> On Dec 4, 2018, at 4:29 PM, Edgecombe, Rick P <rick.p.edgecombe@xxxxxxxxx> wrote: > > On Tue, 2018-12-04 at 16:01 -0800, Nadav Amit wrote: >>> 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”. > I thought you were trying to prevent having any memory that at any time was W+X, > how does vfree help with the module load time issues, where it starts WRX on > x86? I didn’t say it does. The patch I submitted before [1] should deal with the issue of module loading, and I still think it is required. I also addressed the kprobe and ftrace issues that you raised. Perhaps it makes more sense that I will include the patch I proposed for module cleanup to make the patch-set “complete”. If you finish the changes you propose before the patch is applied, it could be dropped. I just want to get rid of this series, as it keeps collecting more and more patches. I suspect it will not be the last version anyhow. [1] https://lkml.org/lkml/2018/11/21/305
