From 33a9813825710fdc2b980d566ee391fd093a36c6 Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Thu, 24 Aug 2023 16:31:42 +0200 Subject: introduce __next_thread(), fix next_tid() vs exec() race Patch series "introduce __next_thread(), change next_thread()". After commit dce8f8ed1de1 ("document while_each_thread(), change first_tid() to use for_each_thread()") + this series 1. We have only one lockless user of next_thread(), task_group_seq_get_next(). I think it should be changed too. 2. We have only one user of task_struct->thread_group, thread_group_empty(). The next patches will change thread_group_empty() and kill ->thread_group. This patch (of 2): next_tid(start) does: rcu_read_lock(); if (pid_alive(start)) { pos = next_thread(start); if (thread_group_leader(pos)) pos = NULL; else get_task_struct(pos); it should return pos = NULL when next_thread() wraps to the 1st thread in the thread group, group leader, and the thread_group_leader() check tries to detect this case. But this can race with exec. To simplify, suppose we have a main thread M and a single sub-thread T, next_tid(T) should return NULL. Now suppose that T execs. If next_tid(T) is called after T changes the leadership and before it does release_task() which removes the old leader from list, then next_thread() returns M and thread_group_leader(M) = F. Lockless use of next_thread() should be avoided. After this change only task_group_seq_get_next() does this, and I believe it should be changed as well. Link: https://lkml.kernel.org/r/20230824143112.GA31208@redhat.com Link: https://lkml.kernel.org/r/20230824143142.GA31222@redhat.com Signed-off-by: Oleg Nesterov Cc: Eric W. Biederman Cc: Peter Zijlstra Signed-off-by: Andrew Morton --- fs/proc/base.c | 6 ++---- 1 file changed, 2 insertions(+), 4 deletions(-) (limited to 'fs') diff --git a/fs/proc/base.c b/fs/proc/base.c index ffd54617c354..c0e971cc6d41 100644 --- a/fs/proc/base.c +++ b/fs/proc/base.c @@ -3840,10 +3840,8 @@ static struct task_struct *next_tid(struct task_struct *start) struct task_struct *pos = NULL; rcu_read_lock(); if (pid_alive(start)) { - pos = next_thread(start); - if (thread_group_leader(pos)) - pos = NULL; - else + pos = __next_thread(start); + if (pos) get_task_struct(pos); } rcu_read_unlock(); -- cgit v1.2.3