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Subject[PATCH v2 1/8] x86/fsgsbase/64: Introduce FS/GS base helper functions
Date
With new helpers, FS/GS base access is centralized.
Eventually, when FSGSBASE instruction enabled, it will
be faster.

The helpers are used on ptrace APIs (PTRACE_ARCH_PRCTL,
PTRACE_SETREG, PTRACE_GETREG, etc). Idea is to keep
the FS/GS-update mechanism organized.

"inactive" GS base refers to base backed up at kernel
entries and of inactive (user) task's.

The bug that returns stale FS/GS base value (when index
is nonzero) is preserved and will be fixed by next
patch.

Based-on-code-from: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
---
arch/x86/include/asm/fsgsbase.h | 47 ++++++++++++++
arch/x86/kernel/process_64.c | 132 +++++++++++++++++++++++++++++-----------
arch/x86/kernel/ptrace.c | 28 +++------
3 files changed, 153 insertions(+), 54 deletions(-)
create mode 100644 arch/x86/include/asm/fsgsbase.h

diff --git a/arch/x86/include/asm/fsgsbase.h b/arch/x86/include/asm/fsgsbase.h
new file mode 100644
index 0000000..f00a8a6
--- /dev/null
+++ b/arch/x86/include/asm/fsgsbase.h
@@ -0,0 +1,47 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_FSGSBASE_H
+#define _ASM_FSGSBASE_H 1
+
+#ifndef __ASSEMBLY__
+
+#ifdef CONFIG_X86_64
+
+#include <asm/msr-index.h>
+
+/*
+ * Read/write a task's fsbase or gsbase. This returns the value that
+ * the FS/GS base would have (if the task were to be resumed). These
+ * work on current or on a different non-running task.
+ */
+unsigned long read_task_fsbase(struct task_struct *task);
+unsigned long read_task_gsbase(struct task_struct *task);
+int write_task_fsbase(struct task_struct *task, unsigned long fsbase);
+int write_task_gsbase(struct task_struct *task, unsigned long gsbase);
+
+/* Helper functions for reading/writing FS/GS base */
+
+static inline unsigned long read_fsbase(void)
+{
+ unsigned long fsbase;
+
+ rdmsrl(MSR_FS_BASE, fsbase);
+ return fsbase;
+}
+
+void write_fsbase(unsigned long fsbase);
+
+static inline unsigned long read_inactive_gsbase(void)
+{
+ unsigned long gsbase;
+
+ rdmsrl(MSR_KERNEL_GS_BASE, gsbase);
+ return gsbase;
+}
+
+void write_inactive_gsbase(unsigned long gsbase);
+
+#endif /* CONFIG_X86_64 */
+
+#endif /* __ASSEMBLY__ */
+
+#endif /* _ASM_FSGSBASE_H */
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 12bb445..ace0158 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -54,6 +54,7 @@
#include <asm/vdso.h>
#include <asm/intel_rdt_sched.h>
#include <asm/unistd.h>
+#include <asm/fsgsbase.h>
#ifdef CONFIG_IA32_EMULATION
/* Not included via unistd.h */
#include <asm/unistd_32_ia32.h>
@@ -278,6 +279,94 @@ static __always_inline void load_seg_legacy(unsigned short prev_index,
}
}

+void write_fsbase(unsigned long fsbase)
+{
+ /* set the selector to 0 to not confuse __switch_to */
+ loadseg(FS, 0);
+ wrmsrl(MSR_FS_BASE, fsbase);
+}
+
+void write_inactive_gsbase(unsigned long gsbase)
+{
+ /* set the selector to 0 to not confuse __switch_to */
+ loadseg(GS, 0);
+ wrmsrl(MSR_KERNEL_GS_BASE, gsbase);
+}
+
+unsigned long read_task_fsbase(struct task_struct *task)
+{
+ unsigned long fsbase;
+
+ if (task == current) {
+ fsbase = read_fsbase();
+ } else {
+ /*
+ * XXX: This will not behave as expected if called
+ * if fsindex != 0. This preserves an existing bug
+ * that will be fixed.
+ */
+ fsbase = task->thread.fsbase;
+ }
+
+ return fsbase;
+}
+
+unsigned long read_task_gsbase(struct task_struct *task)
+{
+ unsigned long gsbase;
+
+ if (task == current) {
+ gsbase = read_inactive_gsbase();
+ } else {
+ /*
+ * XXX: This will not behave as expected if called
+ * if gsindex != 0. Same bug preservation as above
+ * read_task_fsbase.
+ */
+ gsbase = task->thread.gsbase;
+ }
+
+ return gsbase;
+}
+
+int write_task_fsbase(struct task_struct *task, unsigned long fsbase)
+{
+ int cpu;
+
+ /*
+ * Not strictly needed for fs, but do it for symmetry
+ * with gs
+ */
+ if (unlikely(fsbase >= TASK_SIZE_MAX))
+ return -EPERM;
+
+ cpu = get_cpu();
+ task->thread.fsbase = fsbase;
+ if (task == current)
+ write_fsbase(fsbase);
+ task->thread.fsindex = 0;
+ put_cpu();
+
+ return 0;
+}
+
+int write_task_gsbase(struct task_struct *task, unsigned long gsbase)
+{
+ int cpu;
+
+ if (unlikely(gsbase >= TASK_SIZE_MAX))
+ return -EPERM;
+
+ cpu = get_cpu();
+ task->thread.gsbase = gsbase;
+ if (task == current)
+ write_inactive_gsbase(gsbase);
+ task->thread.gsindex = 0;
+ put_cpu();
+
+ return 0;
+}
+
int copy_thread_tls(unsigned long clone_flags, unsigned long sp,
unsigned long arg, struct task_struct *p, unsigned long tls)
{
@@ -618,54 +707,27 @@ static long prctl_map_vdso(const struct vdso_image *image, unsigned long addr)
long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
{
int ret = 0;
- int doit = task == current;
- int cpu;

