> On Dec 5, 2018, at 11:29 PM, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote: > >> On Thu, 6 Dec 2018 at 00:16, Andy Lutomirski <luto@xxxxxxxxxx> wrote: >> >>> On Wed, Dec 5, 2018 at 3:41 AM Will Deacon <will.deacon@xxxxxxx> wrote: >>> >>>> On Tue, Dec 04, 2018 at 12:09:49PM -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 wrAcked-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> > itable 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? >>>>>> >>>>> 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. >>> >>> Of course, I forgot about the linear mapping. On arm64, we've just queued >>> support for reflecting changes to read-only permissions in the linear map >>> [1]. So, whilst the linear map is always non-executable, we will need to >>> make parts of it writable again when freeing the module. >>> >>>> 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. >>> >>> You mean set_memory_rw() here, right? Although, eliding the TLB invalidation >>> would open up a window where the vmap mapping is executable and the linear >>> mapping is writable, which is a bit rubbish. >>> >> >> Right, and Rick pointed out the same issue. Instead, we should set >> the direct map not-present or its ARM equivalent, then do the flush, >> then make it RW. I assume this also works on arm and arm64, although >> I don't know for sure. On x86, the CPU won't cache not-present PTEs. > > If we are going to unmap the linear alias, why not do it at vmalloc() > time rather than vfree() time? That’s not totally nuts. Do we ever have code that expects __va() to work on module data? Perhaps crypto code trying to encrypt static data because our APIs don’t understand virtual addresses. I guess if highmem is ever used for modules, then we should be fine. RO instead of not present might be safer. But I do like the idea of renaming Rick's flag to something like VM_XPFO or VM_NO_DIRECT_MAP and making it do all of this. (It seems like some people call it the linear map and some people call it the direct map. Is there any preference?)
