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 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? > > > > > 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. Will [1] https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/commit/?h=for-next/core&id=c55191e96caa9d787e8f682c5e525b7f8172a3b4
