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Message-ID: <cd488979-d257-42b9-937f-470cc3c57f5e@lucifer.local> Date: Mon, 24 Apr 2023 15:29:57 +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 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/$pid/mem and ptrace to avoid breaking this > > functionality (I observed breakpoints breaking without it which obviously > > is a no go :). I'm not sure if there's a more general way we could check > > for this though? > > More broadly we should make sure these usages of GUP safe somehow so > that it can reliably write to those types of pages without breaking > the current FS contract.. > > I forget exactly, but IIRC, don't you have to hold some kind of page > spinlock while writing to the page memory? > I think perhaps you're thinking of the mm->mmap_lock? Which will be held for the FOLL_GET cases and simply prevent the VMA from disappearing below us but not do much else. > So, users that do this, or can be fixed to do this, can get file > backed pages. It suggests that a flag name is more like > FOLL_CALLER_USES_FILE_WRITE_LOCKING > As stated above, I'm not sure what locking you're referring to, but seems to me that FOLL_GET already implies what you're thinking? I wonder whether we should do this check purely for FOLL_PIN to be honest? As this indicates medium to long-term access without mmap_lock held. This would exclude the /proc/$pid/mem and ptrace paths which use gup_remote(). 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. All PUP cases that do not specify FOLL_LONGTERM also do this, so we could atually go so far as to reduce the patch to simply performing the vma_wants_writenotify() check if (FOLL_PIN | FOLL_LONGTERM) is specified, which covers the io_uring case. Alternatively if we wanted to be safer, we could add a FOLL_ALLOW_FILE_PIN that is checked on FOLL_PIN and ignored on FOLL_LONGTERM? > > 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? > > Anything using FOLL_LONGTERM should not set the flag, GUP should even > block the combination. OK > > And we need to have in mind that the flag indicates the code is > buggy, so if you set it then we should understand how is that caller > expected to be fixed. > > Jason I think we are working towards a much simpler solution in any case!
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