Re: Buggy __free(kfree) usage pattern already in tree

[Peter Zijlstra]

On Tue, Sep 19, 2023 at 02:57:52PM +0200, Peter Zijlstra wrote:
> On Sat, Sep 16, 2023 at 12:13:32AM +0200, Peter Zijlstra wrote:
> 
> > I think I can make it work, I'll go have a play, but perhaps not now,
> > it's past midnight ;-)
> 
> So I have been playing about with this a bit and it keeps falling short
> of 'nice' :/
> 
> What I ended up with is the below. The simple scoped_guard () extension
> is useful, albeit slightly awkward (I'll reply with another patch making
> use of it). But the basic problem is that it will have to have the form:

---
Index: linux-2.6/include/linux/sched/task.h
===================================================================
--- linux-2.6.orig/include/linux/sched/task.h
+++ linux-2.6/include/linux/sched/task.h
@@ -226,4 +226,6 @@ static inline void task_unlock(struct ta
 	spin_unlock(&p->alloc_lock);
 }
 
+DEFINE_GUARD(task_lock, struct task_struct *, task_lock(_T), task_unlock(_T))
+
 #endif /* _LINUX_SCHED_TASK_H */
Index: linux-2.6/include/linux/spinlock.h
===================================================================
--- linux-2.6.orig/include/linux/spinlock.h
+++ linux-2.6/include/linux/spinlock.h
@@ -548,5 +548,31 @@ DEFINE_LOCK_GUARD_1(spinlock_irqsave, sp
 DEFINE_LOCK_GUARD_1_COND(spinlock_irqsave, _try,
 			 spin_trylock_irqsave(_T->lock, _T->flags))
 
+DEFINE_LOCK_GUARD_1(read_lock, rwlock_t,
+		    read_lock(_T->lock),
+		    read_unlock(_T->lock))
+
+DEFINE_LOCK_GUARD_1(read_lock_irq, rwlock_t,
+		    read_lock_irq(_T->lock),
+		    read_unlock_irq(_T->lock))
+
+DEFINE_LOCK_GUARD_1(read_lock_irqsave, rwlock_t,
+		    read_lock_irqsave(_T->lock, _T->flags),
+		    read_unlock_irqrestore(_T->lock, _T->flags),
+		    unsigned long flags)
+
+DEFINE_LOCK_GUARD_1(write_lock, rwlock_t,
+		    write_lock(_T->lock),
+		    write_unlock(_T->lock))
+
+DEFINE_LOCK_GUARD_1(write_lock_irq, rwlock_t,
+		    write_lock_irq(_T->lock),
+		    write_unlock_irq(_T->lock))
+
+DEFINE_LOCK_GUARD_1(write_lock_irqsave, rwlock_t,
+		    write_lock_irqsave(_T->lock, _T->flags),
+		    write_unlock_irqrestore(_T->lock, _T->flags),
+		    unsigned long flags)
+
 #undef __LINUX_INSIDE_SPINLOCK_H
 #endif /* __LINUX_SPINLOCK_H */
Index: linux-2.6/kernel/ptrace.c
===================================================================
--- linux-2.6.orig/kernel/ptrace.c
+++ linux-2.6/kernel/ptrace.c
@@ -386,6 +386,34 @@ static int check_ptrace_options(unsigned
 	return 0;
 }
 
+static inline void ptrace_set_stopped(struct task_struct *task)
+{
+	guard(spinlock)(&task->sighand->siglock);
+
+	/*
+	 * If the task is already STOPPED, set JOBCTL_TRAP_STOP and
+	 * TRAPPING, and kick it so that it transits to TRACED.  TRAPPING
+	 * will be cleared if the child completes the transition or any
+	 * event which clears the group stop states happens.  We'll wait
+	 * for the transition to complete before returning from this
+	 * function.
+	 *
+	 * This hides STOPPED -> RUNNING -> TRACED transition from the
+	 * attaching thread but a different thread in the same group can
+	 * still observe the transient RUNNING state.  IOW, if another
+	 * thread's WNOHANG wait(2) on the stopped tracee races against
+	 * ATTACH, the wait(2) may fail due to the transient RUNNING.
+	 *
+	 * The following task_is_stopped() test is safe as both transitions
+	 * in and out of STOPPED are protected by siglock.
+	 */
+	if (task_is_stopped(task) &&
+	    task_set_jobctl_pending(task, JOBCTL_TRAP_STOP | JOBCTL_TRAPPING)) {
+		task->jobctl &= ~JOBCTL_STOPPED;
+		signal_wake_up_state(task, __TASK_STOPPED);
+	}
+}
+
 static int ptrace_attach(struct task_struct *task, long request,
 			 unsigned long addr,
 			 unsigned long flags)
@@ -393,17 +421,17 @@ static int ptrace_attach(struct task_str
 	bool seize = (request == PTRACE_SEIZE);
 	int retval;
 
