Re: [PATCH 1/2 v2] kprobe: Do not use uaccess functions to access kernel memory that can fault

[Alexei Starovoitov]

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.