On Wed, 2019-08-14 at 09:48 -0700, Dave Hansen wrote: > On 8/14/19 9:27 AM, Yu-cheng Yu wrote: > > On Tue, 2019-08-13 at 15:55 -0700, Andy Lutomirski wrote: > > > On Tue, Aug 13, 2019 at 2:02 PM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote: > > > > When a task does fork(), its shadow stack (SHSTK) must be duplicated > > > > for the child. This patch implements a flow similar to copy-on-write > > > > of an anonymous page, but for SHSTK. > > > > > > > > A SHSTK PTE must be RO and dirty. This dirty bit requirement is used > > > > to effect the copying. In copy_one_pte(), clear the dirty bit from a > > > > SHSTK PTE to cause a page fault upon the next SHSTK access. At that > > > > time, fix the PTE and copy/re-use the page. > > > > > > Is using VM_SHSTK and special-casing all of this really better than > > > using a special mapping or other pseudo-file-backed VMA and putting > > > all the magic in the vm_operations? > > > > A special mapping is cleaner. However, we also need to exclude normal [RO + > > dirty] pages from shadow stack. > > I don't understand what you are saying. > > Are you saying that we need this VM_SHSTK flag in order to exclude > RO+HW-Dirty pages from being created in non-shadow-stack VMAs? We use VM_SHSTK for page fault handling (the special-casing). If we have a special mapping, all these become cleaner (but more code). However, we still need most of the PTE macros (e.g. ptep_set_wrprotect, PAGE_DIRTY_SW, etc.). Yu-cheng