-	retval = -EIO;
 	if (seize) {
 		if (addr != 0)
-			goto out;
+			return -EIO;
 		/*
 		 * This duplicates the check in check_ptrace_options() because
 		 * ptrace_attach() and ptrace_setoptions() have historically
 		 * used different error codes for unknown ptrace options.
 		 */
 		if (flags & ~(unsigned long)PTRACE_O_MASK)
-			goto out;
+			return -EIO;
+
 		retval = check_ptrace_options(flags);
 		if (retval)
 			return retval;
@@ -414,88 +442,58 @@ static int ptrace_attach(struct task_str
 
 	audit_ptrace(task);
 
-	retval = -EPERM;
 	if (unlikely(task->flags & PF_KTHREAD))
-		goto out;
+		return -EPERM;
 	if (same_thread_group(task, current))
-		goto out;
+		return -EPERM;
 
 	/*
 	 * Protect exec's credential calculations against our interference;
 	 * SUID, SGID and LSM creds get determined differently
 	 * under ptrace.
 	 */
-	retval = -ERESTARTNOINTR;
-	if (mutex_lock_interruptible(&task->signal->cred_guard_mutex))
-		goto out;
-
-	task_lock(task);
-	retval = __ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS);
-	task_unlock(task);
-	if (retval)
-		goto unlock_creds;
-
-	write_lock_irq(&tasklist_lock);
-	retval = -EPERM;
-	if (unlikely(task->exit_state))
-		goto unlock_tasklist;
-	if (task->ptrace)
-		goto unlock_tasklist;
-
-	task->ptrace = flags;
-
-	ptrace_link(task, current);
-
-	/* SEIZE doesn't trap tracee on attach */
-	if (!seize)
-		send_sig_info(SIGSTOP, SEND_SIG_PRIV, task);
+	scoped_guard (mutex_intr, &task->signal->cred_guard_mutex) {
 
-	spin_lock(&task->sighand->siglock);
+		scoped_guard (task_lock, task) {
+			retval = __ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS);
+			if (retval)
+				return retval;
+		}
 
-	/*
-	 * If the task is already STOPPED, set JOBCTL_TRAP_STOP and
-	 * TRAPPING, and kick it so that it transits to TRACED.  TRAPPING
-	 * will be cleared if the child completes the transition or any
-	 * event which clears the group stop states happens.  We'll wait
-	 * for the transition to complete before returning from this
-	 * function.
-	 *
-	 * This hides STOPPED -> RUNNING -> TRACED transition from the
-	 * attaching thread but a different thread in the same group can
-	 * still observe the transient RUNNING state.  IOW, if another
-	 * thread's WNOHANG wait(2) on the stopped tracee races against
-	 * ATTACH, the wait(2) may fail due to the transient RUNNING.
-	 *
-	 * The following task_is_stopped() test is safe as both transitions
-	 * in and out of STOPPED are protected by siglock.
-	 */
-	if (task_is_stopped(task) &&
-	    task_set_jobctl_pending(task, JOBCTL_TRAP_STOP | JOBCTL_TRAPPING)) {
-		task->jobctl &= ~JOBCTL_STOPPED;
-		signal_wake_up_state(task, __TASK_STOPPED);
-	}
+		scoped_guard (write_lock, &tasklist_lock) {
+			if (unlikely(task->exit_state))
+				return -EPERM;
+			if (task->ptrace)
+				return -EPERM;
 
-	spin_unlock(&task->sighand->siglock);
+			task->ptrace = flags;
 
-	retval = 0;
-unlock_tasklist:
-	write_unlock_irq(&tasklist_lock);
-unlock_creds:
-	mutex_unlock(&task->signal->cred_guard_mutex);
-out:
-	if (!retval) {
-		/*
-		 * We do not bother to change retval or clear JOBCTL_TRAPPING
-		 * if wait_on_bit() was interrupted by SIGKILL. The tracer will
-		 * not return to user-mode, it will exit and clear this bit in
-		 * __ptrace_unlink() if it wasn't already cleared by the tracee;
-		 * and until then nobody can ptrace this task.
-		 */
-		wait_on_bit(&task->jobctl, JOBCTL_TRAPPING_BIT, TASK_KILLABLE);
-		proc_ptrace_connector(task, PTRACE_ATTACH);
+			ptrace_link(task, current);
+
+			/* SEIZE doesn't trap tracee on attach */
+			if (!seize)
+				send_sig_info(SIGSTOP, SEND_SIG_PRIV, task);
+
+			ptrace_set_stopped(task);
+
+		}
+
+		goto success;
 	}
+	return -ERESTARTNOINTR;
 
-	return retval;
+success:
+	/*
+	 * We do not bother to change retval or clear JOBCTL_TRAPPING
+	 * if wait_on_bit() was interrupted by SIGKILL. The tracer will
+	 * not return to user-mode, it will exit and clear this bit in
+	 * __ptrace_unlink() if it wasn't already cleared by the tracee;
+	 * and until then nobody can ptrace this task.
+	 */
+	wait_on_bit(&task->jobctl, JOBCTL_TRAPPING_BIT, TASK_KILLABLE);
+	proc_ptrace_connector(task, PTRACE_ATTACH);
+
+	return 0;
 }
 
 /**