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

[Linus Torvalds <torvalds@linux-foundation.org>]

On Fri, Feb 3, 2017 at 11:08 AM, Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> 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().

Yeah, access_ok() is bogus. It needs to just check against TASK_SIZE
or whatever.

> 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...

Ok, I definitely agree that it looks like __get_user_pages_fast() just
needs to get rid of the access_ok() and replace it with a proper check
for the user address space range.

Looks like arm[64] and powerpc.are the current users. Adding in some
people involved with the original submission a few years ago.

I do note that the x86 __get_user_pages_fast() thing looks dodgy too.

In particular, we do it right in the *real* get_user_pages_fast(), see
commit 7f8189068726 ("x86: don't use 'access_ok()' as a range check in
get_user_pages_fast()"). But then the same bug was re-introduced when
the "irq safe" version was merged. As well as in the GENERIC_RCU_GUP
version.

Gaah. Apparently PeterZ copied the old buggy version before the fix
when he added __get_user_pages_fast() in commit 465a454f254e ("x86,
mm: Add __get_user_pages_fast()").

I guess it could be considered a merge error (both happened during the
2.6.31 merge window).

                   Linus