> On Dec 4, 2018, at 3:52 PM, Edgecombe, Rick P <rick.p.edgecombe@xxxxxxxxx> wrote: > >> On Tue, 2018-12-04 at 12:09 -0800, Andy Lutomirski wrote: >> On Tue, Dec 4, 2018 at 12:02 PM Edgecombe, Rick P >> <rick.p.edgecombe@xxxxxxxxx> wrote: >>> >>>> On Tue, 2018-12-04 at 16:03 +0000, Will Deacon wrote: >>>> On Mon, Dec 03, 2018 at 05:43:11PM -0800, Nadav Amit wrote: >>>>>> On Nov 27, 2018, at 4:07 PM, Rick Edgecombe < >>>>>> rick.p.edgecombe@xxxxxxxxx> >>>>>> wrote: >>>>>> >>>>>> Since vfree will lazily flush the TLB, but not lazily free the >>>>>> underlying >>>>>> pages, >>>>>> it often leaves stale TLB entries to freed pages that could get re- >>>>>> used. >>>>>> This is >>>>>> undesirable for cases where the memory being freed has special >>>>>> permissions >>>>>> such >>>>>> as executable. >>>>> >>>>> So I am trying to finish my patch-set for preventing transient W+X >>>>> mappings >>>>> from taking space, by handling kprobes & ftrace that I missed (thanks >>>>> again >>>>> for >>>>> pointing it out). >>>>> >>>>> But all of the sudden, I don’t understand why we have the problem that >>>>> this >>>>> (your) patch-set deals with at all. We already change the mappings to >>>>> make >>>>> the memory writable before freeing the memory, so why can’t we make it >>>>> non-executable at the same time? Actually, why do we make the module >>>>> memory, >>>>> including its data executable before freeing it??? >>>> >>>> Yeah, this is really confusing, but I have a suspicion it's a combination >>>> of the various different configurations and hysterical raisins. We can't >>>> rely on module_alloc() allocating from the vmalloc area (see nios2) nor >>>> can we rely on disable_ro_nx() being available at build time. >>>> >>>> If we *could* rely on module allocations always using vmalloc(), then >>>> we could pass in Rick's new flag and drop disable_ro_nx() altogether >>>> afaict -- who cares about the memory attributes of a mapping that's about >>>> to disappear anyway? >>>> >>>> Is it just nios2 that does something different? >>>> >>>> Will >>> >>> Yea it is really intertwined. I think for x86, set_memory_nx everywhere >>> would >>> solve it as well, in fact that was what I first thought the solution should >>> be >>> until this was suggested. It's interesting that from the other thread Masami >>> Hiramatsu referenced, set_memory_nx was suggested last year and would have >>> inadvertently blocked this on x86. But, on the other architectures I have >>> since >>> learned it is a bit different. >>> >>> It looks like actually most arch's don't re-define set_memory_*, and so all >>> of >>> the frob_* functions are actually just noops. In which case allocating RWX >>> is >>> needed to make it work at all, because that is what the allocation is going >>> to >>> stay at. So in these archs, set_memory_nx won't solve it because it will do >>> nothing. >>> >>> On x86 I think you cannot get rid of disable_ro_nx fully because there is >>> the >>> changing of the permissions on the directmap as well. You don't want some >>> other >>> caller getting a page that was left RO when freed and then trying to write >>> to >>> it, if I understand this. >>> >> >> Exactly. >> >> After slightly more thought, I suggest renaming VM_IMMEDIATE_UNMAP to >> VM_MAY_ADJUST_PERMS or similar. It would have the semantics you want, >> but it would also call some arch hooks to put back the direct map >> permissions before the flush. Does that seem reasonable? It would >> need to be hooked up that implement set_memory_ro(), but that should >> be quite easy. If nothing else, it could fall back to set_memory_ro() >> in the absence of a better implementation. > > With arch hooks, I guess we could remove disable_ro_nx then. I think you would > still have to flush twice on x86 to really have no W^X violating window from the > direct map (I think x86 is the only one that sets permissions there?). But this > could be down from sometimes 3. You could also directly vfree non exec RO memory > without set_memory_, like in BPF. Just one flush if you’re careful. Set the memory not-present in the direct map and zap it from the vmap area, then flush, then set it RW in the > > The vfree deferred list would need to be moved since it then couldn't reuse the > allocations since now the vfreed memory might be RO. It could kmalloc, or lookup > the vm_struct. So would probably be a little slower in the interrupt case. Is > this ok? I’m fine with that. For eBPF, we should really have a lookaside list for small allocations.
