> On Dec 4, 2018, at 10:56 AM, Andy Lutomirski <l...@kernel.org> wrote: > > On Mon, Dec 3, 2018 at 5:43 PM Nadav Amit <nadav.a...@gmail.com> wrote: >>> On Nov 27, 2018, at 4:07 PM, Rick Edgecombe <rick.p.edgeco...@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??? > > All the code you're looking at is IMO a very awkward and possibly > incorrect of doing what's actually necessary: putting the direct map > the way it wants to be. > > Can't we shove this entirely mess into vunmap? Have a flag (as part > of vmalloc like in Rick's patch or as a flag passed to a vfree variant > directly) that makes the vunmap code that frees the underlying pages > also reset their permissions? > > Right now, we muck with set_memory_rw() and set_memory_nx(), which > both have very awkward (and inconsistent with each other!) semantics > when called on vmalloc memory. And they have their own flushes, which > is inefficient. Maybe the right solution is for vunmap to remove the > vmap area PTEs, call into a function like set_memory_rw() that resets > the direct maps to their default permissions *without* flushing, and > then to do a single flush for everything. Or, even better, to cause > the change_page_attr code to do the flush and also to flush the vmap > area all at once so that very small free operations can flush single > pages instead of flushing globally.
Thanks for the explanation. I read it just after I realized that indeed the whole purpose of this code is to get cpa_process_alias() update the corresponding direct mapping. This thing (pageattr.c) indeed seems over-engineered and very unintuitive. Right now I have a list of patch-sets that I owe, so I don’t have the time to deal with it. But, I still think that disable_ro_nx() should not call set_memory_x(). IIUC, this breaks W+X of the direct-mapping which correspond with the module memory. Does it ever stop being W+X?? I’ll have another look.
