> 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%7Cd44d6f0765dd49b20db708d6990ee7e8%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C636864686717142892&sdata=gVALdkEULEhj4iJNEWAGxyYWe2lxnHRdamW5ZA2A5RQ%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%7Cd44d6f0765dd49b20db708d6990ee7e8%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C636864686717142892&sdata=nu2J1FtJsZJmt53SKJz8C8ktWE9eycwdAA%2BiCi1TfCc%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?
