On Fri, 2019-04-26 at 10:40 -0700, Andy Lutomirski wrote: > > On Apr 26, 2019, at 8:19 AM, James Bottomley <James.Bottomley@hanse > > npartnership.com> wrote: > > > > On Fri, 2019-04-26 at 08:07 -0700, Andy Lutomirski wrote: > > > > On Apr 26, 2019, at 7:57 AM, James Bottomley > > > > <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > > > > > On Fri, 2019-04-26 at 07:46 -0700, Dave Hansen wrote: > > > > > > On 4/25/19 2:45 PM, Mike Rapoport wrote: > > > > > > After the isolated system call finishes, the mappings > > > > > > created during its execution are cleared. > > > > > > > > > > Yikes. I guess that stops someone from calling write() a > > > > > bunch of times on every filesystem using every block device > > > > > driver and all the DM code to get a lot of code/data faulted > > > > > in. But, it also means not even long-running processes will > > > > > ever have a chance of behaving anything close to normally. > > > > > > > > > > Is this something you think can be rectified or is there > > > > > something fundamental that would keep SCI page tables from > > > > > being cached across different invocations of the same > > > > > syscall? > > > > > > > > There is some work being done to look at pre-populating the > > > > isolated address space with the expected execution footprint of > > > > the system call, yes. It lessens the ROP gadget protection > > > > slightly because you might find a gadget in the pre-populated > > > > code, but it solves a lot of the overhead problem. > > > > > > I’m not even remotely a ROP expert, but: what stops a ROP payload > > > from using all the “fault-in” gadgets that exist — any function > > > that can return on an error without doing to much will fault in > > > the whole page containing the function. > > > > The address space pre-population is still per syscall, so you don't > > get access to the code footprint of a different syscall. So the > > isolated address space is created anew for every system call, it's > > just pre-populated with that system call's expected footprint. > > That’s not what I mean. Suppose I want to use a ROP gadget in > vmalloc(), but vmalloc isn’t in the page tables. Then first push > vmalloc itself into the stack. As long as RDI contains a sufficiently > ridiculous value, it should just return without doing anything. And > it can return right back into the ROP gadget, which is now available. Yes, it's not perfect, but stack space for a smashing attack is at a premium and now you need two stack frames for every gadget you chain instead of one so we've halved your ability to chain gadgets. > > > To improve this, we would want some thing that would try to check > > > whether the caller is actually supposed to call the callee, which > > > is more or less the hard part of CFI. So can’t we just do CFI > > > and call it a day? > > > > By CFI you mean control flow integrity? In theory I believe so, > > yes, but in practice doesn't it require a lot of semantic object > > information which is easy to get from higher level languages like > > java but a bit more difficult for plain C. > > Yes. As I understand it, grsecurity instruments gcc to create some > kind of hash of all function signatures. Then any indirect call can > effectively verify that it’s calling a function of the right type. > And every return verified a cookie. > > On CET CPUs, RET gets checked directly, and I don’t see the benefit > of SCI. Presumably you know something I don't but I thought CET CPUs had been planned for release for ages, but not actually released yet? > > > On top of that, a robust, maintainable implementation of this > > > thing seems very complicated — for example, what happens if > > > vfree() gets called? > > > > Address space Local vs global object tracking is another thing on > > our list. What we'd probably do is verify the global object was > > allowed to be freed and then hand it off safely to the main kernel > > address space. > > This seems exceedingly complicated. It's a research project: we're exploring what's possible so we can choose the techniques that give the best security improvement for the additional overhead. James
