linux-kernel - [PATCH] pidfd: Use new infrastructure to fix deadlocks in execve

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Date:   Sat, 14 Mar 2020 10:12:11 +0100
From:   Bernd Edlinger <bernd.edlinger@...mail.de>
To:     Kees Cook <keescook@...omium.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Jann Horn <jannh@...gle.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Jonathan Corbet <corbet@....net>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Oleg Nesterov <oleg@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Andrei Vagin <avagin@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Yuyang Du <duyuyang@...il.com>,
        David Hildenbrand <david@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Anshuman Khandual <anshuman.khandual@....com>,
        David Howells <dhowells@...hat.com>,
        James Morris <jamorris@...ux.microsoft.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian Kellner <christian@...lner.me>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Aleksa Sarai <cyphar@...har.com>,
        "Dmitry V. Levin" <ldv@...linux.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Sargun Dhillon <sargun@...gun.me>
Subject: [PATCH] pidfd: Use new infrastructure to fix deadlocks in execve

This changes __pidfd_fget to use the new exec_update_mutex
instead of cred_guard_mutex.

This should be safe, as the credentials do not change
before exec_update_mutex is locked.  Therefore whatever
file access is possible with holding the cred_guard_mutex
here is also possbile with the exec_update_mutex.

Signed-off-by: Bernd Edlinger <bernd.edlinger@...mail.de>
---
 kernel/pid.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

This replaces Eric's "[PATCH] pidfd: Stop taking cred_guard_mutex"

diff --git a/kernel/pid.c b/kernel/pid.c
index 0f4ecb5..04821f4 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -584,7 +584,7 @@ static struct file *__pidfd_fget(struct task_struct *task, int fd)
 	struct file *file;
 	int ret;
 
-	ret = mutex_lock_killable(&task->signal->cred_guard_mutex);
+	ret = mutex_lock_killable(&task->signal->exec_update_mutex);
 	if (ret)
 		return ERR_PTR(ret);
 
@@ -593,7 +593,7 @@ static struct file *__pidfd_fget(struct task_struct *task, int fd)
 	else
 		file = ERR_PTR(-EPERM);
 
-	mutex_unlock(&task->signal->cred_guard_mutex);
+	mutex_unlock(&task->signal->exec_update_mutex);
 
 	return file ?: ERR_PTR(-EBADF);
 }
-- 
1.9.1