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...) --Andy -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>