[REVIEW][PATCH 02/26] signal/ptrace: Simplify and fix PTRACE_KILL

["Eric W. Biederman" <ebiederm@xmission.com>]

Since PTRACE_KILL was introduced in 1.1.78 it has only worked if the
process is stopped in do_signal.  On a ptraced but non-stopped process
PTRACE_KILL has always returned success and done nothing.

Separate the noop case of PTRACE_KILL from the case where it does
nothing.  This fixes the fact that taking sighand lock in
ptrace_resume is not safe if the process could be in the middle of
exec or do_exit.  The current test for child->state is insufficient to
prevent that race.

With the code explicitly implementing the noop people maintaining
ptrace no longer need to worry what happens in PTRACE_KILL if the
process is not stopped.

The alternative fix is to change the implementation of PTRACE_KILL
to just be send_sig(SIGKILL, child, 1);  But I don't know if anything
depends on the current documented behavior.

Cc: Oleg Nesterov <oleg@redhat.com>
Cc: stable@vger.kernel.org
Fixes: b72c186999e6 ("ptrace: fix race between ptrace_resume() and wait_task_stopped()")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
 kernel/ptrace.c | 43 ++++++++++++++++++++++++++-----------------
 1 file changed, 26 insertions(+), 17 deletions(-)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 6f357f4fc859..5d6ff7040863 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -212,15 +212,18 @@ static void ptrace_unfreeze_traced(struct task_struct *task)
  *
  * Check whether @child is being ptraced by %current and ready for further
  * ptrace operations.  If @ignore_state is %false, @child also should be in
- * %TASK_TRACED state and on return the child is guaranteed to be traced
- * and not executing.  If @ignore_state is %true, @child can be in any
- * state.
+ * %TASK_TRACED state and on succesful return the child is guaranteed to be
+ * traced and not executing.  If @ignore_state is %true, @child can be in
+ * any state on succesful return.
  *
  * CONTEXT:
  * Grabs and releases tasklist_lock and @child->sighand->siglock.
  *
  * RETURNS:
- * 0 on success, -ESRCH if %child is not ready.
+ * 0 on success,
+ * -ESRCH if %child is not traced
+ * -EAGAIN if %child can not be frozen
+ * -EBUSY if the wait for %child fails
  */
 static int ptrace_check_attach(struct task_struct *child, bool ignore_state)
 {
@@ -240,6 +243,7 @@ static int ptrace_check_attach(struct task_struct *child, bool ignore_state)
 		 * child->sighand can't be NULL, release_task()
 		 * does ptrace_unlink() before __exit_signal().
 		 */
+		ret = -EAGAIN;
 		if (ignore_state || ptrace_freeze_traced(child))
 			ret = 0;
 	}
@@ -253,7 +257,7 @@ static int ptrace_check_attach(struct task_struct *child, bool ignore_state)
 			 * so we should not worry about leaking __TASK_TRACED.
 			 */
 			WARN_ON(child->state == __TASK_TRACED);
-			ret = -ESRCH;
+			ret = -EBUSY;
 		}
 	}
 
@@ -1074,8 +1078,6 @@ int ptrace_request(struct task_struct *child, long request,
 		return ptrace_resume(child, request, data);
 
 	case PTRACE_KILL:
-		if (child->exit_state)	/* already dead */
-			return 0;
 		return ptrace_resume(child, request, SIGKILL);
 
 #ifdef CONFIG_HAVE_ARCH_TRACEHOOK
@@ -1147,14 +1149,17 @@ SYSCALL_DEFINE4(ptrace, long, request, long, pid, unsigned long, addr,
 		goto out_put_task_struct;
 	}
 
-	ret = ptrace_check_attach(child, request == PTRACE_KILL ||
-				  request == PTRACE_INTERRUPT);
-	if (ret < 0)
-		goto out_put_task_struct;
-
-	ret = arch_ptrace(child, request, addr, data);
-	if (ret || request != PTRACE_DETACH)
-		ptrace_unfreeze_traced(child);
+	ret = ptrace_check_attach(child, request == PTRACE_INTERRUPT);
+	if (!ret) {
+		ret = arch_ptrace(child, request, addr, data);
+		if (ret || request != PTRACE_DETACH)
+			ptrace_unfreeze_traced(child);
+	}
+	/* PTRACE_KILL is a noop when not attached */
+	else if ((request == PTRACE_KILL) && (ret != -ESRCH))
+		ret = 0;
+	else
+		ret = -ESRCH;
 
  out_put_task_struct:
 	put_task_struct(child);
@@ -1292,13 +1297,17 @@ COMPAT_SYSCALL_DEFINE4(ptrace, compat_long_t, request, compat_long_t, pid,
 		goto out_put_task_struct;
 	}
 
-	ret = ptrace_check_attach(child, request == PTRACE_KILL ||
-				  request == PTRACE_INTERRUPT);
+	ret = ptrace_check_attach(child, request == PTRACE_INTERRUPT);
 	if (!ret) {
 		ret = compat_arch_ptrace(child, request, addr, data);
 		if (ret || request != PTRACE_DETACH)
 			ptrace_unfreeze_traced(child);
 	}
+	/* PTRACE_KILL is a noop when not attached */
+	else if ((request == PTRACE_KILL) && (ret != -ESRCH))
+		ret = 0;
+	else
+		ret = -ESRCH;
 
  out_put_task_struct:
 	put_task_struct(child);
-- 
2.21.0