> 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%7Cf2513009ef734ecd6b0d08d69913a5ae%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C636864707020821793&sdata=HAbnDcrBne64JyPuVUMKmM7nQk67F%2BFvjuXEn8TmHeo%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%7Cf2513009ef734ecd6b0d08d69913a5ae%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C636864707020821793&sdata=vNRIMKtFDy%2F3z5FlTwDiJY6VGEV%2FMHgQPTdFSFtCo4s%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.
