[PATCH] signal: don't always leave task frozen after ptrace_stop()

[Roman Gushchin]

The ptrace_stop() function contains the cgroup_enter_frozen() call,
but no cgroup_leave_frozen(). When ptrace_stop() is called from the
do_jobctl_trap() path, it's correct, because corresponding
cgroup_leave_frozen() calls in get_signal() will guarantee that
the task won't leave the signal handler loop frozen.

However, if ptrace_stop() is called from ptrace_signal() or
ptrace_notify(), there is no such guarantee, and the task may leave
with the frozen bit set.

It leads to the regression, reported by Alex Xu. Write system call
gets mistakenly interrupted by fake TIF_SIGPENDING, which is set
by recalc_sigpending_tsk() because of the set frozen bit.

The regression can be reproduced by stracing the following simple
program:

  #include <unistd.h>

  int main() {
      write(1, "a", 1);
      return 0;
  }

An attempt to run strace ./a.out leads to the infinite loop:
  [ pre-main omitted ]
  write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
  write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
  write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
  write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
  write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
  write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
  [ repeats forever ]

With this patch applied, it works as expected:
  [ pre-main omitted ]
  write(1, "a", 1)                        = 1
  exit_group(0)                           = ?
  +++ exited with 0 +++

Reported-by: Alex Xu <alex_y_xu@xxxxxxxx>
Fixes: 76f969e8948d ("cgroup: cgroup v2 freezer")
Signed-off-by: Roman Gushchin <guro@xxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
---
 kernel/signal.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 8607b11ff936..f12abbda4c4b 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2016,7 +2016,8 @@ static bool sigkill_pending(struct task_struct *tsk)
  * If we actually decide not to stop at all because the tracer
  * is gone, we keep current->exit_code unless clear_code.
  */
-static void ptrace_stop(int exit_code, int why, int clear_code, kernel_siginfo_t *info)
+static void ptrace_stop(int exit_code, int why, int clear_code,
+			kernel_siginfo_t *info, bool may_remain_frozen)
 	__releases(&current->sighand->siglock)
 	__acquires(&current->sighand->siglock)
 {
@@ -2112,6 +2113,8 @@ static void ptrace_stop(int exit_code, int why, int clear_code, kernel_siginfo_t
 		preempt_enable_no_resched();
 		cgroup_enter_frozen();
 		freezable_schedule();
+		if (!may_remain_frozen)
+			cgroup_leave_frozen(true);
 	} else {
 		/*
 		 * By the time we got the lock, our tracer went away.
@@ -2152,7 +2155,8 @@ static void ptrace_stop(int exit_code, int why, int clear_code, kernel_siginfo_t
 	recalc_sigpending_tsk(current);
 }
 
-static void ptrace_do_notify(int signr, int exit_code, int why)
+static void ptrace_do_notify(int signr, int exit_code, int why,
+			     bool may_remain_frozen)
 {
 	kernel_siginfo_t info;
 
@@ -2163,7 +2167,7 @@ static void ptrace_do_notify(int signr, int exit_code, int why)
 	info.si_uid = from_kuid_munged(current_user_ns(), current_uid());
 
 	/* Let the debugger run.  */
-	ptrace_stop(exit_code, why, 1, &info);
+	ptrace_stop(exit_code, why, 1, &info, may_remain_frozen);
 }
 
 void ptrace_notify(int exit_code)
@@ -2173,7 +2177,7 @@ void ptrace_notify(int exit_code)
 		task_work_run();
 
 	spin_lock_irq(&current->sighand->siglock);
-	ptrace_do_notify(SIGTRAP, exit_code, CLD_TRAPPED);
+	ptrace_do_notify(SIGTRAP, exit_code, CLD_TRAPPED, false);
 	spin_unlock_irq(&current->sighand->siglock);
 }
 
@@ -2328,10 +2332,10 @@ static void do_jobctl_trap(void)
 			signr = SIGTRAP;
 		WARN_ON_ONCE(!signr);
 		ptrace_do_notify(signr, signr | (PTRACE_EVENT_STOP << 8),
-				 CLD_STOPPED);
+				 CLD_STOPPED, true);
 	} else {
 		WARN_ON_ONCE(!signr);
-		ptrace_stop(signr, CLD_STOPPED, 0, NULL);
+		ptrace_stop(signr, CLD_STOPPED, 0, NULL, true);
 		current->exit_code = 0;
 	}
 }
@@ -2385,7 +2389,7 @@ static int ptrace_signal(int signr, kernel_siginfo_t *info)
 	 * comment in dequeue_signal().
 	 */
 	current->jobctl |= JOBCTL_STOP_DEQUEUED;
-	ptrace_stop(signr, CLD_TRAPPED, 0, info);
+	ptrace_stop(signr, CLD_TRAPPED, 0, info, false);
 
 	/* We're back.  Did the debugger cancel the sig?  */
 	signr = current->exit_code;
-- 
2.20.1