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

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Date:   Mon, 23 Jan 2023 18:25:29 +0100
From:   Jan Kara <jack@...e.cz>
To:     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>, Jan Kara <jack@...e.cz>,
        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 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 ;)

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR