Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20221205192304.1957418-1-Liam.Howlett@oracle.com>
 <20221205123250.3fc552d96fcca5dc58be8443@linux-foundation.org>
 <CAG48ez26s0R6DsPKJ1dUomwSwCfhWcpQD6Zb0GU0rbYcFD1hww@mail.gmail.com> <b1b80402-2c90-8006-bcf8-716e6ef719c2@suse.cz>
In-Reply-To: <b1b80402-2c90-8006-bcf8-716e6ef719c2@suse.cz>
From:   Jann Horn <jannh@google.com>
Date:   Mon, 5 Dec 2022 23:22:44 +0100
Message-ID: <CAG48ez2trF-V658GpXC5u2N54omeZ0w_T1XTTcrtg+w0PEMeDw@mail.gmail.com>
Subject: Re: [PATCH v2] mmap: Fix do_brk_flags() modifying obviously incorrect VMAs
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Liam Howlett <liam.howlett@oracle.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Yu Zhao <yuzhao@google.com>, Jason Donenfeld <Jason@zx2c4.com>,
        Matthew Wilcox <willy@infradead.org>,
        SeongJae Park <sj@kernel.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Mon, Dec 5, 2022 at 11:13 PM Vlastimil Babka <vbabka@suse.cz> wrote:
> On 12/5/22 22:55, Jann Horn wrote:
> > On Mon, Dec 5, 2022 at 9:32 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> >> On Mon, 5 Dec 2022 19:23:17 +0000 Liam Howlett <liam.howlett@oracle.com> wrote:
> >> > Add more sanity checks to the VMA that do_brk_flags() will expand.
> >> > Ensure the VMA matches basic merge requirements within the function
> >> > before calling can_vma_merge_after().
> >>
> >> I't unclear what's actually being fixed here.
> >>
> >> Why do you feel we need the above changes?
> >>
> >> > Drop the duplicate checks from vm_brk_flags() since they will be
> >> > enforced later.
> >> >
> >> > Fixes: 2e7ce7d354f2 ("mm/mmap: change do_brk_flags() to expand existing VMA and add do_brk_munmap()")
> >>
> >> Fixes in what way?  Removing the duplicate checks?
> >
> > The old code would expand file VMAs on brk(), which is functionally
> > wrong and also dangerous in terms of locking because the brk() path
> > isn't designed for file VMAs and therefore doesn't lock the file
> > mapping. Checking can_vma_merge_after() ensures that new anonymous
> > VMAs can't be merged into file VMAs.
> >
> > See https://lore.kernel.org/linux-mm/CAG48ez1tJZTOjS_FjRZhvtDA-STFmdw8PEizPDwMGFd_ui0Nrw@mail.gmail.com/
> > .
>
> I guess the point is that if we fix it still within 6.1, we don't have to
> devise how exactly this is exploitable,

Yeah, that was sort of my thinking.

> but due to the insufficient locking
> it most likely is, right?

To be honest, I don't really know how bad this is - pretty much the
only thing we're doing here is to change the VMA end. I don't know if
that messes up the address_space's interval tree or something?
I have no clue how that data structure looks.