linux-kernel - Re: [PATCH v3 09/10] mm/mremap: permit mremap() move of multiple VMAs

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Message-ID: <655f696f-d748-43bb-8d3f-4bef6faf6385@lucifer.local>
Date: Fri, 25 Jul 2025 20:59:41 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Jann Horn <jannh@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Peter Xu <peterx@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Pedro Falcato <pfalcato@...e.de>,
        Rik van Riel <riel@...riel.com>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v3 09/10] mm/mremap: permit mremap() move of multiple VMAs

On Fri, Jul 25, 2025 at 09:10:25PM +0200, Jann Horn wrote:
> On Fri, Jul 25, 2025 at 7:28 PM Lorenzo Stoakes
> <lorenzo.stoakes@...cle.com> wrote:
> > On Fri, Jul 25, 2025 at 07:11:49PM +0200, Jann Horn wrote:
> > > On Fri, Jul 11, 2025 at 1:38 PM Lorenzo Stoakes
> > > <lorenzo.stoakes@...cle.com> wrote:
> > > > Note that any failures encountered will result in a partial move. Since an
> > > > mremap() can fail at any time, this might result in only some of the VMAs
> > > > being moved.
> > > >
> > > > Note that failures are very rare and typically require an out of a memory
> > > > condition or a mapping limit condition to be hit, assuming the VMAs being
> > > > moved are valid.
> > >
> > > Hrm. So if userspace tries to move a series of VMAs with mremap(), and
> > > the operation fails, and userspace assumes the old syscall semantics,
> > > userspace could assume that its memory is still at the old address,
> > > when that's actually not true; and if userspace tries to access it
> > > there, userspace UAF happens?
> >
> > At 6pm on the last day of the cycle? :) dude :) this long week gets ever
> > longer...
>
> To be clear, I very much do not expect you to instantly reply to
> random patch review mail I send you late on a Friday evening. :P

Haha sure, just keen to clarify things on this!

>
> > Otherwise for mapping limit we likely hit it right away. I moved all the
> > checks up front for standard VMA/param errors.
>
> Ah, I missed that part.

Yeah the refactoring part of the series prior to this patch goes to great
lengths to prepare for this (including in moving tests earlier - all of
which I confirmed were good to be done earlier).

:)