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Howlett" , Vlastimil Babka , Jann Horn , Pedro Falcato , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Arnd Bergmann , Greg Kroah-Hartman , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Lance Yang , syzbot+c181d3198e98f8aef8b9@syzkaller.appspotmail.com Subject: Re: [PATCH] drivers/char/mem: mmap readonly MAP_SHARED-/dev/zero correctly Message-ID: References: <20260924-fix-dev-zero-readonly-shared-v1-1-153c2111e323@kernel.org> <9efc7bc1-8d9d-49ac-b856-003e81558411@kernel.org> <20260924195826.70a87d6fb78d754bc73892cd@linux-foundation.org> <95e91b4a-ca16-49a3-9a1a-5b38afa6f08d@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: azund6c71787uy9y5j7p1sn13s5tcn9x X-Rspamd-Queue-Id: 65BBA160010 X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1790608114-516233 X-HE-Meta: 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 YI7euopZ Ul/FNIOjsR9pzK2r7hY7nfErbXQFEubNK4Y+1y8A7NFLCGWWxMkiEOvkBTPaWmUim6eBp+51sznQ685OytnNmyGiHixQXE+FI+rjMPgD7s4DxZ2ZRWLgTLYbO2/qxhSAkUOFg6AYGWUeYiZjrOwlAT5jCUnvXaDNH1ZgeoL3hgpOxcP5+/fgh6G7xRADnquU89aNcLQxGeskJrDRPrEUkK7SsUXcsgDSLJCyQXR0EkqBbdDBYbYAEZeXFCNGlkPsM+w+tzJTyO3jK0vRY4Gr+Sk8w0Gc98WYHenbAEcDupmycseNo28QPWDG2NO1tXPeh2QWrhGbAgpyVbb1HGBi/AGauhTavPVbVXNKJJH9pc+FsM8fhppknjnVexAqOmT11LH28DW9fCtxTlWxmq8+lEjUNrAwkiwxtw/+nmRhjztfc9PAghH1Hy/QCcl0B8SOH6j/3ZIXUyAIlshQ= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Sep 28, 2026 at 01:59:47PM +0200, David Hildenbrand (Arm) wrote: > > > > > It then becomes a question about how to get readonly memory. > > > >> > >> We can > >> > >> a) Disallow allocating an anon_vma and fail gracefully. So only a shared > >> zeropage could ever get mapped there. Might break the s390x > >> mm_forbids_zeropage(). But given that's only used in hypervisors like QEMU, > >> unlikely. > > > > Something like: > > > > /* about to maybe prep anon_vma */ > > > > if (!vma_cow_mapping(vma) && vma_test(vma, VMA_MAYSHARE_BIT)) { > > /* don't prep anon give zero page */ > > } > > > > ? > > Something like that, but really just failing directly from anon-vma preparation > in a !vma_cow_mapping(vma), maybe using a special error code. > > But the less we have to hack around this special case, the better. > > > > > I think though it's surely the only case (I hope!) where you can possibly be > > both anon (as in missing vm_ops) and !CoW? I hope? :) > > > > So it feels better to fix it at the source. > > > > OTOH maybe it's worth special-casing so we don't allocate on read. > > > > But that brings me to c)... > > > > > >> > >> b) Do what Lorenzo proposes. This will allocate real memory. Someone decided to > >> use MAP_SHARED, for unknown reasons, so I'd assume it's unlikely that > >> something breaks, but you have a point. > > > > I would say this patch is the right fix for the moment to fix the assert, and we > > can chase up with other approaches afterwards. > > It will get backported, so we better be careful. > > IIUC, the real change is only for /dev/zero users that > > fd = open("/dev/zero", O_RDONLY); > mmap(fd, MAP_SHARED, PROT_READ); > > Which is rather something odd to do indeed. > > Letting AI do some digging ... it says there are not known users (I don't trust > it, but I suspect at least there are no prominent users, which makes sense ...). > > > Getting shared memory when requesting MAP_SHARED does sound like the right thing > to do ... > > So let's try this: > > Acked-by: David Hildenbrand (Arm) Thanks! Yeah let's revisit this one later more broadly. > > -- > Cheers, > > David -- Cheers, Lorenzo