[PATCH v4 10/12] ptrace: Don't change __state

Eric W. Biederman
2022-05-05T18:26:43Z

Stop playing with tsk->__state to remove TASK_WAKEKILL while a ptrace
command is executing.

Instead remove TASK_WAKEKILL from the definition of TASK_TRACED, and
implement a new jobctl flag TASK_PTRACE_FROZEN. This new flag is set
in jobctl_freeze_task and cleared when ptrace_stop is awoken or in
jobctl_unfreeze_task (when ptrace_stop remains asleep).

In signal_wake_up add __TASK_TRACED to state along with TASK_WAKEKILL
when the wake up is for a fatal signal. Skip adding __TASK_TRACED
when TASK_PTRACE_FROZEN is not set. This has the same effect as
changing TASK_TRACED to __TASK_TRACED as all of the wake_ups that use
TASK_KILLABLE go through signal_wake_up.

Handle a ptrace_stop being called with a pending fatal signal.
Previously it would have been handled by schedule simply failing to
sleep. As TASK_WAKEKILL is no longer part of TASK_TRACED schedule
will sleep with a fatal_signal_pending. The code in signal_wake_up
guarantees that the code will be awaked by any fatal signal that
codes after TASK_TRACED is set.

Previously the __state value of __TASK_TRACED was changed to
TASK_RUNNING when woken up or back to TASK_TRACED when the code was
left in ptrace_stop. Now when woken up ptrace_stop now clears
JOBCTL_PTRACE_FROZEN and when left sleeping ptrace_unfreezed_traced
clears JOBCTL_PTRACE_FROZEN.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
include/linux/sched.h | 2 +-
include/linux/sched/jobctl.h | 2 ++
include/linux/sched/signal.h | 5 +++--
kernel/ptrace.c | 21 ++++++++-------------
kernel/sched/core.c | 5 +----
kernel/signal.c | 14 ++++++--------
6 files changed, 21 insertions(+), 28 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index d5e3c00b74e1..610f2fdb1e2c 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -103,7 +103,7 @@ struct task_group;
/* Convenience macros for the sake of set_current_state: */
#define TASK_KILLABLE (TASK_WAKEKILL | TASK_UNINTERRUPTIBLE)
#define TASK_STOPPED (TASK_WAKEKILL | __TASK_STOPPED)
-#define TASK_TRACED (TASK_WAKEKILL | __TASK_TRACED)
+#define TASK_TRACED __TASK_TRACED

#define TASK_IDLE (TASK_UNINTERRUPTIBLE | TASK_NOLOAD)

diff --git a/include/linux/sched/jobctl.h b/include/linux/sched/jobctl.h
index fa067de9f1a9..d556c3425963 100644
--- a/include/linux/sched/jobctl.h
+++ b/include/linux/sched/jobctl.h
@@ -19,6 +19,7 @@ struct task_struct;
#define JOBCTL_TRAPPING_BIT 21 /* switching to TRACED */
#define JOBCTL_LISTENING_BIT 22 /* ptracer is listening for events */
#define JOBCTL_TRAP_FREEZE_BIT 23 /* trap for cgroup freezer */
+#define JOBCTL_PTRACE_FROZEN_BIT 24 /* frozen for ptrace */

#define JOBCTL_STOP_DEQUEUED (1UL << JOBCTL_STOP_DEQUEUED_BIT)
#define JOBCTL_STOP_PENDING (1UL << JOBCTL_STOP_PENDING_BIT)
@@ -28,6 +29,7 @@ struct task_struct;
#define JOBCTL_TRAPPING (1UL << JOBCTL_TRAPPING_BIT)
#define JOBCTL_LISTENING (1UL << JOBCTL_LISTENING_BIT)
#define JOBCTL_TRAP_FREEZE (1UL << JOBCTL_TRAP_FREEZE_BIT)
+#define JOBCTL_PTRACE_FROZEN (1UL << JOBCTL_PTRACE_FROZEN_BIT)

#define JOBCTL_TRAP_MASK (JOBCTL_TRAP_STOP | JOBCTL_TRAP_NOTIFY)
#define JOBCTL_PENDING_MASK (JOBCTL_STOP_PENDING | JOBCTL_TRAP_MASK)
diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index 3c8b34876744..e66948abbee4 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -435,9 +435,10 @@ extern void calculate_sigpending(void);

extern void signal_wake_up_state(struct task_struct *t, unsigned int state);

