linux-kernel - Re: [PATCH v2 01/17] mm/gup: Fixup p*_access

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Date:   Thu, 14 Dec 2017 21:04:56 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de, x86@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirsky <luto@...nel.org>,
        Borislav Petkov <bpetkov@...e.de>,
        Greg KH <gregkh@...uxfoundation.org>, keescook@...gle.com,
        hughd@...gle.com, Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        David Laight <David.Laight@...lab.com>,
        Eduardo Valentin <eduval@...zon.com>, aliguori@...zon.com,
        Will Deacon <will.deacon@....com>, linux-mm@...ck.org,
        kirill.shutemov@...ux.intel.com, dan.j.williams@...el.com
Subject: Re: [PATCH v2 01/17] mm/gup: Fixup p*_access_permitted()

On 12/14/2017 12:54 PM, Peter Zijlstra wrote:
>> That short-circuits the page fault pretty quickly.  So, basically, the
>> rule is: if the hardware says you tripped over pkey permissions, you
>> die.  We don't try to do anything to the underlying page *before* saying
>> that you die.
> That only works when you trip the fault from hardware. Not if you do a
> software fault using gup().
> 
> AFAIK __get_user_pages(FOLL_FORCE|FOLL_WRITE|FOLL_GET) will loop
> indefinitely on the case I described.

So, the underlying bug here is that we now a get_user_pages_remote() and
then go ahead and do the p*_access_permitted() checks against the
current PKRU.  This was introduced recently with the addition of the new
p??_access_permitted() calls.

We have checks in the VMA path for the "remote" gups and we avoid
consulting PKRU for them.  This got missed in the pkeys selftests
because I did a ptrace read, but not a *write*.  I also didn't
explicitly test it against something where a COW needed to be done.

I've got some additions to the selftests and a fix where we pass FOLL_*
flags around a bit more instead of just 'write'.  I'll get those out as
soon as I do a bit more testing.