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Shutemov" References: <20230718234512.1690985-1-seanjc@google.com> <20230718234512.1690985-7-seanjc@google.com> From: Paolo Bonzini Subject: Re: [RFC PATCH v11 06/29] KVM: Introduce KVM_SET_USER_MEMORY_REGION2 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 7E32F120013 X-Stat-Signature: k5xbctmdkdgr5ftn7ywj3dpegmacg8ae X-HE-Tag: 1690819096-868146 X-HE-Meta: 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 s4cCswCi y/Mv+4wIxzaiCOJHKNLDmBLNMKlZWoPPwpFrMCK19YOUtHCDIYpE557iJ3R19dPy3qQVVl31j0Q4sHLeQ3iTe4HDZWPc6ttWkb9stbE0N3QSXB/WY0MwF5yE8OQCOWwkMYV7MvhQHcuwirny1MNLRQZMPFXHQRYAF5oshlT2CbHTERFgw69DdREZ280Dml3Jk8ytEQwGjd3I7EOgkiTo2uvsS3GOzZoLJB3An+L27ROFbVv82goZkBga59K/+Q5htEY5+o42sJfecv8TSFcIExl3p8DVzoj0SbcPNU2TP8WRE2cvdffE1ujrqw4EDdOv98Tjwsbz5KTV5FdYrNZW8G2ywC0PxTTZuRy7qZdADXya5ZN5/jDt2nSaz2E5UzUueX/Co7J4p72APlE1xvaUNYV7ACCG2ie2IF1KB6XRCWPzZmpUwJ2ycql7tNGmGBA9z7vahXqUuL9qhpTIyCFqDRrAaKw== 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 7/29/23 02:03, Sean Christopherson wrote: > KVM would need to do multiple uaccess reads, but that's not a big > deal. Am I missing something, or did past us just get too clever and > miss the obvious solution? You would have to introduce struct kvm_userspace_memory_region2 anyway, though not a new ioctl, for two reasons: 1) the current size of the struct is part of the userspace API via the KVM_SET_USER_MEMORY_REGION #define, so introducing a new struct is the easiest way to preserve this 2) the struct can (at least theoretically) enter the ABI of a shared library, and such mismatches are really hard to detect and resolve. So it's better to add the padding to a new struct, and keep struct kvm_userspace_memory_region backwards-compatible. As to whether we should introduce a new ioctl: doing so makes KVM_SET_USER_MEMORY_REGION's detection of bad flags a bit more robust; it's not like we cannot introduce new flags at all, of course, but having out-of-bounds reads as a side effect of new flags is a bit nasty. Protecting programs from their own bugs gets into diminishing returns very quickly, but introducing a new ioctl can make exploits a bit harder when struct kvm_userspace_memory_region is on the stack and adjacent to an attacker-controlled location. Paolo