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[185.231.240.75]) by mx.google.com with ESMTPS id l6si9843773ljh.184.2019.03.26.01.19.51 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 26 Mar 2019 01:19:52 -0700 (PDT) Received-SPF: pass (google.com: domain of ktkhai@virtuozzo.com designates 185.231.240.75 as permitted sender) client-ip=185.231.240.75; Authentication-Results: mx.google.com; spf=pass (google.com: domain of ktkhai@virtuozzo.com designates 185.231.240.75 as permitted sender) smtp.mailfrom=ktkhai@virtuozzo.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=virtuozzo.com Received: from [172.16.25.169] by relay.sw.ru with esmtp (Exim 4.91) (envelope-from ) id 1h8hJB-00082L-TY; Tue, 26 Mar 2019 11:19:46 +0300 Subject: Re: [PATCH 1/2] userfaultfd: use RCU to free the task struct when fork fails From: Kirill Tkhai To: Andrea Arcangeli , Andrew Morton Cc: linux-mm@kvack.org, zhong jiang , syzkaller-bugs@googlegroups.com, syzbot+cbb52e396df3e565ab02@syzkaller.appspotmail.com, Mike Rapoport , Mike Kravetz , Peter Xu , Dmitry Vyukov References: <20190325225636.11635-1-aarcange@redhat.com> <20190325225636.11635-2-aarcange@redhat.com> <29e5c5ed-5efb-fe02-45a5-97903c88e0ec@virtuozzo.com> <303ae124-dc01-0989-7cca-39ec7331d1be@virtuozzo.com> Message-ID: Date: Tue, 26 Mar 2019 11:19:45 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.5.1 MIME-Version: 1.0 In-Reply-To: <303ae124-dc01-0989-7cca-39ec7331d1be@virtuozzo.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 26.03.2019 11:18, Kirill Tkhai wrote: > On 26.03.2019 11:07, Kirill Tkhai wrote: >> On 26.03.2019 01:56, Andrea Arcangeli wrote: >>> MEMCG depends on the task structure not to be freed under >>> rcu_read_lock() in get_mem_cgroup_from_mm() after it dereferences >>> mm->owner. >>> >>> An alternate possible fix would be to defer the delivery of the >>> userfaultfd contexts to the monitor until after fork() is guaranteed >>> to succeed. Such a change would require more changes because it would >>> create a strict ordering dependency where the uffd methods would need >>> to be called beyond the last potentially failing branch in order to be >>> safe. This solution as opposed only adds the dependency to common code >>> to set mm->owner to NULL and to free the task struct that was pointed >>> by mm->owner with RCU, if fork ends up failing. The userfaultfd >>> methods can still be called anywhere during the fork runtime and the >>> monitor will keep discarding orphaned "mm" coming from failed forks in >>> userland. >>> >>> This race condition couldn't trigger if CONFIG_MEMCG was set =n at >>> build time. >>> >>> Fixes: 893e26e61d04 ("userfaultfd: non-cooperative: Add fork() event") >>> Cc: stable@kernel.org >>> Tested-by: zhong jiang >>> Reported-by: syzbot+cbb52e396df3e565ab02@syzkaller.appspotmail.com >>> Signed-off-by: Andrea Arcangeli >>> --- >>> kernel/fork.c | 34 ++++++++++++++++++++++++++++++++-- >>> 1 file changed, 32 insertions(+), 2 deletions(-) >>> >>> diff --git a/kernel/fork.c b/kernel/fork.c >>> index 9dcd18aa210b..a19790e27afd 100644 >>> --- a/kernel/fork.c >>> +++ b/kernel/fork.c >>> @@ -952,6 +952,15 @@ static void mm_init_aio(struct mm_struct *mm) >>> #endif >>> } >>> >>> +static __always_inline void mm_clear_owner(struct mm_struct *mm, >>> + struct task_struct *p) >>> +{ >>> +#ifdef CONFIG_MEMCG >>> + if (mm->owner == p) >>> + WRITE_ONCE(mm->owner, NULL); >>> +#endif >>> +} >>> + >>> static void mm_init_owner(struct mm_struct *mm, struct task_struct *p) >>> { >>> #ifdef CONFIG_MEMCG >>> @@ -1331,6 +1340,7 @@ static struct mm_struct *dup_mm(struct task_struct *tsk) >>> free_pt: >>> /* don't put binfmt in mmput, we haven't got module yet */ >>> mm->binfmt = NULL; >>> + mm_init_owner(mm, NULL); >>> mmput(mm); >>> >>> fail_nomem: >>> @@ -1662,6 +1672,24 @@ static inline void rcu_copy_process(struct task_struct *p) >>> #endif /* #ifdef CONFIG_TASKS_RCU */ >>> } >>> >>> +#ifdef CONFIG_MEMCG >>> +static void __delayed_free_task(struct rcu_head *rhp) >>> +{ >>> + struct task_struct *tsk = container_of(rhp, struct task_struct, rcu); >>> + >>> + free_task(tsk); >>> +} >>> +#endif /* CONFIG_MEMCG */ >>> + >>> +static __always_inline void delayed_free_task(struct task_struct *tsk) >>> +{ >>> +#ifdef CONFIG_MEMCG >>> + call_rcu(&tsk->rcu, __delayed_free_task); >>> +#else /* CONFIG_MEMCG */ >>> + free_task(tsk); >>> +#endif /* CONFIG_MEMCG */ >>> +} >>> + >>> /* >>> * This creates a new process as a copy of the old one, >>> * but does not actually start it yet. >>> @@ -2123,8 +2151,10 @@ static __latent_entropy struct task_struct *copy_process( >>> bad_fork_cleanup_namespaces: >>> exit_task_namespaces(p); >>> bad_fork_cleanup_mm: >>> - if (p->mm) >>> + if (p->mm) { >>> + mm_clear_owner(p->mm, p); >>> mmput(p->mm); >>> + } >>> bad_fork_cleanup_signal: >>> if (!(clone_flags & CLONE_THREAD)) >>> free_signal_struct(p->signal); >>> @@ -2155,7 +2185,7 @@ static __latent_entropy struct task_struct *copy_process( >>> bad_fork_free: >>> p->state = TASK_DEAD; >>> put_task_stack(p); >>> - free_task(p); >>> + delayed_free_task(p); >> >> Can't call_rcu(&p->rcu, delayed_put_task_struct) be used instead this? > > I mean: > > refcount_set(&tsk->usage, 2); I.e., refcount_set(&tsk->usage, 1); > call_rcu(&p->rcu, delayed_put_task_struct); > > And: > > diff --git a/kernel/fork.c b/kernel/fork.c > index 3c516c6f7ce4..27cdf61b51a1 100644 > --- a/kernel/fork.c > +++ b/kernel/fork.c > @@ -715,7 +715,9 @@ static inline void put_signal_struct(struct signal_struct *sig) > > void __put_task_struct(struct task_struct *tsk) > { > - WARN_ON(!tsk->exit_state); > + if (!tsk->exit_state) > + /* Cleanup of copy_process() */ > + goto free; > WARN_ON(refcount_read(&tsk->usage)); > WARN_ON(tsk == current); > > @@ -727,6 +729,7 @@ void __put_task_struct(struct task_struct *tsk) > put_signal_struct(tsk->signal); > > if (!profile_handoff_task(tsk)) > +free: > free_task(tsk); > } > EXPORT_SYMBOL_GPL(__put_task_struct); >