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Fri, 19 May 2023 10:32:49 -0700 (PDT) MIME-Version: 1.0 References: <20230511182426.1898675-1-axelrasmussen@google.com> <32fdc2c8-b86b-92f3-1d5e-64db6be29126@redhat.com> In-Reply-To: From: Axel Rasmussen Date: Fri, 19 May 2023 10:32:13 -0700 Message-ID: Subject: Re: [PATCH 1/3] mm: userfaultfd: add new UFFDIO_SIGBUS ioctl To: Peter Xu Cc: Jiaqi Yan , David Hildenbrand , James Houghton , Alexander Viro , Andrew Morton , Christian Brauner , Hongchen Zhang , Huang Ying , "Liam R. Howlett" , Miaohe Lin , "Mike Rapoport (IBM)" , Nadav Amit , Naoya Horiguchi , Shuah Khan , ZhangPeng , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, Anish Moorthy Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 358D440019 X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: o7w54iwpx58s9r9m61rgcuh6ojn6xusz X-HE-Tag: 1684517569-401639 X-HE-Meta: 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 GoFBhOhz ikjh2xjCcbR83gCKrnoSa+CRndXlLqDLK11w2Kix8Uk+TRqCbmLRNySIaLqzsGCl3CQpiXopyjyFrmAbXOyIRjqVRhEfuZWlSN7H8Wjg9DaERHx6BhiPqFJaaohGVFzGYBaghR62B5dzFrbDLoDTDMFHzbNH078TqJ5uQzw60ca8oSxUwEFBtMdffQeIwoBMlpNJyBm0Gy6lyHgbnN6a2M+rirBentsEZ6x4XrlTH/7Ks3kvHe0wvXW0p1vkg4GiXeAX7XTxpwSmUNYzaod5rlv+/gWOS0i8l8YD+edFBLs/4+wJAsTDxfR0ZPez1OZZhVyRp+VjtJsFuYnpcqadW16pRHPK8rTgmdTcgwqM3vLQACOUDMyVUZdzGwnMYCWFeXDie X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, May 19, 2023 at 9:20=E2=80=AFAM Peter Xu wrote: > > Hi, Jiaqi, > > On Fri, May 19, 2023 at 08:04:09AM -0700, Jiaqi Yan wrote: > > I don't think CAP_ADMIN is something we can work around: a VMM must be > > a good citizen to avoid introducing any vulnerability to the host or > > guest. > > > > On the other hand, "Userfaults allow the implementation of on-demand > > paging from userland and more generally they allow userland to take > > control of various memory page faults, something otherwise only the > > kernel code could do." [3]. I am not familiar with the UFFD internals, > > but our use case seems to match what UFFD wants to provide: without > > affecting the whole world, give a specific userspace (without > > CAP_ADMIN) the ability to handle page faults (indirectly emulate a > > HWPOISON page (in my mind I treat it as SetHWPOISON(page) + > > TestHWPOISON(page) operation in kernel's PF code)). So is it fair to > > say what Axel provided here is "provide !ADMIN somehow"? > > > > [3]https://docs.kernel.org/admin-guide/mm/userfaultfd.html > > Userfault keywords on "user", IMHO. We don't strictly need userfault to > resolve anything regarding CAP_ADMIN problems. MADV_DONTNEED also dosn't > need CAP_ADMIN, same to any new madvise() if we want to make it useful fo= r > injecting poisoned ptes with !ADMIN and limit it within current->mm. > > But I think you're right that userfaultfd always tried to avoid having > ADMIN and keep everything within its own scope of permissions. > > So again, totally no objection on make it uffd specific for now if you gu= ys > are all happy with it, but just to be clear that it's (to me) mostly for > avoiding another WAKE, and afaics that's not really for solving the ADMIN > issue here. How about this plan: Since the concrete use case we have (postcopy live migration) is UFFD-specific, let's leave it as a UFFDIO_* operation for now. If in the future we come up with a non-UFFD use case, we can add a new MADV_* which does this operation at that point. From my perspective they could even share most of the same implementation code. I don't think it's a big problem keeping the UFFDIO_* version too at that point, because it still provides some (perhaps small) value: - Combines the operation + waking into one syscall - It allows us to support additional UFFD flags which modify / extend the operation in UFFD-specific ways, if we want to add those in the future Seem reasonable? If so, I'll send a v2 with documentation updates. > > Thanks, > > -- > Peter Xu >