linux-kernel - Re: [PATCH v2] mmap: Fix do_brk_flags() modifying obviously incorrect VMAs

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Date:   Tue, 6 Dec 2022 17:12:55 +0000
From:   Liam Howlett <liam.howlett@...cle.com>
To:     Vlastimil Babka <vbabka@...e.cz>
CC:     Jann Horn <jannh@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Yu Zhao <yuzhao@...gle.com>, Jason Donenfeld <Jason@...c4.com>,
        Matthew Wilcox <willy@...radead.org>,
        SeongJae Park <sj@...nel.org>
Subject: Re: [PATCH v2] mmap: Fix do_brk_flags() modifying obviously incorrect
 VMAs

* Vlastimil Babka <vbabka@...e.cz> [221205 17:26]:
> On 12/5/22 23:13, Vlastimil Babka wrote:
> > On 12/5/22 22:55, Jann Horn wrote:
> >> On Mon, Dec 5, 2022 at 9:32 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
> >>> On Mon, 5 Dec 2022 19:23:17 +0000 Liam Howlett <liam.howlett@...cle.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/
> 
> And yeah, that URL should have been a Link: in the patch. And the scenario
> it's fixing described in a bit more detail?

Yes, sorry.  I should have made a better effort in describing what I was
fixing.  It seems I understated what was happening.

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