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>
