On Fri, Feb 3, 2017 at 11:08 AM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> 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
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