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From: Jason Gunthorpe <jgg@ziepe.ca>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Paul Mackerras <paulus@samba.org>,
	linux-sparc <sparclinux@vger.kernel.org>,
	Alexander Gordeev <agordeev@linux.ibm.com>,
	Claudio Imbrenda <imbrenda@linux.ibm.com>,
	Will Deacon <will@kernel.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	linux-s390 <linux-s390@vger.kernel.org>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Richard Weinberger <richard@nod.at>, linux-x86 <x86@kernel.org>,
	Russell King <linux@armlinux.org.uk>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Ingo Molnar <mingo@redhat.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>,
	Gerald Schaefer <gerald.schaefer@linux.ibm.com>,
	Heiko Carstens <hca@linux.ibm.com>, Arnd Bergmann <arnd@arndb.de>,
	John Hubbard <jhubbard@nvidia.com>, Jeff Dike <jdike@addtoit.com>,
	linux-um <linux-um@lists.infradead.org>,
	Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-arm <linux-arm-kernel@lists.infradead.org>,
	linux-mm <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-power <linuxppc-dev@lists.ozlabs.org>,
	Mike Rapoport <rppt@kernel.org>
Subject: Re: [RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table folding
Date: Thu, 10 Sep 2020 15:13:19 -0300	[thread overview]
Message-ID: <20200910181319.GO87483@ziepe.ca> (raw)
In-Reply-To: <CAHk-=wh4SuNvThq1nBiqk0N-fW6NsY5w=VawC=rJs7ekmjAhjA@mail.gmail.com>

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@linux.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

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  reply	other threads:[~2020-09-10 18:14 UTC|newest]

Thread overview: 62+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-07 18:00 [RFC PATCH v2 0/3] mm/gup: fix gup_fast with dynamic page table folding Gerald Schaefer
2020-09-07 18:00 ` [RFC PATCH v2 1/3] " Gerald Schaefer
2020-09-08  5:06   ` Christophe Leroy
2020-09-08 12:09     ` Christian Borntraeger
2020-09-08 12:40       ` Christophe Leroy
2020-09-08 13:38         ` Gerald Schaefer
2020-09-08 14:30   ` Dave Hansen
2020-09-08 17:59     ` Gerald Schaefer
2020-09-09 12:29     ` Gerald Schaefer
2020-09-09 16:18       ` Dave Hansen
2020-09-09 17:25         ` Gerald Schaefer
2020-09-09 18:03           ` Jason Gunthorpe
2020-09-10  9:39             ` Alexander Gordeev
2020-09-10 13:02               ` Jason Gunthorpe
2020-09-10 13:28                 ` Gerald Schaefer
2020-09-10 15:10                   ` Jason Gunthorpe
2020-09-10 17:07                     ` Gerald Schaefer
2020-09-10 17:19                       ` Jason Gunthorpe
2020-09-10 17:57                 ` Gerald Schaefer
2020-09-10 23:21                   ` Jason Gunthorpe
2020-09-10 17:35               ` Linus Torvalds
2020-09-10 18:13                 ` Jason Gunthorpe [this message]
2020-09-10 18:33                   ` Linus Torvalds
2020-09-10 19:10                     ` Gerald Schaefer
2020-09-10 19:32                       ` Linus Torvalds
2020-09-10 21:59                         ` Jason Gunthorpe
2020-09-11  7:09                           ` peterz
2020-09-11 11:19                             ` Jason Gunthorpe
2020-09-11 19:03                             ` [PATCH] " Vasily Gorbik
2020-09-11 19:09                               ` Linus Torvalds
2020-09-11 19:40                               ` Jason Gunthorpe
2020-09-11 20:05                                 ` Jason Gunthorpe
2020-09-11 20:36                                   ` [PATCH v2] " Vasily Gorbik
2020-09-15 17:09                                     ` Vasily Gorbik
2020-09-15 17:14                                     ` Jason Gunthorpe
2020-09-15 17:18                                     ` Mike Rapoport
2020-09-15 17:31                                     ` John Hubbard
2020-09-10 21:22                   ` [RFC PATCH v2 1/3] " John Hubbard
2020-09-10 22:11                     ` Jason Gunthorpe
2020-09-10 22:17                       ` John Hubbard
2020-09-11 12:19                       ` Alexander Gordeev
2020-09-11 16:45                         ` Linus Torvalds
2020-09-10 13:11             ` Gerald Schaefer
2020-09-07 18:00 ` [RFC PATCH v2 2/3] mm: make pXd_addr_end() functions page-table entry aware Gerald Schaefer
2020-09-08  5:14   ` Christophe Leroy
2020-09-08  7:46     ` Alexander Gordeev
2020-09-08  8:16       ` Christophe Leroy
2020-09-08 14:15         ` Alexander Gordeev
2020-09-09  8:38           ` Christophe Leroy
2020-09-08 14:25     ` Alexander Gordeev
2020-09-08 13:26   ` Jason Gunthorpe
2020-09-08 14:33   ` Dave Hansen
2020-09-07 18:00 ` [RFC PATCH v2 3/3] mm: make generic pXd_addr_end() macros inline functions Gerald Schaefer
2020-09-07 20:15   ` Mike Rapoport
2020-09-08  5:19   ` Christophe Leroy
2020-09-08 15:48     ` Alexander Gordeev
2020-09-08 17:20       ` Christophe Leroy
2020-09-07 20:12 ` [RFC PATCH v2 0/3] mm/gup: fix gup_fast with dynamic page table folding Mike Rapoport
2020-09-08  5:22   ` Christophe Leroy
2020-09-08 17:36     ` Gerald Schaefer
2020-09-09 16:12       ` Gerald Schaefer
2020-09-08  4:42 ` Christophe Leroy

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