> 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. 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 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().
