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Date:   Mon, 9 Apr 2018 13:12:32 -0700
From:   Davidlohr Bueso <dave@stgolabs.net>
To:     Eric Biggers <ebiggers3@gmail.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Manfred Spraul <manfred@colorfullife.com>,
        "Eric W . Biederman" <ebiederm@xmission.com>,
        syzkaller-bugs@googlegroups.com
Subject: Re: [PATCH] ipc/shm: fix use-after-free of shm file via
 remap_file_pages()
Message-ID: <20180409201232.3rweldbjtvxjj5ql@linux-n805>
Mail-Followup-To: Eric Biggers <ebiggers3@gmail.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Manfred Spraul <manfred@colorfullife.com>,
        "Eric W . Biederman" <ebiederm@xmission.com>,
        syzkaller-bugs@googlegroups.com
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 <20180409094813.bsjc3u2hnsrdyiuk@black.fi.intel.com>
 <20180409185016.GA203367@gmail.com>
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On Mon, 09 Apr 2018, Eric Biggers wrote:

>It's necessary because if we don't hold a reference to sfd->file, then it can be
>a stale pointer when we compare it in __shm_open().  In particular, if the new
>struct file happened to be allocated at the same address as the old one, then
>'sfd->file == shp->shm_file' so the mmap would be allowed.  But, it will be a
>different shm segment than was intended.  The caller may not even have
>permissions to map it normally, yet it would be done anyway.
>
>In the end it's just broken to have a pointer to something that can be freed out
>from under you...

So this is actually handled by shm_nattch, serialized by the ipc perm->lock.
shm_destroy() is called when 0, which in turn does the fput(shm_file). Note
that shm_file is given a count of 1 when a new segment is created (deep in
get_empty_filp()). So I don't think the pointer is going anywhere, or am I missing
something?

Thanks,
Davidlohr