1 files changed, 29 insertions, 6 deletions

diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c
index 92c20fee6781..e8b9863ef8c4 100644
--- a/arch/x86/kernel/step.c
+++ b/arch/x86/kernel/step.c
@@ -105,6 +105,20 @@ static int is_setting_trap_flag(struct task_struct *child, struct pt_regs *regs)
 static int enable_single_step(struct task_struct *child)
 {
         struct pt_regs *regs = task_pt_regs(child);
+        unsigned long oflags;
+
+        /*
+         * If we stepped into a sysenter/syscall insn, it trapped in
+         * kernel mode; do_debug() cleared TF and set TIF_SINGLESTEP.
+         * If user-mode had set TF itself, then it's still clear from
+         * do_debug() and we need to set it again to restore the user
+         * state so we don't wrongly set TIF_FORCED_TF below.
+         * If enable_single_step() was used last and that is what
+         * set TIF_SINGLESTEP, then both TF and TIF_FORCED_TF are
+         * already set and our bookkeeping is fine.
+         */
+        if (unlikely(test_tsk_thread_flag(child, TIF_SINGLESTEP)))
+                regs->flags |= X86_EFLAGS_TF;
 
         /*
          * Always set TIF_SINGLESTEP - this guarantees that
@@ -113,11 +127,7 @@ static int enable_single_step(struct task_struct *child)
          */
         set_tsk_thread_flag(child, TIF_SINGLESTEP);
 
-        /*
-         * If TF was already set, don't do anything else
-         */
-        if (regs->flags & X86_EFLAGS_TF)
-                return 0;
+        oflags = regs->flags;
 
         /* Set TF on the kernel stack.. */
         regs->flags |= X86_EFLAGS_TF;
@@ -126,9 +136,22 @@ static int enable_single_step(struct task_struct *child)
          * ..but if TF is changed by the instruction we will trace,
          * don't mark it as being "us" that set it, so that we
          * won't clear it by hand later.
+         *
+         * Note that if we don't actually execute the popf because
+         * of a signal arriving right now or suchlike, we will lose
+         * track of the fact that it really was "us" that set it.
          */
-        if (is_setting_trap_flag(child, regs))
+        if (is_setting_trap_flag(child, regs)) {
+                clear_tsk_thread_flag(child, TIF_FORCED_TF);
                 return 0;
+        }
+
+        /*
+         * If TF was already set, check whether it was us who set it.
+         * If not, we should never attempt a block step.
+         */
+        if (oflags & X86_EFLAGS_TF)
+                return test_tsk_thread_flag(child, TIF_FORCED_TF);
 
         set_tsk_thread_flag(child, TIF_FORCED_TF);