Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes: > On Wed, Oct 19, 2016 at 10:29 AM, Jann Horn <jann@xxxxxxxxx> wrote: >> On Wed, Oct 19, 2016 at 11:52:50AM -0500, Eric W. Biederman wrote: >>> Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes: >>> > Simply ptrace yourself, exec the >>> > program, and then dump the program out. A program that really wants >>> > to be unreadable should have a stub: the stub is setuid and readable, >>> > but all the stub does is to exec the real program, and the real >>> > program should have mode 0500 or similar. >>> > >>> > ISTM the "right" check would be to enforce that the program's new >>> > creds can read the program, but that will break backwards >>> > compatibility. >>> >>> Last I looked I had the impression that exec of a setuid program kills >>> the ptrace. >>> >>> If we are talking about a exec of a simple unreadable executable (aka >>> something that sets undumpable but is not setuid or setgid). Then I >>> agree it should break the ptrace as well and since those programs are as >>> rare as hens teeth I don't see any problem with changing the ptrace behavior >>> in that case. >> >> Nope. check_unsafe_exec() sets LSM_UNSAFE_* flags in bprm->unsafe, and then >> the flags are checked by the LSMs and cap_bprm_set_creds() in commoncap.c. >> cap_bprm_set_creds() just degrades the execution to a non-setuid-ish one, >> and e.g. ptracers stay attached. > > I think you're right. I ought to be completely sure because I rewrote > that code back in 2005 or so back when I thought kernel programming > was only for the cool kids. It was probably my first kernel patch > ever and it closed an awkward-to-exploit root hole. But it's been a > while. (Too bad my second (IIRC) kernel patch was more mundane and > fixed the mute button on "new" Lenovo X60-era laptops and spend > several years in limbo...) Ah yes and this is only a problem if the ptracer does not have CAP_SYS_PTRACE. If the tracer does not have sufficient permissions any opinions on failing the exec or kicking out the ptracer? I am leaning towards failing the exec as it is more obvious if someone cares. Dropping the ptracer could be a major mystery. Eric -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
