Date: Tue, 4 Dec 2018 14:48:40 -0800 From: Nadav Amit <nadav.amit@...il.com> To: Andy Lutomirski <luto@...nel.org> Cc: Rick Edgecombe <rick.p.edgecombe@...el.com>, Andrew Morton <akpm@...ux-foundation.org>, Will Deacon <will.deacon@....com>, Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, "Naveen N . Rao" <naveen.n.rao@...ux.vnet.ibm.com>, Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>, "David S. Miller" <davem@...emloft.net>, Masami Hiramatsu <mhiramat@...nel.org>, Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...hat.com>, Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, jeyu@...nel.org, Network Development <netdev@...r.kernel.org>, Ard Biesheuvel <ard.biesheuvel@...aro.org>, Jann Horn <jannh@...gle.com>, Kristen Carlson Accardi <kristen@...ux.intel.com>, Dave Hansen <dave.hansen@...el.com>, "Dock, Deneen T" <deneen.t.dock@...el.com>, Peter Zijlstra <peterz@...radead.org> Subject: Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages > On Dec 4, 2018, at 11:48 AM, Andy Lutomirski <luto@...nel.org> wrote: > > On Tue, Dec 4, 2018 at 11:45 AM Nadav Amit <nadav.amit@...il.com> wrote: >>> On Dec 4, 2018, at 10:56 AM, Andy Lutomirski <luto@...nel.org> wrote: >>> >>> On Mon, Dec 3, 2018 at 5:43 PM Nadav Amit <nadav.amit@...il.com> wrote: >>>>> On Nov 27, 2018, at 4:07 PM, Rick Edgecombe <rick.p.edgecombe@...el.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. > > 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. ]
Powered by blists - more mailing lists
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.