> On Dec 4, 2018, at 3:51 PM, Edgecombe, Rick P <rick.p.edgecombe@xxxxxxxxx> wrote: > > On Tue, 2018-12-04 at 12:36 -0800, Nadav Amit wrote: >>> On 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. >>> >>> The other reasoning was that calling set_memory_nx isn't doing what we are >>> actually trying to do which is prevent the pages from getting released too >>> early. >>> >>> A more clear solution for all of this might involve refactoring some of the >>> set_memory_ de-allocation logic out into __weak functions in either modules >>> or >>> vmalloc. As Jessica points out in the other thread though, modules does a >>> lot >>> more stuff there than the other module_alloc callers. I think it may take >>> some >>> thought to centralize AND make it optimal for every >>> module_alloc/vmalloc_exec >>> user and arch. >>> >>> But for now with the change in vmalloc, we can block the executable mapping >>> freed page re-use issue in a cross platform way. >> >> Please understand me correctly - I didn’t mean that your patches are not >> needed. > Ok, I think I understand. I have been pondering these same things after Masami > Hiramatsu's comments on this thread the other day. > >> All I did is asking - how come the PTEs are executable when they are cleared >> they are executable, when in fact we manipulate them when the module is >> removed. > I think the directmap used to be RWX so maybe historically its trying to return > it to its default state? Not sure. > >> I think I try to deal with a similar problem to the one you encounter - >> broken W^X. The only thing that bothered me in regard to your patches (and >> only after I played with the code) is that there is still a time-window in >> which W^X is broken due to disable_ro_nx(). > Totally agree there is overlap in the fixes and we should sync. > > What do you think about Andy's suggestion for doing the vfree cleanup in vmalloc > with arch hooks? So the allocation goes into vfree fully setup and vmalloc frees > it and on x86 resets the direct map. As long as you do it, I have no problem ;-) You would need to consider all the callers of module_memfree(), and probably to untangle at least part of the mess in pageattr.c . If you are up to it, just say so, and I’ll drop this patch. All I can say is “good luck with all that”.
