On Thu, May 27, 2021 at 08:50:30AM +0100, Steven Price wrote: > On 24/05/2021 19:11, Catalin Marinas wrote: > > I had some (random) thoughts on how to make things simpler, maybe. I > > think most of these races would have been solved if we required PROT_MTE > > in the VMM but this has an impact on the VMM if it wants to use MTE > > itself. If such requirement was in place, all KVM needed to do is check > > PG_mte_tagged. > > > > So what we actually need is a set_pte_at() in the VMM to clear the tags > > and set PG_mte_tagged. Currently, we only do this if the memory type is > > tagged (PROT_MTE) but it's not strictly necessary. > > > > As an optimisation for normal programs, we don't want to do this all the > > time but the visible behaviour wouldn't change (well, maybe for ptrace > > slightly). However, it doesn't mean we couldn't for a VMM, with an > > opt-in via prctl(). This would add a MMCF_MTE_TAG_INIT bit (couldn't > > think of a better name) to mm_context_t.flags and set_pte_at() would > > behave as if the pte was tagged without actually mapping the memory in > > user space as tagged (protection flags not changed). Pages that don't > > support tagging are still safe, just some unnecessary ignored tag > > writes. This would need to be set before the mmap() for the guest > > memory. > > > > If we want finer-grained control we'd have to store this information in > > the vma flags, in addition to VM_MTE (e.g. VM_MTE_TAG_INIT) but without > > affecting the actual memory type. The easiest would be another pte bit, > > though we are short on them. A more intrusive (not too bad) approach is > > to introduce a set_pte_at_vma() and read the flags directly in the arch > > code. In most places where set_pte_at() is called on a user mm, the vma > > is also available. > > > > Anyway, I'm not saying we go this route, just thinking out loud, get > > some opinions. > > Does get_user_pages() actually end up calling set_pte_at() normally? Not always, at least as how it's called from hva_to_pfn(). My reading of the get_user_page_fast_only() is that it doesn't touch the pte, just walks the page tables and pins the page. Of course, it expects a valid pte to have been set in the VMM already, otherwise it doesn't pin any page and the caller falls back to the slow path. The slow path, get_user_pages_unlocked(), passes FOLL_TOUCH and set_pte_at() will be called either in follow_pfn_pte() if it was valid or via faultin_page() -> handle_mm_fault(). > If not then on the normal user_mem_abort() route although we can > easily check VM_MTE_TAG_INIT there's no obvious place to hook in to > ensure that the pages actually allocated have the PG_mte_tagged flag. I don't think it helps if we checked such vma flag in user_mem_abort(), we'd still have the race with set_pte_at() on the page flags. What I was trying to avoid is touching the page flags in too many places, so deferring this always to set_pte_at() in the VMM. > I'm also not sure how well this would work with the MMU notifiers path > in KVM. With MMU notifiers (i.e. the VMM replacing a page in the > memslot) there's not even an obvious hook to enforce the VMA flag. So I > think we'd end up with something like the sanitise_mte_tags() function > to at least check that the PG_mte_tagged flag is set on the pages > (assuming that the trigger for the MMU notifier has done the > corresponding set_pte_at()). Admittedly this might close the current > race documented there. If we kept this check to the VMM set_pte_at(), I think we can ignore the notifiers. > It also feels wrong to me to tie this to a process with prctl(), it > seems much more normal to implement this as a new mprotect() flag as > this is really a memory property not a process property. And I think > we'll find some scary corner cases if we try to associate everything > back to a process - although I can't instantly think of anything that > will actually break. I agree, tying it to the process looks wrong, only that it's less intrusive. I don't think it would break anything, only potential performance regression. A process would still need to pass PROT_MTE to be able to get tag checking. That's basically what I had in an early MTE implementation with clear_user_page() always zeroing the tags. I agree with you that a vma flag would be better but it's more complicated without an additional pte bit. We could also miss some updates as mprotect() for example checks for pte_same() before calling set_pte_at() (it would need to check the updated vma flags). I'll review the latest series but I'm tempted to move the logic in santise_mte_tags() to mte.c and take the big lock in there if PG_mte_tagged is not already set. If we hit performance issues, we can optimise this later to have the page flag set already on creation (new PROT flag, prctl etc.). -- Catalin _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
