linux-kernel - Re: [PATCH v7 0/8] iov_iter: Improve page extraction (ref, pin or just list)

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Message-ID: <e5eb9d67-dbbe-a0d9-1973-d6e6bee4db3a@redhat.com>
Date:   Tue, 24 Jan 2023 11:24:06 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Jan Kara <jack@...e.cz>, Matthew Wilcox <willy@...radead.org>
Cc:     David Howells <dhowells@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, Jeff Layton <jlayton@...nel.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 0/8] iov_iter: Improve page extraction (ref, pin or
 just list)

On 23.01.23 18:25, Jan Kara wrote:
> On Mon 23-01-23 16:42:56, Matthew Wilcox wrote:
>> On Mon, Jan 23, 2023 at 04:38:47PM +0000, David Howells wrote:
>>> Matthew Wilcox <willy@...radead.org> wrote:
>>> Also you only mention DIO read - but what about "start DIO write; fork(); touch
>>> buffer" in the parent - now the write buffer belongs to the child and they can
>>> affect the parent's write.
>>
>> I'm struggling to see the problem here.  If the child hasn't exec'd, the
>> parent and child are still in the same security domain.  The parent
>> could have modified the buffer before calling fork().
> 
> Sadly they are not. Android in particular starts applications by forking
> one big binary (zygote) that has multiple apps linked together and relies
> on the fact the child cannot influence the parent after the fork. We've
> already had CVEs with GUP & COW & fork due to this. David Hildebrand has a
> lot of memories regarding this I believe ;)

:)

Once FOLL_PIN is used most of the issues go away and we don't have to 
play any games with VM flags or similar ...

With FOLL_PIN, I consider anon a solved problem and not worth any new 
fancy ideas.

-- 
Thanks,

David / dhildenb