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 check is going to cause an issue. In fact, it may actually help speed things up as the read will be optimized for either user or kernel address reading. -- Steve
