Date: Thu, 6 Dec 2018 12:26:18 -0800 From: Andy Lutomirski <luto@...nel.org> To: Rick Edgecombe <rick.p.edgecombe@...el.com> Cc: Andrew Lutomirski <luto@...nel.org>, Tycho Andersen <tycho@...ho.ws>, LKML <linux-kernel@...r.kernel.org>, Daniel Borkmann <daniel@...earbox.net>, Ard Biesheuvel <ard.biesheuvel@...aro.org>, Alexei Starovoitov <ast@...nel.org>, Steven Rostedt <rostedt@...dmis.org>, Jessica Yu <jeyu@...nel.org>, Linux-MM <linux-mm@...ck.org>, Jann Horn <jannh@...gle.com>, Nadav Amit <nadav.amit@...il.com>, "Dock, Deneen T" <deneen.t.dock@...el.com>, Peter Zijlstra <peterz@...radead.org>, Kristen Carlson Accardi <kristen@...ux.intel.com>, Andrew Morton <akpm@...ux-foundation.org>, Will Deacon <will.deacon@....com>, Ingo Molnar <mingo@...hat.com>, Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Masami Hiramatsu <mhiramat@...nel.org>, "Naveen N . Rao" <naveen.n.rao@...ux.vnet.ibm.com>, "David S. Miller" <davem@...emloft.net>, Network Development <netdev@...r.kernel.org>, Dave Hansen <dave.hansen@...el.com> Subject: Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages On Thu, Dec 6, 2018 at 12:20 PM Edgecombe, Rick P <rick.p.edgecombe@...el.com> wrote: > > On Thu, 2018-12-06 at 11:19 -0800, Andy Lutomirski wrote: > > On Thu, Dec 6, 2018 at 11:01 AM Tycho Andersen <tycho@...ho.ws> wrote: > > > > > > On Thu, Dec 06, 2018 at 10:53:50AM -0800, Andy Lutomirski wrote: > > > > > If we are going to unmap the linear alias, why not do it at vmalloc() > > > > > time rather than vfree() time? > > > > > > > > That’s not totally nuts. Do we ever have code that expects __va() to > > > > work on module data? Perhaps crypto code trying to encrypt static > > > > data because our APIs don’t understand virtual addresses. I guess if > > > > highmem is ever used for modules, then we should be fine. > > > > > > > > RO instead of not present might be safer. But I do like the idea of > > > > renaming Rick's flag to something like VM_XPFO or VM_NO_DIRECT_MAP and > > > > making it do all of this. > > > > > > Yeah, doing it for everything automatically seemed like it was/is > > > going to be a lot of work to debug all the corner cases where things > > > expect memory to be mapped but don't explicitly say it. And in > > > particular, the XPFO series only does it for user memory, whereas an > > > additional flag like this would work for extra paranoid allocations > > > of kernel memory too. > > > > > > > I just read the code, and I looks like vmalloc() is already using > > highmem (__GFP_HIGH) if available, so, on big x86_32 systems, for > > example, we already don't have modules in the direct map. > > > > So I say we go for it. This should be quite simple to implement -- > > the pageattr code already has almost all the needed logic on x86. The > > only arch support we should need is a pair of functions to remove a > > vmalloc address range from the address map (if it was present in the > > first place) and a function to put it back. On x86, this should only > > be a few lines of code. > > > > What do you all think? This should solve most of the problems we have. > > > > If we really wanted to optimize this, we'd make it so that > > module_alloc() allocates memory the normal way, then, later on, we > > call some function that, all at once, removes the memory from the > > direct map and applies the right permissions to the vmalloc alias (or > > just makes the vmalloc alias not-present so we can add permissions > > later without flushing), and flushes the TLB. And we arrange for > > vunmap to zap the vmalloc range, then put the memory back into the > > direct map, then free the pages back to the page allocator, with the > > flush in the appropriate place. > > > > I don't see why the page allocator needs to know about any of this. > > It's already okay with the permissions being changed out from under it > > on x86, and it seems fine. Rick, do you want to give some variant of > > this a try? > Hi, > > Sorry, I've been having email troubles today. > > I found some cases where vmap with PAGE_KERNEL_RO happens, which would not set > NP/RO in the directmap, so it would be sort of inconsistent whether the > directmap of vmalloc range allocations were readable or not. I couldn't see any > places where it would cause problems today though. > > I was ready to assume that all TLBs don't cache NP, because I don't know how > usages where a page fault is used to load something could work without lots of > flushes. Or the architecture just fixes up the spurious faults, I suppose. I'm only well-educated on the x86 mmu. > If that's the case, then all archs with directmap permissions could > share a single vmalloc special permission flush implementation that works like > Andy described originally. It could be controlled with an > ARCH_HAS_DIRECT_MAP_PERMS. We would just need something like set_pages_np and > set_pages_rw on any archs with directmap permissions. So seems simpler to me > (and what I have been doing) unless I'm missing the problem. Hmm. The only reason I've proposed anything fancier was because I was thinking of minimizing flushes, but I think I'm being silly. This sequence ought to work optimally: - vmalloc(..., VM_HAS_DIRECT_MAP_PERMS); /* no flushes */ - Write some data, via vmalloc's return address. - Use some set_memory_whatever() functions to update permissions, which will flush, hopefully just once. - Run the module code! - vunmap -- this will do a single flush that will fix everything. This does require that set_pages_np() or set_memory_np() or whatever exists and that it's safe to do that, then flush, and then set_pages_rw(). So maybe you want set_pages_np_noflush() and set_pages_rw_noflush() to make it totally clear what's supposed to happen. --Andy
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