Re: [PATCH v3 0/2] iov_iter: allow iov_iter_get_pages_alloc to allocate more pages per call

[Al Viro <viro@ZenIV.linux.org.uk>]

On Fri, Feb 03, 2017 at 10:29:23AM -0800, Linus Torvalds wrote:
> On Thu, Feb 2, 2017 at 11:29 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > get_user_pages() relies upon find_extend_vma() to reject kernel
> > addresses; the fast side of get_user_pages_fast() doesn't have anything
> > of that sort
> 
> It is *supposed* to have it.
> 
> See pte_allows_gup(), for example. In particular, it requires the
> _PAGE_USER bit in the PTE (and the devpte case should require
> _PAGE_BIT_DEVMAP).

On x86 it does.  I don't see anything equivalent in mm/gup.c one, and the
only kinda-sorta similar thing (access_ok() in __get_user_pages_fast()
there) is vulnerable to e.g. access via kernel_write().  The comment in
there
 * Before activating this code, please be aware that the following assumptions
 * are currently made:
 *
 *  *) HAVE_RCU_TABLE_FREE is enabled, and tlb_remove_table is used to free
 *      pages containing page tables.
 *
 *  *) ptes can be read atomically by the architecture.
 *
 *  *) access_ok is sufficient to validate userspace address ranges.
 *
 * The last two assumptions can be relaxed by the addition of helper functions.

doesn't look promising - access_ok() is never sufficient.  Something like
_PAGE_USER tests in x86 one solves that problem, but if anything similar
works for HAVE_GENERIC_RCU_GUP I don't see it.  Thus the question re
what am I missing here...