x86: debugctlmsr kprobes

This adjusts the x86 kprobes implementation to cope with per-thread
MSR_IA32_DEBUGCTLMSR being set for user mode.  I haven't delved deep
enough into the kprobes code to be really sure this covers all the
cases where the user-mode BTF setting needs to be cleared or restored.
It looks about right to me.
Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/kprobes_32.c

@@ -217,8 +217,21 @@ static void __kprobes set_current_kprobe(struct kprobe *p, struct pt_regs *regs,
 		kcb->kprobe_saved_eflags &= ~IF_MASK;
 }
 
+static __always_inline void clear_btf(void)
+{
+	if (test_thread_flag(TIF_DEBUGCTLMSR))
+		wrmsr(MSR_IA32_DEBUGCTLMSR, 0, 0);
+}
+
+static __always_inline void restore_btf(void)
+{
+	if (test_thread_flag(TIF_DEBUGCTLMSR))
+		wrmsr(MSR_IA32_DEBUGCTLMSR, current->thread.debugctlmsr, 0);
+}
+
 static void __kprobes prepare_singlestep(struct kprobe *p, struct pt_regs *regs)
 {
+	clear_btf();
 	regs->eflags |= TF_MASK;
 	regs->eflags &= ~IF_MASK;
 	/*single step inline if the instruction is an int3*/
@@ -542,6 +555,8 @@ static void __kprobes resume_execution(struct kprobe *p,
 	regs->eip = orig_eip + (regs->eip - copy_eip);
 
 no_change:
+	restore_btf();
+
 	return;
 }
 

arch/x86/kernel/kprobes_64.c

@@ -256,8 +256,21 @@ static void __kprobes set_current_kprobe(struct kprobe *p, struct pt_regs *regs,
 		kcb->kprobe_saved_rflags &= ~IF_MASK;
 }
 
+static __always_inline void clear_btf(void)
+{
+	if (test_thread_flag(TIF_DEBUGCTLMSR))
+		wrmsrl(MSR_IA32_DEBUGCTLMSR, 0);
+}
+
+static __always_inline void restore_btf(void)
+{
+	if (test_thread_flag(TIF_DEBUGCTLMSR))
+		wrmsrl(MSR_IA32_DEBUGCTLMSR, current->thread.debugctlmsr);
+}
+
 static void __kprobes prepare_singlestep(struct kprobe *p, struct pt_regs *regs)
 {
+	clear_btf();
 	regs->eflags |= TF_MASK;
 	regs->eflags &= ~IF_MASK;
 	/*single step inline if the instruction is an int3*/
@@ -527,6 +540,7 @@ static void __kprobes resume_execution(struct kprobe *p,
 
 	regs->rip = orig_rip + (regs->rip - copy_rip);
 no_change:
+	restore_btf();
 
 	return;
 }