> On Feb 22, 2019, at 3:59 PM, Andy Lutomirski <l...@kernel.org> wrote: > > On Fri, Feb 22, 2019 at 3:02 PM Jann Horn <ja...@google.com> wrote: >> On Fri, Feb 22, 2019 at 11:39 PM Nadav Amit <na...@vmware.com> wrote: >>>> On Feb 22, 2019, at 2:21 PM, Nadav Amit <na...@vmware.com> wrote: >>>> >>>>> On Feb 22, 2019, at 2:17 PM, Jann Horn <ja...@google.com> wrote: >>>>> >>>>> On Fri, Feb 22, 2019 at 11:08 PM Nadav Amit <na...@vmware.com> wrote: >>>>>>> On Feb 22, 2019, at 1:43 PM, Jann Horn <ja...@google.com> wrote: >>>>>>> >>>>>>> (adding some people from the text_poke series to the thread, removing >>>>>>> stable@) >>>>>>> >>>>>>> On Fri, Feb 22, 2019 at 8:55 PM Andy Lutomirski <l...@amacapital.net> >>>>>>> wrote: >>>>>>>>> On Feb 22, 2019, at 11:34 AM, Alexei Starovoitov >>>>>>>>> <alexei.starovoi...@gmail.com> wrote: >>>>>>>>>> On Fri, Feb 22, 2019 at 02:30:26PM -0500, Steven Rostedt wrote: >>>>>>>>>> On Fri, 22 Feb 2019 11:27:05 -0800 >>>>>>>>>> Alexei Starovoitov <alexei.starovoi...@gmail.com> wrote: >>>>>>>>>> >>>>>>>>>>>> On Fri, Feb 22, 2019 at 09:43:14AM -0800, Linus Torvalds wrote: >>>>>>>>>>>> >>>>>>>>>>>> Then we should still probably fix up "__probe_kernel_read()" to not >>>>>>>>>>>> allow user accesses. The easiest way to do that is actually likely >>>>>>>>>>>> to >>>>>>>>>>>> use the "unsafe_get_user()" functions *without* doing a >>>>>>>>>>>> uaccess_begin(), which will mean that modern CPU's will simply >>>>>>>>>>>> fault >>>>>>>>>>>> on a kernel access to user space. >>>>>>>>>>> >>>>>>>>>>> On bpf side the bpf_probe_read() helper just calls >>>>>>>>>>> probe_kernel_read() >>>>>>>>>>> and users pass both user and kernel addresses into it and expect >>>>>>>>>>> that the helper will actually try to read from that address. >>>>>>>>>>> >>>>>>>>>>> If __probe_kernel_read will suddenly start failing on all user >>>>>>>>>>> addresses >>>>>>>>>>> it will break the expectations. >>>>>>>>>>> How do we solve it in bpf_probe_read? >>>>>>>>>>> Call probe_kernel_read and if that fails call unsafe_get_user >>>>>>>>>>> byte-by-byte >>>>>>>>>>> in the loop? >>>>>>>>>>> That's doable, but people already complain that bpf_probe_read() is >>>>>>>>>>> slow >>>>>>>>>>> and shows up in their perf report. >>>>>>>>>> >>>>>>>>>> We're changing kprobes to add a specific flag to say that we want to >>>>>>>>>> differentiate between kernel or user reads. Can this be done with >>>>>>>>>> bpf_probe_read()? If it's showing up in perf report, I doubt a single >>>>>>>>> >>>>>>>>> so you're saying you will break existing kprobe scripts? >>>>>>>>> I don't think it's a good idea. >>>>>>>>> It's not acceptable to break bpf_probe_read uapi. >>>>>>>> >>>>>>>> If so, the uapi is wrong: a long-sized number does not reliably >>>>>>>> identify an address if you don’t separately know whether it’s a user >>>>>>>> or kernel address. s390x and 4G:4G x86_32 are the notable exceptions. >>>>>>>> I have lobbied for RISC-V and future x86_64 to join the crowd. I >>>>>>>> don’t know whether I’ll win this fight, but the uapi will probably >>>>>>>> have to change for at least s390x. >>>>>>>> >>>>>>>> What to do about existing scripts is a different question. >>>>>>> >>>>>>> This lack of logical separation between user and kernel addresses >>>>>>> might interact interestingly with the text_poke series, specifically >>>>>>> "[PATCH v3 05/20] x86/alternative: Initialize temporary mm for >>>>>>> patching" >>>>>>> (https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Flkml%2F20190221234451.17632-6-rick.p.edgecombe%40intel.com%2F&data=02%7C01%7Cnamit%40vmware.com%7Cbab53e52cc5c4ac4419008d69921d1f1%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C636864767879731941&sdata=2tqD7udTCfNbcNLcj5SFpZt8WwK5NwtgaWMKm1Ye1EE%3D&reserved=0) >>>>>>> and "[PATCH v3 06/20] x86/alternative: Use temporary mm for text >>>>>>> poking" >>>>>>> (https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Flkml%2F20190221234451.17632-7-rick.p.edgecombe%40intel.com%2F&data=02%7C01%7Cnamit%40vmware.com%7Cbab53e52cc5c4ac4419008d69921d1f1%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C636864767879731941&sdata=7%2BLShgLKnra6xzSkxdJrCclCacfdE5IdczwScW83nuE%3D&reserved=0), >>>>>>> right? If someone manages to get a tracing BPF program to trigger in a >>>>>>> task that has switched to the patching mm, could they use >>>>>>> bpf_probe_write_user() - which uses probe_kernel_write() after >>>>>>> checking that KERNEL_DS isn't active and that access_ok() passes - to >>>>>>> overwrite kernel text that is mapped writable in the patching mm? >>>>>> >>>>>> Yes, this is a good point. I guess text_poke() should be defined with >>>>>> “__kprobes” and open-code memcpy. >>>>>> >>>>>> Does it sound reasonable? >>>>> >>>>> Doesn't __text_poke() as implemented in the proposed patch use a >>>>> couple other kernel functions, too? Like switch_mm_irqs_off() and >>>>> pte_clear() (which can be a call into a separate function on paravirt >>>>> kernels)? >>>> >>>> I will move the pte_clear() to be done after the poking mm was unloaded. >>>> Give me a few minutes to send a sketch of what I think should be done. >>> >>> Err.. You are right, I don’t see an easy way of preventing a kprobe from >>> being set on switch_mm_irqs_off(), and open-coding this monster is too ugly. >>> >>> The reasonable solution seems to me as taking all the relevant pieces of >>> code (and data) that might be used during text-poking and encapsulating >>> them, so they >>> will be set in a memory area which cannot be kprobe'd. This can also be >>> useful to write-protect data structures of code that calls text_poke(), >>> e.g., static-keys. It can also protect data on that stack that is used >>> during text_poke() from being overwritten from another core. >>> >>> This solution is somewhat similar to Igor Stoppa’s idea of using “enclaves” >>> when doing write-rarely operations. >>> >>> Right now, I think that text_poke() will keep being susceptible to such >>> an attack, unless you have a better suggestion. >> >> A relatively simple approach might be to teach BPF not to run kprobe >> programs and such in contexts where current->mm isn't the active mm? >> Maybe using nmi_uaccess_okay(), or something like that? It looks like >> perf_callchain_user() also already uses that. Except that a lot of >> this code is x86-specific... > > This sounds like exactly the right solution. If you're running from > some unknown context (like NMI or tracing), then you should check > nmi_uaccess_okay(). I think we should just promote that to be a > non-arch-specific function (that returns true by default) and check it > the relevant bpf_probe_xyz() functions.
I can do that, but notice that switch_mm_irqs_off() writes to cpu_tlbstate.loaded_mm before it actually writes to CR3. So there are still a couple of instructions (and the load_new_mm_cr3()) in between that a kprobe can be set on, no? I can mark them as non-kprobable.
