On Sun, Jan 15, 2012 at 1:32 PM, Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote: > On 1/15/2012 12:59 PM, Andrew Lutomirski wrote: >> >> On Sun, Jan 15, 2012 at 12:16 PM, Casey Schaufler >> <casey@xxxxxxxxxxxxxxxx> wrote: >>> >>> On 1/14/2012 12:22 PM, Linus Torvalds wrote: >>>> >>>> And yes, I really seriously do believe that is both safer and simpler >>>> than some model that says "you can drop stuff", and then you have to >>>> start making up rules for what "dropping" means. >>>> >>>> Does "dropping" mean allowing setuid(geteuid()) for example? That *is* >>>> dropping the uid in a _POSIX_SAVED_IDS environment. And I'm saying >>>> that no, we should not even allow that. It's simply all too "subtle". >>> >>> >>> I am casting my two cents worth behind Linus. Dropping >>> privilege can be every bit as dangerous as granting privilege >>> in the real world of atrocious user land code. Especially in >>> the case of security policy enforcing user land code. >> >> Can you think of *any* plausible attack that is possible with my patch >> (i.e. no_new_privs allows setuid, setresuid, and capset) that would be >> prevented or even mitigated if I blocked those syscalls? I can't. >> (The sendmail-style attack is impossible with no_new_privs.) > > > I am notoriously bad at coming up with this sort of example. > I will try, I may not hit the mark, but it should be close. > > The application is running with saved uid != euid when > no-new-privs is set. It execs a new binary, which keeps > the saved and effective uids. The program calls setreuid, > which succeeds. It opens the saved userid's files. If you don't trust that binary, then why are you execing it with saved uid != euid in the first place? If you are setting no_new_privs, then you are new code and should have at least some basic awareness of the semantics. The exact same "exploit" is possible if you have CAP_DAC_OVERRIDE with either no_new_privs semantics -- if you have a privilege and you run untrusted code, then you had better remove that privilege somehow for the untrusted code. IOW, *drop privileges if you are a sandbox*. Otherwise you're screwed with or without no_new_privs. Another way of saying this is: no_new_privs is not a sandbox. It's just a way to make it safe for sandboxes and other such weird things processes can do to themselves safe across execve. If you want a sandbox, use seccomp mode 2, which will require you to set no_new_privs. --Andy -- 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
