linux-kernel - Re: ARM: vmsplit 4g/4g

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Date:   Mon, 8 Jun 2020 16:43:57 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     afzal mohammed <afzal.mohd.ma@...il.com>
Cc:     Russell King <linux@...linux.org.uk>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: ARM: vmsplit 4g/4g

On Mon, Jun 8, 2020 at 1:18 PM afzal mohammed <afzal.mohd.ma@...il.com> wrote:
> On Sun, Jun 07, 2020 at 09:26:26PM +0200, Arnd Bergmann wrote:
>
> > I think you have to use get_user_pages() though instead of
> > get_user_pages_fast(),
> > in order to be able to check the permission bits to prevent doing a
> > copy_to_user()
> > into read-only mappings.
>
> i was not aware of this, is it documented somewhere ?, afaiu,
> difference b/n get_user_pages_fast() & get_user_pages() is that fast
> version will try to pin pages w/o acquiring mmap_sem if possible.

There is another difference: get_user_pages_fast() does not return
a  vm_area_struct pointer, which is where you would check the access
permissions. I suppose those pointers could not be returned to callers
that don't already hold the mmap_sem.

> > Do you want me to review the uaccess patch to look for any missing
> > corner cases, or do you want to do the whole set of user access helpers
> > first?
>
> i will cleanup and probably post RFC initially for the changes
> handling copy_{from,to}_user() to get feedback.

Ok.

      Arnd