switch (option) {
- case ARCH_SET_GS:
- if (arg2 >= TASK_SIZE_MAX)
- return -EPERM;
- cpu = get_cpu();
- task->thread.gsindex = 0;
- task->thread.gsbase = arg2;
- if (doit) {
- load_gs_index(0);
- ret = wrmsrl_safe(MSR_KERNEL_GS_BASE, arg2);
- }
- put_cpu();
+ case ARCH_SET_GS: {
+ ret = write_task_gsbase(task, arg2);
break;
- case ARCH_SET_FS:
- /* Not strictly needed for fs, but do it for symmetry
- with gs */
- if (arg2 >= TASK_SIZE_MAX)
- return -EPERM;
- cpu = get_cpu();
- task->thread.fsindex = 0;
- task->thread.fsbase = arg2;
- if (doit) {
- /* set the selector to 0 to not confuse __switch_to */
- loadsegment(fs, 0);
- ret = wrmsrl_safe(MSR_FS_BASE, arg2);
- }
- put_cpu();
+ }
+ case ARCH_SET_FS: {
+ ret = write_task_fsbase(task, arg2);
break;
+ }
case ARCH_GET_FS: {
unsigned long base;

- if (doit)
- rdmsrl(MSR_FS_BASE, base);
- else
- base = task->thread.fsbase;
+ base = read_task_fsbase(task);
ret = put_user(base, (unsigned long __user *)arg2);
break;
}
case ARCH_GET_GS: {
unsigned long base;

- if (doit)
- rdmsrl(MSR_KERNEL_GS_BASE, base);
- else
- base = task->thread.gsbase;
+ base = read_task_gsbase(task);
ret = put_user(base, (unsigned long __user *)arg2);
break;
}
diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index ed5c4cd..b2f0beb 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -39,6 +39,7 @@
#include <asm/hw_breakpoint.h>
#include <asm/traps.h>
#include <asm/syscall.h>
+#include <asm/fsgsbase.h>

#include "tls.h"

@@ -396,12 +397,11 @@ static int putreg(struct task_struct *child,
if (value >= TASK_SIZE_MAX)
return -EIO;
/*
- * When changing the segment base, use do_arch_prctl_64
- * to set either thread.fs or thread.fsindex and the
- * corresponding GDT slot.
+ * When changing the FS base, use the same
+ * mechanism as for do_arch_prctl_64
*/
if (child->thread.fsbase != value)
- return do_arch_prctl_64(child, ARCH_SET_FS, value);
+ return write_task_fsbase(child, value);
return 0;
case offsetof(struct user_regs_struct,gs_base):
/*
@@ -410,7 +410,7 @@ static int putreg(struct task_struct *child,
if (value >= TASK_SIZE_MAX)
return -EIO;
if (child->thread.gsbase != value)
- return do_arch_prctl_64(child, ARCH_SET_GS, value);
+ return write_task_gsbase(child, value);
return 0;
#endif
}
@@ -434,20 +434,10 @@ static unsigned long getreg(struct task_struct *task, unsigned long offset)
return get_flags(task);

#ifdef CONFIG_X86_64
- case offsetof(struct user_regs_struct, fs_base): {
- /*
- * XXX: This will not behave as expected if called on
- * current or if fsindex != 0.
- */
- return task->thread.fsbase;
- }
- case offsetof(struct user_regs_struct, gs_base): {
- /*
- * XXX: This will not behave as expected if called on
- * current or if fsindex != 0.
- */
- return task->thread.gsbase;
- }
+ case offsetof(struct user_regs_struct, fs_base):
+ return read_task_fsbase(task);
+ case offsetof(struct user_regs_struct, gs_base):
+ return read_task_gsbase(task);
#endif
}

--
2.7.4
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