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Message-ID: <c8fff8b3-ead6-4f52-bf17-f2ef2e752b57@lucifer.local> Date: Mon, 24 Apr 2023 13:38:49 +0100 From: Lorenzo Stoakes <lstoakes@...il.com> To: Jason Gunthorpe <jgg@...dia.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 09:28:07AM -0300, Jason Gunthorpe wrote: > On Mon, Apr 24, 2023 at 11:17:55AM +0100, Lorenzo Stoakes wrote: > > On Mon, Apr 24, 2023 at 02:43:56AM -0700, Christoph Hellwig wrote: > > > I'm pretty sure DIRECT I/O reads that write into file backed mappings > > > are out there in the wild. > > I wonder if that is really the case? I know people tried this with > RDMA and it didn't get very far before testing uncovered data > corruption and kernel crashes.. Maybe O_DIRECT has a much smaller race > window so people can get away with it? > > > I know Jason is keen on fixing this at a fundamental level and this flag is > > ultimately his suggestion, so it certainly doesn't stand in the way of this > > work moving forward. > > Yeah, the point is to close it off, because while we wish it was > fixed properly, it isn't. We are still who knows how far away from it. > > In the mean time this is a fairly simple way to oops the kernel, > especially with cases like io_uring and RDMA. So, I view it as a > security problem. > > My general dislike was that io_uring protected itself from the > security problem and we left all the rest of the GUP users out to dry. > > So, my suggestion was to mark the places where we want to allow this, > eg O_DIRECT, and block everwhere else. Lorenzo, I would significantly > par back the list you have. 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? A perhaps slightly unpleasant solution might be to not enforce this when FOLL_FORCE is specified which is mostly a ptrace + friends thing then we could drop all those exceptions. I wouldn't be totally opposed to dropping it for RDMA too, because I suspect accessing file-backed mappings for that is pretty iffy. Do you have a sense of which in the list you feel could be pared back? > > I also suggest we force block it at some kernel lockdown level.. > > Alternatively, perhaps we abuse FOLL_LONGTERM and prevent it from > working with filebacked pages since, I think, the ease of triggering a > bug goes up the longer the pages are pinned. > This would solve the io_uring case and it is certainly more of a concern when the pin is intended to be kept around, though it feels a bit icky as a non-FOLL_LONGTERM pin could surely be problematic too? > Jason
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