From: Vegard Nossum <vegard.nossum@oracle.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: linux-kernel@vger.kernel.org,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Frederic Weisbecker <fweisbec@gmail.com>,
Jamie Iles <jamie.iles@oracle.com>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
Andy Lutomirski <luto@kernel.org>
Subject: Re: [PATCH] kthread: fix use-after-free if kthread fork fails
Date: Fri, 5 May 2017 19:17:31 +0200 [thread overview]
Message-ID: <939ac33e-ebe1-2b1e-425f-aced558ad5e3@oracle.com> (raw)
In-Reply-To: <20170505164428.GA500@redhat.com>
[-- Attachment #1: Type: text/plain, Size: 1505 bytes --]
On 05/05/17 18:44, Oleg Nesterov wrote:
> On 05/05, Vegard Nossum wrote:
>>
>> If a kthread forks (e.g. usermodehelper since commit 1da5c46fa965) but
>> fails in copy_process() between calling dup_task_struct() and setting
>> p->set_child_tid, then the value of p->set_child_tid will be inherited
>> from the parent and get prematurely freed by free_kthread_struct().
>
> Aaah... thanks!
>
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -518,6 +518,13 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
>> atomic_set(&tsk->stack_refcount, 1);
>> #endif
>>
>> + /*
>> + * Forking kthreads (e.g. usermodehelper) should not inherit this
>> + * field since it's a pointer to a 'struct kthread' which is not
>> + * reference counted.
>> + */
>> + tsk->set_child_tid = NULL;
>> +
>
> Can't we just move both
>
> p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? child_tidptr : NULL;
> /*
> * Clear TID on mm_release()?
> */
> p->clear_child_tid = (clone_flags & CLONE_CHILD_CLEARTID) ? child_tidptr : NULL;
>
> lines here?
clone_flags is not available in dup_task_struct(), but we could move
those lines higher in copy_process(). The reason we didn't do it was
that we thought it was a little fragile/unobvious that this has to
happen before free_task() is called and that it was safer to clear it in
dup_task_struct() (which also contains zeroing of other fields).
The newly attached patch has been tested and seems to work, if you
prefer it.
Vegard
[-- Attachment #2: set_child_tid.patch --]
[-- Type: text/x-patch, Size: 1093 bytes --]
diff --git a/kernel/fork.c b/kernel/fork.c
index fbdc29365b83..c52e22fdf7ca 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1561,6 +1561,18 @@ static __latent_entropy struct task_struct *copy_process(
if (!p)
goto fork_out;
+ /*
+ * This _must_ happen before we call free_task(), i.e. before we jump
+ * to any of the bad_fork_* labels. This is to avoid freeing
+ * p->set_child_tid which is (ab)used as a kthread's data pointer for
+ * kernel threads (PF_KTHREAD).
+ */
+ p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? child_tidptr : NULL;
+ /*
+ * Clear TID on mm_release()?
+ */
+ p->clear_child_tid = (clone_flags & CLONE_CHILD_CLEARTID) ? child_tidptr : NULL;
+
ftrace_graph_init_task(p);
rt_mutex_init_task(p);
@@ -1727,11 +1739,6 @@ static __latent_entropy struct task_struct *copy_process(
}
}
- p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? child_tidptr : NULL;
- /*
- * Clear TID on mm_release()?
- */
- p->clear_child_tid = (clone_flags & CLONE_CHILD_CLEARTID) ? child_tidptr : NULL;
#ifdef CONFIG_BLOCK
p->plug = NULL;
#endif
next prev parent reply other threads:[~2017-05-05 17:18 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-05-05 16:20 [PATCH] kthread: fix use-after-free if kthread fork fails Vegard Nossum
2017-05-05 16:44 ` Oleg Nesterov
2017-05-05 17:17 ` Vegard Nossum [this message]
2017-05-06 19:51 ` Oleg Nesterov
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