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

[Masami Hiramatsu]

On Tue, 26 Feb 2019 10:24:47 -0500
Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:

> On Fri, Feb 22, 2019 at 11:27:05AM -0800, Alexei Starovoitov 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.
> 
> Slightly related and FWIW, BCC's eBPF-based opensnoop tool [1] installs a
> kprobe on do_sys_open to monitor calls to the open syscall globally.
> 
> do_sys_open() has prototype:
> 
> long do_sys_open(int dfd, const char __user *filename, int flags, umode_t mode);
> 
> This causes a "blank" filename to be displayed by opensnoop when I run it on
> my Pixel 3 (arm64), possibly because this is a user pointer. However, it
> works fine on x86-64.
> 
> So it seems to me that on arm64, reading user pointers directly still doesn't
> work even if there is a distinction between user/kernel addresses. In that
> case reading the user pointer using user accessors (possibly using
> bpf_probe_user_read helper) should be needed to fix this issue (as Yonghong
> also privately discussed with me).

OK, it sounds like the same issue. Please add a bpf_user_read() and use it
for __user pointer.

Thank you,


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>