Re: [PATCH 1/2] fs: Check f_cred instead of current's creds in should_remove_suid()

[Kees Cook]

On Wed, Jan 25, 2017 at 1:43 PM, Ben Hutchings <ben@xxxxxxxxxxxxxxx> wrote:
> On Wed, 2017-01-25 at 13:06 -0800, Andy Lutomirski wrote:
>> If an unprivileged program opens a setgid file for write and passes
>> the fd to a privileged program and the privileged program writes to
>> it, we currently fail to clear the setgid bit.  Fix it by checking
>> f_cred instead of current's creds whenever a struct file is
>> involved.
> [...]
>
> What if, instead, a privileged program passes the fd to an un
> unprivileged program?  It sounds like a bad idea to start with, but at
> least currently the unprivileged program is going to clear the setgid
> bit when it writes.  This change would make that behaviour more
> dangerous.
>
> Perhaps there should be a capability check on both the current
> credentials and file credentials?  (I realise that we've considered
> file credential checks to be sufficient elsewhere, but those cases
> involved virtual files with special semantics, where it's clearer that
> a privileged process should not pass them to an unprivileged process.)

We need a set of self-tests for this whole area. :( There are so many
corner cases. We still have an unfixed corner case with mmap writes
not clearing set*id bits that I tried to solve last year...

-Kees

-- 
Kees Cook
Nexus Security