x86/fsgsbase/64: Introduce FS/GS base helper functions

Introduce FS/GS base access functionality via <asm/fsgsbase.h>,
not yet used by anything directly.

Factor out task_seg_base() from x86/ptrace.c and rename it to
x86_fsgsbase_read_task() to make it part of the new helpers.

This will allow us to enhance FSGSBASE support and eventually enable
the FSBASE/GSBASE instructions.

An "inactive" GS base refers to a base saved at kernel entry
and being part of an inactive, non-running/stopped user-task.
(The typical ptrace model.)

Here are the new functions:

  x86_fsbase_read_task()
  x86_gsbase_read_task()
  x86_fsbase_write_task()
  x86_gsbase_write_task()
  x86_fsbase_read_cpu()
  x86_fsbase_write_cpu()
  x86_gsbase_read_cpu_inactive()
  x86_gsbase_write_cpu_inactive()

As an advantage of the unified namespace we can now see all FS/GSBASE
API use in the kernel via the following 'git grep' pattern:

  $ git grep x86_.*sbase

[ mingo: Wrote new changelog. ]

Based-on-code-from: Andy Lutomirski <luto@kernel.org>
Suggested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Markus T Metzger <markus.t.metzger@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Shankar <ravi.v.shankar@intel.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1537312139-5580-3-git-send-email-chang.seok.bae@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 124 insertions, 0 deletions

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index ea5ea850348d..2a53ff8d1baf 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>
@@ -286,6 +287,129 @@ static __always_inline void load_seg_legacy(unsigned short prev_index,
 	}
 }
 
+unsigned long x86_fsgsbase_read_task(struct task_struct *task,
+				     unsigned short selector)
+{
+	unsigned short idx = selector >> 3;
+	unsigned long base;
+
+	if (likely((selector & SEGMENT_TI_MASK) == 0)) {
+		if (unlikely(idx >= GDT_ENTRIES))
+			return 0;
+
+		/*
+		 * There are no user segments in the GDT with nonzero bases
+		 * other than the TLS segments.
+		 */
+		if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
+			return 0;
+
+		idx -= GDT_ENTRY_TLS_MIN;
+		base = get_desc_base(&task->thread.tls_array[idx]);
+	} else {
+#ifdef CONFIG_MODIFY_LDT_SYSCALL
+		struct ldt_struct *ldt;
+
+		/*
+		 * If performance here mattered, we could protect the LDT
+		 * with RCU.  This is a slow path, though, so we can just
+		 * take the mutex.
+		 */
+		mutex_lock(&task->mm->context.lock);
+		ldt = task->mm->context.ldt;
+		if (unlikely(idx >= ldt->nr_entries))
+			base = 0;
+		else
+			base = get_desc_base(ldt->entries + idx);
+		mutex_unlock(&task->mm->context.lock);
+#else
+		base = 0;
+#endif
+	}
+
+	return base;
+}
+
+void x86_fsbase_write_cpu(unsigned long fsbase)
+{
+	/*
+	 * Set the selector to 0 as a notion, that the segment base is
+	 * overwritten, which will be checked for skipping the segment load
+	 * during context switch.
+	 */
+	loadseg(FS, 0);
+	wrmsrl(MSR_FS_BASE, fsbase);
+}
+
+void x86_gsbase_write_cpu_inactive(unsigned long gsbase)
+{
+	/* Set the selector to 0 for the same reason as %fs above. */
+	loadseg(GS, 0);
+	wrmsrl(MSR_KERNEL_GS_BASE, gsbase);
+}
+
+unsigned long x86_fsbase_read_task(struct task_struct *task)
+{
+	unsigned long fsbase;
+
+	if (task == current)
+		fsbase = x86_fsbase_read_cpu();
+	else if (task->thread.fsindex == 0)
+		fsbase = task->thread.fsbase;
+	else
+		fsbase = x86_fsgsbase_read_task(task, task->thread.fsindex);
+
+	return fsbase;
+}
+
+unsigned long x86_gsbase_read_task(struct task_struct *task)
+{
+	unsigned long gsbase;
+
+	if (task == current)
+		gsbase = x86_gsbase_read_cpu_inactive();
+	else if (task->thread.gsindex == 0)
+		gsbase = task->thread.gsbase;
+	else
+		gsbase = x86_fsgsbase_read_task(task, task->thread.gsindex);
+
+	return gsbase;
+}
+
+int x86_fsbase_write_task(struct task_struct *task, unsigned long fsbase)
+{
+	/*
+	 * Not strictly needed for %fs, but do it for symmetry
+	 * with %gs
+	 */
+	if (unlikely(fsbase >= TASK_SIZE_MAX))
+		return -EPERM;
+
+	preempt_disable();
+	task->thread.fsbase = fsbase;
+	if (task == current)
+		x86_fsbase_write_cpu(fsbase);
+	task->thread.fsindex = 0;
+	preempt_enable();
+
+	return 0;
+}
+
+int x86_gsbase_write_task(struct task_struct *task, unsigned long gsbase)
+{
+	if (unlikely(gsbase >= TASK_SIZE_MAX))
+		return -EPERM;
+
+	preempt_disable();
+	task->thread.gsbase = gsbase;
+	if (task == current)
+		x86_gsbase_write_cpu_inactive(gsbase);
+	task->thread.gsindex = 0;
+	preempt_enable();
+
+	return 0;
+}
+
 int copy_thread_tls(unsigned long clone_flags, unsigned long sp,
 		unsigned long arg, struct task_struct *p, unsigned long tls)
 {