(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://lore.kernel.org/lkml/20190221234451.17632-6-rick.p.edgeco...@intel.com/) and "[PATCH v3 06/20] x86/alternative: Use temporary mm for text poking" (https://lore.kernel.org/lkml/20190221234451.17632-7-rick.p.edgeco...@intel.com/), 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?
