Messages in this thread | | | Date | Tue, 4 Dec 2018 16:03:04 +0000 | From | Will Deacon <> | Subject | Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages |
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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@intel.com> 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
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