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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Eddy Wu <eddy_wu@trendmicro.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: [PATCH 4.9 109/117] fork: fix copy_process(CLONE_PARENT) race with the exiting ->real_parent
Date:   Mon,  9 Nov 2020 13:55:35 +0100
Message-Id: <20201109125030.872108197@linuxfoundation.org>
In-Reply-To: <20201109125025.630721781@linuxfoundation.org>
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From: Eddy Wu <itseddy0402@gmail.com>

commit b4e00444cab4c3f3fec876dc0cccc8cbb0d1a948 upstream.

current->group_leader->exit_signal may change during copy_process() if
current->real_parent exits.

Move the assignment inside tasklist_lock to avoid the race.

Signed-off-by: Eddy Wu <eddy_wu@trendmicro.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 kernel/fork.c |   10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1734,14 +1734,9 @@ static __latent_entropy struct task_stru
 	/* ok, now we should be set up.. */
 	p->pid = pid_nr(pid);
 	if (clone_flags & CLONE_THREAD) {
-		p->exit_signal = -1;
 		p->group_leader = current->group_leader;
 		p->tgid = current->tgid;
 	} else {
-		if (clone_flags & CLONE_PARENT)
-			p->exit_signal = current->group_leader->exit_signal;
-		else
-			p->exit_signal = (clone_flags & CSIGNAL);
 		p->group_leader = p;
 		p->tgid = p->pid;
 	}
@@ -1786,9 +1781,14 @@ static __latent_entropy struct task_stru
 	if (clone_flags & (CLONE_PARENT|CLONE_THREAD)) {
 		p->real_parent = current->real_parent;
 		p->parent_exec_id = current->parent_exec_id;
+		if (clone_flags & CLONE_THREAD)
+			p->exit_signal = -1;
+		else
+			p->exit_signal = current->group_leader->exit_signal;
 	} else {
 		p->real_parent = current;
 		p->parent_exec_id = current->self_exec_id;
+		p->exit_signal = (clone_flags & CSIGNAL);
 	}
 
 	spin_lock(&current->sighand->siglock);