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Date:   Fri, 15 Mar 2019 17:39:44 -0400
From:   Andrea Arcangeli <aarcange@redhat.com>
To:     zhong jiang <zhongjiang@huawei.com>
Cc:     Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Peter Xu <peterx@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dmitry Vyukov <dvyukov@google.com>,
        syzbot <syzbot+cbb52e396df3e565ab02@syzkaller.appspotmail.com>,
        Michal Hocko <mhocko@kernel.org>, cgroups@vger.kernel.org,
        Johannes Weiner <hannes@cmpxchg.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        David Rientjes <rientjes@google.com>,
        Hugh Dickins <hughd@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Mel Gorman <mgorman@suse.de>, Vlastimil Babka <vbabka@suse.cz>
Subject: Re: KASAN: use-after-free Read in get_mem_cgroup_from_mm
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On Fri, Mar 08, 2019 at 03:10:08PM +0800, zhong jiang wrote:
> I can reproduce the issue in arm64 qemu machine.  The issue will leave after applying the
> patch.
> 
> Tested-by: zhong jiang <zhongjiang@huawei.com>

Thanks a lot for the quick testing!

> Meanwhile,  I just has a little doubt whether it is necessary to use RCU to free the task struct or not.
> I think that mm->owner alway be NULL after failing to create to process. Because we call mm_clear_owner.

I wish it was enough, but the problem is that the other CPU may be in
the middle of get_mem_cgroup_from_mm() while this runs, and it would
dereference mm->owner while it is been freed without the call_rcu
affter we clear mm->owner. What prevents this race is the
rcu_read_lock() in get_mem_cgroup_from_mm() and the corresponding
call_rcu to free the task struct in the fork failure path (again only
if CONFIG_MEMCG=y is defined). Considering you can reproduce this tiny
race on arm64 qemu (perhaps tcg JIT timing variantions helps?), you
might also in theory be able to still reproduce the race condition if
you remove the call_rcu from delayed_free_task and you replace it with
free_task.