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

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Message-ID: <20190222192703.epvgxghwybte7gxs@ast-mbp.dhcp.thefacebook.com>
Date:   Fri, 22 Feb 2019 11:27:05 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andy Lutomirski <luto@...capital.net>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        stable <stable@...r.kernel.org>,
        Changbin Du <changbin.du@...il.com>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...nel.org>, daniel@...earbox.net,
        netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH 1/2 v2] kprobe: Do not use uaccess functions to access
 kernel memory that can fault

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.