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

From: Andy Lutomirski
Date: Tue Aug 01 2017 - 09:13:21 EST

On Mon, Jul 31, 2017 at 3:43 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Wed, Jul 19, 2017 at 9:53 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>> 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.
>> Can you just put the bprm->secureexec |=
>> security_bprm_secureexec(bprm); assignment in prepare_binprm() right
>> after security_bprm_set_creds()? This would make patch 1 make sense
>> and make this make sense too, I think. Or is there some reason why it
>> wouldn't work? If the latter, I think the patch descriptions and
>> comments should maybe be fixed up.
> Yeah, I'll make this change for the next version. It makes things a
> little less ugly in the series. In this version I was trying to focus
> on eliminating the LSM hook instead of first moving it (to
> setup_new_exec()) and then moving it a second time (to the
> bprm_set_creds() hook).
> Have you had a chance to review the later consolidation patches? So
> far no one else has reviewed those. (David, any chance you have some
> time too?) I'd love to get at least some Reviewed-bys for them...

I looked briefly. I'll try to look more closely tomorrow.