linux-kernel - Re: [PATCH v2] mm/gup: disallow GUP writing to file-backed mappings by default

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Message-ID: <ZEa+L5ivNDhCmgj4@nvidia.com>
Date:   Mon, 24 Apr 2023 14:36:47 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     Christoph Hellwig <hch@...radead.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>,
        Matthew Wilcox <willy@...radead.org>,
        Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>,
        Leon Romanovsky <leon@...nel.org>,
        Christian Benvenuti <benve@...co.com>,
        Nelson Escobar <neescoba@...co.com>,
        Bernard Metzler <bmt@...ich.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Bjorn Topel <bjorn@...nel.org>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Christian Brauner <brauner@...nel.org>,
        Richard Cochran <richardcochran@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        linux-fsdevel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH v2] mm/gup: disallow GUP writing to file-backed mappings
 by default

On Mon, Apr 24, 2023 at 03:29:57PM +0100, Lorenzo Stoakes wrote:
> On Mon, Apr 24, 2023 at 10:39:25AM -0300, Jason Gunthorpe wrote:
> > On Mon, Apr 24, 2023 at 01:38:49PM +0100, Lorenzo Stoakes wrote:
> >
> > > I was being fairly conservative in that list, though we certainly need to
> > > set the flag for /proc/$pid/mem and ptrace to avoid breaking this
> > > functionality (I observed breakpoints breaking without it which obviously
> > > is a no go :). I'm not sure if there's a more general way we could check
> > > for this though?
> >
> > More broadly we should make sure these usages of GUP safe somehow so
> > that it can reliably write to those types of pages without breaking
> > the current FS contract..
> >
> > I forget exactly, but IIRC, don't you have to hold some kind of page
> > spinlock while writing to the page memory?
> >
> 
> I think perhaps you're thinking of the mm->mmap_lock? Which will be held
> for the FOLL_GET cases and simply prevent the VMA from disappearing below
> us but not do much else.

No not mmap_lock, I want to say there is a per-page lock that
interacts with the write protect, or at worst this needs to use the
page table spinlocks.

> I wonder whether we should do this check purely for FOLL_PIN to be honest?
> As this indicates medium to long-term access without mmap_lock held. This
> would exclude the /proc/$pid/mem and ptrace paths which use gup_remote().

Everything is buggy. FOLL_PIN is part of a someday solution to solve
it.

> That and a very specific use of uprobes are the only places that use
> FOLL_GET in this instance and each of them are careful in any case to
> handle setting the dirty page flag.

That is actually the bug :) Broadly the bug is to make a page dirty
without holding the right locks to actually dirty it.

Jason