On Tue, Dec 4, 2018 at 11:45 AM Nadav Amit <nadav.amit@xxxxxxxxx> wrote: > > > On Dec 4, 2018, at 10:56 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote: > > > > On Mon, Dec 3, 2018 at 5:43 PM Nadav Amit <nadav.amit@xxxxxxxxx> 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??? > > > > 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. > Dunno. I did once chase down a bug where some memory got freed while it was still read-only, and the results were hilarious and hard to debug, since the explosion happened long after the buggy code finished. --Andy
