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Date: Mon, 05 Apr 2021 11:58:51 -0500
From: ebiederm@...ssion.com (Eric W. Biederman)
To: Alexey Gladkov <gladkov.alexey@...il.com>
Cc: LKML <linux-kernel@...r.kernel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Linux Containers <containers@...ts.linux-foundation.org>, linux-mm@...ck.org, Alexey Gladkov <legion@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, Christian Brauner <christian.brauner@...ntu.com>, Jann Horn <jannh@...gle.com>, Jens Axboe <axboe@...nel.dk>, Kees Cook <keescook@...omium.org>, Linus Torvalds <torvalds@...ux-foundation.org>, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH v9 6/8] Reimplement RLIMIT_SIGPENDING on top of ucounts
A small bug below.
Eric
> diff --git a/kernel/signal.c b/kernel/signal.c
> index f2a1b898da29..1b537d9de447 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -413,49 +413,44 @@ void task_join_group_stop(struct task_struct *task)
> static struct sigqueue *
> __sigqueue_alloc(int sig, struct task_struct *t, gfp_t flags, int override_rlimit)
> {
> - struct sigqueue *q = NULL;
> - struct user_struct *user;
> - int sigpending;
> + struct sigqueue *q = kmem_cache_alloc(sigqueue_cachep, flags);
>
> - /*
> - * Protect access to @t credentials. This can go away when all
> - * callers hold rcu read lock.
> - *
> - * NOTE! A pending signal will hold on to the user refcount,
> - * and we get/put the refcount only when the sigpending count
> - * changes from/to zero.
> - */
> - rcu_read_lock();
> - user = __task_cred(t)->user;
> - sigpending = atomic_inc_return(&user->sigpending);
> - if (sigpending == 1)
> - get_uid(user);
> - rcu_read_unlock();
> + if (likely(q != NULL)) {
> + bool overlimit;
>
> - if (override_rlimit || likely(sigpending <= task_rlimit(t, RLIMIT_SIGPENDING))) {
> - q = kmem_cache_alloc(sigqueue_cachep, flags);
> - } else {
> - print_dropped_signal(sig);
> - }
> -
> - if (unlikely(q == NULL)) {
> - if (atomic_dec_and_test(&user->sigpending))
> - free_uid(user);
> - } else {
> INIT_LIST_HEAD(&q->list);
> q->flags = 0;
> - q->user = user;
> +
> + /*
> + * Protect access to @t credentials. This can go away when all
> + * callers hold rcu read lock.
> + */
> + rcu_read_lock();
> + q->ucounts = get_ucounts(task_ucounts(t));
> + if (q->ucounts) {
> + overlimit = inc_rlimit_ucounts_and_test(q->ucounts, UCOUNT_RLIMIT_SIGPENDING,
> + 1, task_rlimit(t, RLIMIT_SIGPENDING));
> +
> + if (override_rlimit || likely(!overlimit)) {
> + rcu_read_unlock();
> + return q;
> + }
> + }
> + rcu_read_unlock();
I believe you need to call __sigqueue_free here.
> }
>
> - return q;
> + print_dropped_signal(sig);
> + return NULL;
> }
>
> static void __sigqueue_free(struct sigqueue *q)
> {
> if (q->flags & SIGQUEUE_PREALLOC)
> return;
> - if (atomic_dec_and_test(&q->user->sigpending))
> - free_uid(q->user);
> + if (q->ucounts) {
> + dec_rlimit_ucounts(q->ucounts, UCOUNT_RLIMIT_SIGPENDING, 1);
> + put_ucounts(q->ucounts);
> + }
> kmem_cache_free(sigqueue_cachep, q);
> }
>
Eric
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