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From:   "Eric W. Biederman" <ebiederm@xmission.com>
To:     linux-kernel@vger.kernel.org
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        Jann Horn <jannh@google.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>
Date:   Tue, 26 Apr 2022 17:52:10 -0500
Message-Id: <20220426225211.308418-8-ebiederm@xmission.com>
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Subject: [PATCH 8/9] ptrace: Use siglock instead of tasklist_lock in ptrace_check_attach
Precedence: bulk

Now that siglock protects tsk->parent and tsk->ptrace there is no need
to grab tasklist_lock in ptrace_check_attach.  The siglock can handle
all of the locking needs of ptrace_check_attach.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
 kernel/ptrace.c | 29 ++++++++++++++---------------
 1 file changed, 14 insertions(+), 15 deletions(-)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 0634da7ac685..842511ee9a9f 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -189,17 +189,14 @@ static bool ptrace_freeze_traced(struct task_struct *task)
 {
 	bool ret = false;
 
-	/* Lockless, nobody but us can set this flag */
 	if (task->jobctl & JOBCTL_LISTENING)
 		return ret;
 
-	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);
 		ret = true;
 	}
-	spin_unlock_irq(&task->sighand->siglock);
 
 	return ret;
 }
@@ -237,33 +234,35 @@ static void ptrace_unfreeze_traced(struct task_struct *task)
  * state.
  *
  * CONTEXT:
- * Grabs and releases tasklist_lock and @child->sighand->siglock.
+ * Grabs and releases @child->sighand->siglock.
  *
  * RETURNS:
  * 0 on success, -ESRCH if %child is not ready.
  */
 static int ptrace_check_attach(struct task_struct *child, bool ignore_state)
 {
+	unsigned long flags;
 	int ret = -ESRCH;
 
 	/*
-	 * We take the read lock around doing both checks to close a
+	 * We take the siglock around doing both checks to close a
 	 * possible race where someone else was tracing our child and
 	 * detached between these two checks.  After this locked check,
 	 * we are sure that this is our traced child and that can only
 	 * be changed by us so it's not changing right after this.
 	 */
-	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().
-		 */
-		if (ignore_state || ptrace_freeze_traced(child))
-			ret = 0;
+	if (lock_task_sighand(child, &flags)) {
+		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().
+			 */
+			if (ignore_state || ptrace_freeze_traced(child))
+				ret = 0;
+		}
+		unlock_task_sighand(child, &flags);
 	}
-	read_unlock(&tasklist_lock);
 
 	if (!ret && !ignore_state &&
 	    WARN_ON_ONCE(!wait_task_inactive(child, 0)))
-- 
2.35.3