linux-kernel - Re: [PATCH v3 0/2] iov_iter: allow iov_iter_get_pages

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Date:   Fri, 3 Feb 2017 11:28:48 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Al Viro <viro@...iv.linux.org.uk>,
        Steve Capper <steve.capper@...aro.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Hugh Dickins <hughd@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Cc:     Jeff Layton <jlayton@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
        ceph-devel <ceph-devel@...r.kernel.org>,
        lustre-devel@...ts.lustre.org,
        V9FS Developers <v9fs-developer@...ts.sourceforge.net>,
        Jan Kara <jack@...e.cz>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v3 0/2] iov_iter: allow iov_iter_get_pages_alloc to
 allocate more pages per call

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