On Tue, 2018-12-04 at 14:48 -0800, Nadav Amit 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. > Hold on...so on architectures that don't have set_memory_ but do have something like NX, wont the executable stale TLB continue to live to re-used pages, and so it doesn't fix the problem this patch is trying to address generally? I see at least a couple archs use vmalloc and have an exec bit, but don't define set_memory_*.
