[PATCH 26/27] x86: debugctlmsr kprobes

[Roland McGrath <roland@redhat.com>]

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>
---
 arch/x86/kernel/kprobes_32.c |   15 +++++++++++++++
 arch/x86/kernel/kprobes_64.c |   15 +++++++++++++++
 2 files changed, 30 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kernel/kprobes_32.c b/arch/x86/kernel/kprobes_32.c
index d87a523..f151f06 100644
--- a/arch/x86/kernel/kprobes_32.c
+++ b/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;
 }
 
diff --git a/arch/x86/kernel/kprobes_64.c b/arch/x86/kernel/kprobes_64.c
index 3db3611..d3be418 100644
--- a/arch/x86/kernel/kprobes_64.c
+++ b/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*/
@@ -534,6 +547,8 @@ static void __kprobes resume_execution(struct kprobe *p,
 	} else {
 		regs->rip = orig_rip + (regs->rip - copy_rip);
 	}
+
+	restore_btf();
 }
 
 int __kprobes post_kprobe_handler(struct pt_regs *regs)
-
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