linux-kernel - Re: [PATCH v2] mm: userfaultfd: avoid passing an invalid range to vma

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Date:   Mon, 15 May 2023 23:10:18 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH v2] mm: userfaultfd: avoid passing an invalid range to
 vma_merge()

On Mon, May 15, 2023 at 05:39:25PM -0400, Peter Xu wrote:
> On Mon, May 15, 2023 at 08:32:32PM +0100, Lorenzo Stoakes wrote:
> > As well as fixing the repro described in [1] this also continues to pass
> > uffd unit tests.
>
> Side note on testing, not directly relevant to the patch itself..
>
> I'm wondering whether do we have tests somewhere to just test vma
> operations on split and merge, then verify it using smap or whatever.
>
> The uffd unit test in this case is probably not gonna trigger anything
> because we always mostly register with a whole vma over the testing ranges,
> so not immediately helpful.
>
> The trick here is we have quite a few ops that will call vma merge/split in
> different ways, but logically we can still category them into: (1) add/del
> vmas, or (2) modify vma flags, so at least for case (2) we can have a
> framework to cover all the cases (mbind, mprotect, uffd reg/unreg, mlock,
> etc.), the difference will be the flags we'll be looking at for different
> cases, however how vmas merge/split should be somehow in the same pattern.
>

I totally agree we need more testing on this. We do have some basic
self-tests for various things but I don't think we test this specifically
or certainly not in the way I'd prefer (somehow pull vma_merge() + friends
into userland and instrument with heavy unit tests).

I do intend to try to do something with this soon.

Something with /proc/$pid/[s]maps could be a good straightforward thing, I
will try to write some small test (we already have a little repro for the
reported issue) for this anyway.

> --
> Peter Xu
>