linux-kernel - [PATCH 12/16] ptrace: Stop protecting ptrace_set_signr with tasklist

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Message-Id: <20220518225355.784371-12-ebiederm@xmission.com>
Date:   Wed, 18 May 2022 17:53:51 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     linux-kernel@...r.kernel.org
Cc:     rjw@...ysocki.net, Oleg Nesterov <oleg@...hat.com>,
        mingo@...nel.org, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, mgorman@...e.de,
        bigeasy@...utronix.de, Will Deacon <will@...nel.org>,
        tj@...nel.org, linux-pm@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        linux-um@...ts.infradead.org, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        linux-xtensa@...ux-xtensa.org, Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, linux-ia64@...r.kernel.org,
        Robert OCallahan <roc@...nos.co>, Kyle Huey <khuey@...nos.co>,
        Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        Jason Wessel <jason.wessel@...driver.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Douglas Anderson <dianders@...omium.org>,
        Douglas Miller <dougmill@...ux.vnet.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: [PATCH 12/16] ptrace: Stop protecting ptrace_set_signr with tasklist_lock

Now that ptrace_set_signr no longer sets task->exit_code the race
documented in commit b72c186999e6 ("ptrace: fix race between
ptrace_resume() and wait_task_stopped()") is no longer possible, as
task->exit_code is only updated by wait during a ptrace_stop.

As there is no possibilty of a race and ptrace_freeze_traced is
all of the protection ptrace_set_signr needs to operate without
contention move ptrace_set_signr outside of tasklist_lock
and remove the documentation about the race that is no more.

Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
---
 kernel/ptrace.c | 19 ++++---------------
 1 file changed, 4 insertions(+), 15 deletions(-)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index e0ecb1536dfc..d0527b6e2b29 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -595,17 +595,14 @@ static int ptrace_detach(struct task_struct *child, unsigned int data)
 	/* Architecture-specific hardware disable .. */
 	ptrace_disable(child);
 
+	ptrace_set_signr(child, data);
+
 	write_lock_irq(&tasklist_lock);
 	/*
 	 * We rely on ptrace_freeze_traced(). It can't be killed and
 	 * untraced by another thread, it can't be a zombie.
 	 */
 	WARN_ON(!child->ptrace || child->exit_state);
-	/*
-	 * tasklist_lock avoids the race with wait_task_stopped(), see
-	 * the comment in ptrace_resume().
-	 */
-	ptrace_set_signr(child, data);
 	__ptrace_detach(current, child);
 	write_unlock_irq(&tasklist_lock);
 
@@ -869,17 +866,9 @@ static int ptrace_resume(struct task_struct *child, long request,
 		user_disable_single_step(child);
 	}
 
-	/*
-	 * Change ->exit_code and ->state under siglock to avoid the race
-	 * with wait_task_stopped() in between; a non-zero ->exit_code will
-	 * wrongly look like another report from tracee.
-	 *
-	 * Note that we need siglock even if ->exit_code == data and/or this
-	 * status was not reported yet, the new status must not be cleared by
-	 * wait_task_stopped() after resume.
-	 */
-	spin_lock_irq(&child->sighand->siglock);
 	ptrace_set_signr(child, data);
+
+	spin_lock_irq(&child->sighand->siglock);
 	child->jobctl &= ~JOBCTL_TRACED;
 	wake_up_state(child, __TASK_TRACED);
 	spin_unlock_irq(&child->sighand->siglock);
-- 
2.35.3