Messages in this thread | | | Date | Wed, 13 Dec 2017 19:32:09 +0100 | From | Peter Zijlstra <> | Subject | Re: [patch 05/16] mm: Allow special mappings with user access cleared |
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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@infradead.org> 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.
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