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

[Andrew Lutomirski]

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.)

Also, how would you even block setuid(2) in a non-confusing manner?
The semantics and error returns are already such a disaster than it's
barely worth it for anything to check the return value.

>
> This even more important in environments that support fine
> granularity of privilege, including capabilities and SELinux.
> Under SELinux a domain transition can increase, decrease or
> completely change a process' access rights and there is really
> no way for the kernel to tell which it is because that's all
> encoded in the arbitrary SELinux policy. Smack does not try
> to maintain a notion of hierarchy of privilege, so the notion
> of any change being equivalent to any other is in line with
> the Smack philosophy.
>

My patch does not (barring bugs) allow selinux domain transitions.  I
certainly think that all security transitions that vary across
distributions should be blocked by no_new_privs.

--Andy
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