Re: [patch 05/16] mm: Allow special mappings with user access cleared

[Peter Zijlstra]

On Wed, Dec 13, 2017 at 10:08:30AM -0800, Linus Torvalds wrote:
> On Wed, Dec 13, 2017 at 7:54 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > Which is why get_user_pages() _should_ enforce this.
> >
> > What use are protection keys if you can trivially circumvent them?
> 
> No, we will *not* worry about protection keys in get_user_pages().
> 
> They are not "security". They are a debug aid and safety against random mis-use.
> 
> In particular, they are very much *NOT* about "trivially circumvent
> them". The user could just change their mapping thing, for chrissake!
> 
> We already allow access to PROT_NONE for gdb and friends, very much on purpose.
> 
> We're not going to make the VM more complex for something that
> absolutely nobody cares about, and has zero security issues.

OK, that might have been my phrasing that was off -- mostly because I
was looking at it from the VM_NOUSER angle, but currently:

  - gup_pte_range() has pte_access_permitted()

  - follow_page_pte() has pte_access_permitted() for FOLL_WRITE only

All I'm saying is that that is inconsistent and we should change
follow_page_pte() to use pte_access_permitted() for FOLL_GET, such that
__get_user_pages_fast() and __get_user_pages() have matching semantics.

Now, if VM_NOUSER were to live, the above change would ensure write(2)
cannot read from such VMAs, where the existing test for FOLL_WRITE
already disallows read(2) from writing to them.

But even without VM_NOUSER it makes the VM more consistent than it is
today.