-static inline void signal_wake_up(struct task_struct *t, bool resume)
+static inline void signal_wake_up(struct task_struct *t, bool fatal)
{
- signal_wake_up_state(t, resume ? TASK_WAKEKILL : 0);
+ fatal = fatal && !(t->jobctl & JOBCTL_PTRACE_FROZEN);
+ signal_wake_up_state(t, fatal ? TASK_WAKEKILL | __TASK_TRACED : 0);
}
static inline void ptrace_signal_wake_up(struct task_struct *t, bool resume)
{
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 05953ac9f7bd..83ed28262708 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -197,7 +197,7 @@ static bool ptrace_freeze_traced(struct task_struct *task)
spin_lock_irq(&task->sighand->siglock);
if (task_is_traced(task) && !looks_like_a_spurious_pid(task) &&
!__fatal_signal_pending(task)) {
- WRITE_ONCE(task->__state, __TASK_TRACED);
+ task->jobctl |= JOBCTL_PTRACE_FROZEN;
ret = true;
}
spin_unlock_irq(&task->sighand->siglock);
@@ -207,23 +207,19 @@ static bool ptrace_freeze_traced(struct task_struct *task)

static void ptrace_unfreeze_traced(struct task_struct *task)
{
- if (READ_ONCE(task->__state) != __TASK_TRACED)
- return;
-
- WARN_ON(!task->ptrace || task->parent != current);
+ unsigned long flags;

/*
- * PTRACE_LISTEN can allow ptrace_trap_notify to wake us up remotely.
- * Recheck state under the lock to close this race.
+ * The child may be awake and may have cleared
+ * JOBCTL_PTRACE_FROZEN (see ptrace_resume). The child will
+ * not set JOBCTL_PTRACE_FROZEN or enter __TASK_TRACED anew.
*/
- spin_lock_irq(&task->sighand->siglock);
- if (READ_ONCE(task->__state) = __TASK_TRACED) {
+ if (lock_task_sighand(task, &flags)) {
+ task->jobctl &= ~JOBCTL_PTRACE_FROZEN;
if (__fatal_signal_pending(task))
wake_up_state(task, __TASK_TRACED);
- else
- WRITE_ONCE(task->__state, TASK_TRACED);
+ unlock_task_sighand(task, &flags);
}
- spin_unlock_irq(&task->sighand->siglock);
}

/**
@@ -256,7 +252,6 @@ static int ptrace_check_attach(struct task_struct *child, bool ignore_state)
*/
read_lock(&tasklist_lock);
if (child->ptrace && child->parent = current) {
- WARN_ON(READ_ONCE(child->__state) = __TASK_TRACED);
/*
* child->sighand can't be NULL, release_task()
* does ptrace_unlink() before __exit_signal().
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d575b4914925..3c351707e830 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6304,10 +6304,7 @@ static void __sched notrace __schedule(unsigned int sched_mode)

/*
* We must load prev->state once (task_struct::state is volatile), such
- * that:
- *
- * - we form a control dependency vs deactivate_task() below.
- * - ptrace_{,un}freeze_traced() can change ->state underneath us.
+ * that we form a control dependency vs deactivate_task() below.
*/
prev_state = READ_ONCE(prev->__state);
if (!(sched_mode & SM_MASK_PREEMPT) && prev_state) {
diff --git a/kernel/signal.c b/kernel/signal.c
index d2d0c753156c..a58b68a2d3c6 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2209,14 +2209,12 @@ static int ptrace_stop(int exit_code, int why, unsigned long message,
}

/*
- * schedule() will not sleep if there is a pending signal that
- * can awaken the task.
- *
- * After this point ptrace_signal_wake_up will clear TASK_TRACED
- * if ptrace_unlink happens. Handle previous ptrace_unlinks
- * here to prevent ptrace_stop sleeping in schedule.
+ * After this point ptrace_signal_wake_up or signal_wake_up
+ * will clear TASK_TRACED if ptrace_unlink happens or a fatal
+ * signal comes in. Handle previous ptrace_unlinks and fatal
+ * signals here to prevent ptrace_stop sleeping in schedule.
*/
- if (!current->ptrace)
+ if (!current->ptrace || __fatal_signal_pending(current))
return exit_code;

set_special_state(TASK_TRACED);
@@ -2305,7 +2303,7 @@ static int ptrace_stop(int exit_code, int why, unsigned long message,
current->exit_code = 0;

/* LISTENING can be set only during STOP traps, clear it */
- current->jobctl &= ~JOBCTL_LISTENING;
+ current->jobctl &= ~(JOBCTL_LISTENING | JOBCTL_PTRACE_FROZEN);

/*
* Queued signals ignored us while we were stopped for tracing.
--
2.35.3