> On Dec 4, 2018, at 5:09 PM, Edgecombe, Rick P <rick.p.edgecombe@xxxxxxxxx> wrote: > > 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_*. Again, this does not come instead of your patch (the one in this thread). And if you follow Andy’s suggestion, the patch I propose will not be needed. However, in the meantime - I see no reason to mark data as executable, even for a brief period of time.
