linux-kernel - Re: [RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table folding

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Message-ID: <20200910181319.GO87483@ziepe.ca>
Date:   Thu, 10 Sep 2020 15:13:19 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Alexander Gordeev <agordeev@...ux.ibm.com>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Dave Hansen <dave.hansen@...el.com>,
        John Hubbard <jhubbard@...dia.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@...linux.org.uk>,
        Mike Rapoport <rppt@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Jeff Dike <jdike@...toit.com>,
        Richard Weinberger <richard@....at>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Arnd Bergmann <arnd@...db.de>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        linux-x86 <x86@...nel.org>,
        linux-arm <linux-arm-kernel@...ts.infradead.org>,
        linux-power <linuxppc-dev@...ts.ozlabs.org>,
        linux-sparc <sparclinux@...r.kernel.org>,
        linux-um <linux-um@...ts.infradead.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>
Subject: Re: [RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table
 folding

On Thu, Sep 10, 2020 at 10:35:38AM -0700, Linus Torvalds wrote:
> On Thu, Sep 10, 2020 at 2:40 AM Alexander Gordeev
> <agordeev@...ux.ibm.com> wrote:
> >
> > It is only gup_fast case that exposes the issue. It hits because
> > pointers to stack copies are passed to gup_pXd_range iterators, not
> > pointers to real page tables itself.
> 
> Can we possibly change fast-gup to not do the stack copies?
>
> I'd actually rather do something like that, than the "addr_end" thing.

> As you say, none of the other page table walking code does what the
> GUP code does, and I don't think it's required.

As I understand it, the requirement is because fast-gup walks without
the page table spinlock, or mmap_sem held so it must READ_ONCE the
*pXX.

It then checks that it is a valid page table pointer, then calls
pXX_offset().

The arch implementation of pXX_offset() derefs again the passed pXX
pointer. So it defeats the READ_ONCE and the 2nd load could observe
something that is no longer a page table pointer and crash.

Passing it the address of the stack value is a way to force
pXX_offset() to use the READ_ONCE result which has already been tested
to be a page table pointer.

Other page walking code that holds the mmap_sem tends to use
pmd_trans_unstable() which solves this problem by injecting a
barrier. The load hidden in pte_offset() after a pmd_trans_unstable()
can't be re-ordered and will only see a page table entry under the
mmap_sem.

However, I think that logic would have been much clearer following the
GUP model of READ_ONCE vs extra reads and a hidden barrier. At least
it took me a long time to work it out :(

I also think there are real bugs here where places are reading *pXX
multiple times without locking the page table. One was found recently
in the wild in the huge tlb code IIRC.

The mm/pagewalk.c has these missing READ_ONCE bugs too.

So.. To change away from the stack option I think we'd have to pass
the READ_ONCE value to pXX_offset() as an extra argument instead of it
derefing the pointer internally.

Jason