linux-kernel - [PATCH v6 02/16] exec: Factor unshare_sighand out of de

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Message-ID: <AM6PR03MB51708AECEA6E05CAE2FDC166E4F50@AM6PR03MB5170.eurprd03.prod.outlook.com>
Date:   Fri, 20 Mar 2020 21:24:50 +0100
From:   Bernd Edlinger <bernd.edlinger@...mail.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.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>,
        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>
Subject: [PATCH v6 02/16] exec: Factor unshare_sighand out of de_thread and
 call it separately

This makes the code clearer and makes it easier to implement a mutex
that is not taken over any locations that may block indefinitely waiting
for userspace.

Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
Reviewed-by: Bernd Edlinger <bernd.edlinger@...mail.de>
Reviewed-by: Kees Cook <keescook@...omium.org>
Acked-by: Christian Brauner <christian.brauner@...ntu.com>
Reviewed-by: Kirill Tkhai <ktkhai@...tuozzo.com>
---
 fs/exec.c | 39 ++++++++++++++++++++++++++-------------
 1 file changed, 26 insertions(+), 13 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index c3f3479..ff74b9a 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1194,6 +1194,23 @@ static int de_thread(struct task_struct *tsk)
 	flush_itimer_signals();
 #endif
 
+	BUG_ON(!thread_group_leader(tsk));
+	return 0;
+
+killed:
+	/* protects against exit_notify() and __exit_signal() */
+	read_lock(&tasklist_lock);
+	sig->group_exit_task = NULL;
+	sig->notify_count = 0;
+	read_unlock(&tasklist_lock);
+	return -EAGAIN;
+}
+
+
+static int unshare_sighand(struct task_struct *me)
+{
+	struct sighand_struct *oldsighand = me->sighand;
+
 	if (refcount_read(&oldsighand->count) != 1) {
 		struct sighand_struct *newsighand;
 		/*
@@ -1210,23 +1227,13 @@ static int de_thread(struct task_struct *tsk)
 
 		write_lock_irq(&tasklist_lock);
 		spin_lock(&oldsighand->siglock);
-		rcu_assign_pointer(tsk->sighand, newsighand);
+		rcu_assign_pointer(me->sighand, newsighand);
 		spin_unlock(&oldsighand->siglock);
 		write_unlock_irq(&tasklist_lock);
 
 		__cleanup_sighand(oldsighand);
 	}
-
-	BUG_ON(!thread_group_leader(tsk));
 	return 0;
-
-killed:
-	/* protects against exit_notify() and __exit_signal() */
-	read_lock(&tasklist_lock);
-	sig->group_exit_task = NULL;
-	sig->notify_count = 0;
-	read_unlock(&tasklist_lock);
-	return -EAGAIN;
 }
 
 char *__get_task_comm(char *buf, size_t buf_size, struct task_struct *tsk)
@@ -1264,14 +1271,20 @@ int flush_old_exec(struct linux_binprm * bprm)
 	int retval;
 
 	/*
-	 * Make sure we have a private signal table and that
-	 * we are unassociated from the previous thread group.
+	 * Make this the only thread in the thread group.
 	 */
 	retval = de_thread(me);
 	if (retval)
 		goto out;
 
 	/*
+	 * Make the signal table private.
+	 */
+	retval = unshare_sighand(me);
+	if (retval)
+		goto out;
+
+	/*
 	 * Must be called _before_ exec_mmap() as bprm->mm is
 	 * not visibile until then. This also enables the update
 	 * to be lockless.
-- 
1.9.1