On Fri, Aug 15, 2014 at 12:07 PM, Alexei Starovoitov <ast@xxxxxxxxxxxx> wrote: > On Fri, Aug 15, 2014 at 11:53 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: >> On Fri, Aug 15, 2014 at 10:51 AM, Alexei Starovoitov <ast@xxxxxxxxxxxx> wrote: >>> On Fri, Aug 15, 2014 at 10:25 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: >>>> On Wed, Aug 13, 2014 at 12:57 AM, Alexei Starovoitov <ast@xxxxxxxxxxxx> wrote: >>>>> 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. Agreed. TBH, I would just add a comment to the open implementation saying that, if unprivileged or less privileged open is allowed, then this needs to be fixed. > 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... That won't help. The same attack can be done with SCM_RIGHTS, and cloexec can be cleared. -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html