Re: [PATCH 1/2] mm: Fix vma_is_anonymous() false-positives

From: Michal Hocko <mhocko@kernel.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Dmitry Vyukov <dvyukov@google.com>,
	Oleg Nesterov <oleg@redhat.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	stable@vger.kernel.org
Subject: Re: [PATCH 1/2] mm: Fix vma_is_anonymous() false-positives
Date: Mon, 16 Jul 2018 15:30:28 +0200	[thread overview]
Message-ID: <20180716133028.GQ17280@dhcp22.suse.cz> (raw)
In-Reply-To: <20180710134858.3506f097104859b533c81bf3@linux-foundation.org>

On Tue 10-07-18 13:48:58, Andrew Morton wrote:
> On Tue, 10 Jul 2018 16:48:20 +0300 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:
> 
> > vma_is_anonymous() relies on ->vm_ops being NULL to detect anonymous
> > VMA. This is unreliable as ->mmap may not set ->vm_ops.
> > 
> > False-positive vma_is_anonymous() may lead to crashes:
> > 
> > ...
> > 
> > This can be fixed by assigning anonymous VMAs own vm_ops and not relying
> > on it being NULL.
> > 
> > If ->mmap() failed to set ->vm_ops, mmap_region() will set it to
> > dummy_vm_ops. This way we will have non-NULL ->vm_ops for all VMAs.
> 
> Is there a smaller, simpler fix which we can use for backporting
> purposes and save the larger rework for development kernels?

Why cannot we simply keep anon vma with null vm_ops and set dummy_vm_ops
for all users who do not initialize it in their mmap callbacks?
Basically have a sanity check&fixup in call_mmap?
-- 
Michal Hocko
SUSE Labs