Re: [PATCH RFC v4 net-next 17/26] tracing: allow eBPF programs to be attached to events

On Fri, Aug 15, 2014 at 11:53 AM, Andy Lutomirski [off-list ref] wrote:

On Fri, Aug 15, 2014 at 10:51 AM, Alexei Starovoitov [off-list ref] wrote:

quoted

On Fri, Aug 15, 2014 at 10:25 AM, Andy Lutomirski [off-list ref] wrote:

quoted

On Wed, Aug 13, 2014 at 12:57 AM, Alexei Starovoitov [off-list ref] wrote:

quoted

User interface:
fd = open("/sys/kernel/debug/tracing/__event__/filter")

write(fd, "bpf_123")

I didn't follow all the code flow leading to parsing the "bpf_123"
string, but if it works the way I imagine it does, it's a security
problem.  In general, write(2) should never do anything that involves
any security-relevant context of the caller.

Ideally, you would look up fd 123 in the file table of whomever called
open.  If that's difficult to implement efficiently, then it would be
nice to have some check that the callers of write(2) and open(2) are
the same task and that exec wasn't called in between.

This isn't a very severe security issue because you need privilege to
open the thing in the first place, but it would still be nice to
address.

hmm. you need to be root to open the events anyway.
pretty much the whole tracing for root only, since any kernel data
structures can be printed, stored into maps and so on.
So I don't quite follow your security concern here.

Even say root opens a tracepoint and does exec() of another
app that uploads ebpf program, gets program_fd and does
write into tracepoint fd. The root app that did this open() is
doing exec() on purpose. It's not like it's exec-ing something
it doesn't know about.

As long as everyone who can debugfs/tracing/whatever has all
privileges, then this is fine.

If not, then it's a minor capability or MAC bypass.  Suppose you only
have one capability or, more realistically, limited MAC permissions.

Hard to think of MAC abbreviation other than in networking way... ;)
MAC bypass... kinda sounds like L3 networking without L2... ;)

You can still open the tracing file, pass it to an unwitting program
with elevated permission (e.g. using selinux's entrypoint mechanism),
and trick that program into writing bpf_123.

hmm, but to open tracing file you'd need to be root already...
otherwise yeah, if non-root could open it and pass it, then it
would be nasty.

Admittedly, it's unlikely that fd 123 will be an *eBPF* fd, but the
attack is possible.

I don't think that fixing this should be a prerequisite for merging,
since the risk is so small.  Nonetheless, it would be nice.  (This
family of attacks has lead to several root vulnerabilities in the
past.)

Ok. I think keeping a track of pid between open and write is kinda
ugly. Should we add some new CAP flag and check it for all file
ops? Another option is to conditionally make open() of tracing
files as cloexec...