Re: [PATCH PLACEHOLDER 1/3] fs/exec: "always_unprivileged" patch

[Andrew Lutomirski <luto@mit.edu>]

On Sun, Jan 15, 2012 at 1:32 PM, Casey Schaufler <casey@schaufler-ca.com> wrote:
> On 1/15/2012 12:59 PM, Andrew Lutomirski wrote:
>>
>> On Sun, Jan 15, 2012 at 12:16 PM, Casey Schaufler
>> <casey@schaufler-ca.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