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%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.
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...
