On Mon, Sep 30, 2019 at 11:31:29AM -0700, Kees Cook wrote: > On Sat, Sep 28, 2019 at 07:37:27PM -0400, Steven Rostedt wrote: > > On Wed, 28 Aug 2019 21:07:24 -0700 > > Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote: > > > > > > > > This won’t make me much more comfortable, since CAP_BPF lets it do an ever-growing set of nasty things. I’d much rather one or both of two things happen: > > > > > > > > 1. Give it CAP_TRACING only. It can leak my data, but it’s rather hard for it to crash my laptop, lose data, or cause other shenanigans. > > > > > > > > 2. Improve it a bit do all the privileged ops are wrapped by capset(). > > > > > > > > Does this make sense? I’m a security person on occasion. I find > > > > vulnerabilities and exploit them deliberately and I break things by > > > > accident on a regular basis. In my considered opinion, CAP_TRACING > > > > alone, even extended to cover part of BPF as I’ve described, is > > > > decently safe. Getting root with just CAP_TRACING will be decently > > > > challenging, especially if I don’t get to read things like sshd’s > > > > memory, and improvements to mitigate even that could be added. I > > > > am quite confident that attacks starting with CAP_TRACING will have > > > > clear audit signatures if auditing is on. I am also confident that > > > > CAP_BPF *will* allow DoS and likely privilege escalation, and this > > > > will only get more likely as BPF gets more widely used. And, if > > > > BPF-based auditing ever becomes a thing, writing to the audit > > > > daemon’s maps will be a great way to cover one’s tracks. > > > > > > CAP_TRACING, as I'm proposing it, will allow full tracefs access. > > > I think Steven and Massami prefer that as well. > > > That includes kprobe with probe_kernel_read. > > > That also means mini-DoS by installing kprobes everywhere or running > > > too much ftrace. > > > > I was talking with Kees at Plumbers about this, and we were talking > > about just using simple file permissions. I started playing with some > > patches to allow the tracefs be visible but by default it would only be > > visible by root. > > > > rwx------ > > > > Then a start up script (or perhaps mount options) could change the > > group owner, and change this to: > > > > rwxrwx--- > > > > Where anyone in the group assigned (say "tracing") gets full access to > > the file system. > > > > The more I was playing with this, the less I see the need for > > CAP_TRACING for ftrace and reading the format files. > > Nice! Thanks for playing with this. I like it because it gives us a way > to push policy into userspace (group membership, etc), and provides a > clean way (hopefully) do separate "read" (kernel memory confidentiality) > from "write" (kernel memory integrity), which wouldn't have been possible > with a single new CAP_... tracefs is a file system, so clearly file based acls are much better fit for all tracefs operations. But that is not the case for ftrace overall. bpf_trace_printk() calls trace_printk() that dumps into trace pipe. Technically it's ftrace operation, but it cannot be controlled by tracefs and by file permissions. That's the motivation to guard bpf_trace_printk() usage from bpf program with CAP_TRACING. Both 'trace' and 'trace_pipe' have quirky side effects. Like opening 'trace' file will make all parallel trace_printk() to be ignored. While reading 'trace_pipe' file will clear it. The point that traditional 'read' and 'write' ACLs don't map as-is to tracefs, so I would be careful categorizing things into confidentiality vs integrity only based on access type.
