On Dec 4, 2018, at 2:48 PM, Nadav Amit <nadav.amit@xxxxxxxxx> wrote: >> On Dec 4, 2018, at 11:48 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote: >> >> 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. > > This piece of code causes me pain and misery. > > So, it turns out that the direct map is *not* changed if you just change > the NX-bit. See change_page_attr_set_clr(): > > /* No alias checking for _NX bit modifications */ > checkalias = (pgprot_val(mask_set) | pgprot_val(mask_clr)) != _PAGE_NX; > > How many levels of abstraction are broken in the way? What would happen > if somebody tries to change the NX-bit and some other bit in the PTE? > Luckily, I don’t think someone does… at least for now. > > So, again, I think the change I proposed makes sense. nios2 does not have > set_memory_x() and it will not be affected. > > [ I can add a comment, although I don’t have know if nios2 has an NX bit, > and I don’t find any code that defines PTEs. Actually where is pte_present() > of nios2 being defined? Whatever. ] > At least rename the function, then. The last thing we need is for disable_ro_nx to *enable* NX.
