_GET_FS					0x1003
+#define ARCH_GET_GS					0x1004
 
-#define ARCH_GET_CPUID		0x1011
-#define ARCH_SET_CPUID		0x1012
+#define ARCH_GET_CPUID					0x1011
+#define ARCH_SET_CPUID					0x1012
 
-#define ARCH_SET_STATE_ENABLE	0x1021
-#define ARCH_GET_STATE_ENABLE	0x1022
+#define ARCH_GET_FEATURES_WITH_KERNEL_ASSISTANCE	0x1020
+#define ARCH_SET_STATE_ENABLE				0x1021
+#define ARCH_GET_STATE_ENABLE				0x1022
 
-#define ARCH_MAP_VDSO_X32	0x2001
-#define ARCH_MAP_VDSO_32	0x2002
-#define ARCH_MAP_VDSO_64	0x2003
+#define ARCH_MAP_VDSO_X32				0x2001
+#define ARCH_MAP_VDSO_32				0x2002
+#define ARCH_MAP_VDSO_64				0x2003
 
 #endif /* _ASM_X86_PRCTL_H */
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 2c62872e4fef..6622366d51ae 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -1297,6 +1297,25 @@ int realloc_xstate_buffer(struct fpu *fpu, u64 mask)
 	return 0;
 }
 
+/**
+ * get_features_mask_uabi - Get feature list that the kernel supports
+ * Return:	A bitmap to indicate which state is available
+ *
+ */
+u64 get_features_mask_uabi(void)
+{
+	if (!cpu_feature_enabled(X86_FEATURE_FPU))
+		return 0;
+
+	if (use_xsave())
+		return xfeatures_mask_uabi();
+
+	if (use_fxsr())
+		return XFEATURE_MASK_FPSSE;
+
+	return XFEATURE_MASK_FP;
+}
+
 /**
  * set_group_state_perm - Set a per-process permission for dynamic user
  *			  states.
@@ -1306,16 +1325,7 @@ int realloc_xstate_buffer(struct fpu *fpu, u64 mask)
  */
 long set_group_state_perm(struct task_struct *tsk, u64 state_perm)
 {
-	u64 features_mask;
-
-	if (!cpu_feature_enabled(X86_FEATURE_FPU))
-		features_mask = 0;
-	else if (use_xsave())
-		features_mask = xfeatures_mask_uabi();
-	else if (use_fxsr())
-		features_mask = XFEATURE_MASK_FPSSE;
-	else
-		features_mask = XFEATURE_MASK_FP;
+	u64 features_mask = get_features_mask_uabi();
 
 	if (state_perm & ~features_mask)
 		return -EINVAL;
@@ -1365,17 +1375,16 @@ void reset_dynamic_state_perm(struct task_struct *tsk)
  */
 u64 get_group_state_perm(struct task_struct *tsk)
 {
+	u64 mask = get_features_mask_uabi();
+
 	if (!cpu_feature_enabled(X86_FEATURE_FPU))
 		return 0;
 
 	if (use_xsave())
-		return (xfeatures_mask_uabi() & ~xfeatures_mask_user_dynamic) |
+		return (mask & ~xfeatures_mask_user_dynamic) |
 		       get_group_dynamic_state_perm(tsk);
-
-	if (use_fxsr())
-		return XFEATURE_MASK_FPSSE;
-
-	return XFEATURE_MASK_FP;
+	else
+		return mask;
 }
 
 static void copy_feature(bool from_xstate, struct membuf *to, void *xstate,
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index beab1bf76d1a..544b0c4f3aed 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -1023,6 +1023,8 @@ long do_arch_prctl_common(struct task_struct *task, int option,
 		return get_cpuid_mode();
 	case ARCH_SET_CPUID:
 		return set_cpuid_mode(task, arg2);
+	case ARCH_GET_FEATURES_WITH_KERNEL_ASSISTANCE:
+		return put_user(get_features_mask_uabi(), (unsigned long __user *)arg2);
 	case ARCH_SET_STATE_ENABLE:
 		return set_group_state_perm(task, arg2);
 	case ARCH_GET_STATE_ENABLE:
-- 
2.17.1