On Tue, 26 Feb 2019 10:24:47 -0500 Joel Fernandes joel@joelfernandes.org 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,