[lustre-devel] [PATCH v3 0/2] iov_iter: allow iov_iter_get_pages_alloc to allocate more pages per call
viro at ZenIV.linux.org.uk
Fri Feb 3 11:08:17 PST 2017
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 at 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
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
* 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...
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