|From:||Andrew Lutomirski <luto-AT-mit.edu>|
|To:||Casey Schaufler <casey-AT-schaufler-ca.com>|
|Subject:||Re: [PATCH PLACEHOLDER 1/3] fs/exec: "always_unprivileged" patch|
|Date:||Sun, 15 Jan 2012 14:07:32 -0800|
|Cc:||Linus Torvalds <torvalds-AT-linux-foundation.org>, Jamie Lokier <jamie-AT-shareable.org>, Will Drewry <wad-AT-chromium.org>, linux-kernel-AT-vger.kernel.org, keescook-AT-chromium.org, john.johansen-AT-canonical.com, serge.hallyn-AT-canonical.com, coreyb-AT-linux.vnet.ibm.com, pmoore-AT-redhat.com, eparis-AT-redhat.com, djm-AT-mindrot.org, segoon-AT-openwall.com, rostedt-AT-goodmis.org, jmorris-AT-namei.org, scarybeasts-AT-gmail.com, avi-AT-redhat.com, penberg-AT-cs.helsinki.fi, viro-AT-zeniv.linux.org.uk, mingo-AT-elte.hu, akpm-AT-linux-foundation.org, khilman-AT-ti.com, borislav.petkov-AT-amd.com, amwang-AT-redhat.com, oleg-AT-redhat.com, ak-AT-linux.intel.com, eric.dumazet-AT-gmail.com, gregkh-AT-suse.de, dhowells-AT-redhat.com, daniel.lezcano-AT-free.fr, linux-fsdevel-AT-vger.kernel.org, linux-security-module-AT-vger.kernel.org, olofj-AT-chromium.org, mhalcrow-AT-google.com, dlaor-AT-redhat.com, corbet-AT-lwn.net, alan-AT-lxorguk.ukuu.org.uk|
On Sun, Jan 15, 2012 at 1:32 PM, Casey Schaufler <firstname.lastname@example.org> wrote: > On 1/15/2012 12:59 PM, Andrew Lutomirski wrote: >> >> On Sun, Jan 15, 2012 at 12:16 PM, Casey Schaufler >> <email@example.com> 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
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