Re: [PATCH v3 06/15] commoncap: Refactor to remove bprm_secureexec hook

[Kees Cook]

On Tue, Jul 18, 2017 at 6:10 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> On Tue, Jul 18, 2017 at 3:25 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> The commoncap implementation of the bprm_secureexec hook is the only LSM
>> that depends on the final call to its bprm_set_creds hook (since it may
>> be called for multiple files, it ignores bprm->called_set_creds). As a
>> result, it cannot safely _clear_ bprm->secureexec since other LSMs may
>> have set it.  Instead, remove the bprm_secureexec hook by introducing a
>> new flag to bprm specific to commoncap: cap_elevated. This is similar to
>> cap_effective, but that is used for a specific subset of elevated
>> privileges, and exists solely to track state from bprm_set_creds to
>> bprm_secureexec. As such, it will be removed in the next patch.
>>
>> Here, set the new bprm->cap_elevated flag when setuid/setgid has happened
>> from bprm_fill_uid() or fscapabilities have been prepared. This temporarily
>> moves the bprm_secureexec hook to a static inline. The helper will be
>> removed in the next patch; this makes the step easier to review and bisect,
>> since this does not introduce any changes to inputs nor outputs to the
>> "elevated privileges" calculation.
>>
>> The new flag is merged with the bprm->secureexec flag in setup_new_exec()
>> since this marks the end of any further prepare_binprm() calls.
>
> Reviewed-by: Andy Lutomirski <luto@xxxxxxxxxx>
>
> with the redundant caveat that...
>
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -1330,6 +1330,13 @@ EXPORT_SYMBOL(would_dump);
>>
>>  void setup_new_exec(struct linux_binprm * bprm)
>>  {
>> +       /*
>> +        * Once here, prepare_binrpm() will not be called any more, so
>> +        * the final state of setuid/setgid/fscaps can be merged into the
>> +        * secureexec flag.
>> +        */
>> +       bprm->secureexec |= bprm->cap_elevated;
>> +
>
> ...the weird placement of the other assignments to bprm->secureexec
> makes this exceedingly confusing.

Any thoughts on how I could improve this? The main take-away is that
commoncap's secureexec is special, and this was the cleanest way I
could find to deal with it...

-Kees

-- 
Kees Cook
Pixel Security