[PATCH 1/2] exec: Add a per bprm->file version of per_clear

[Eric W. Biederman]

Kees Cook <keescook at chromium.org> writes:

> On Fri, May 29, 2020 at 11:46:40AM -0500, Eric W. Biederman wrote:
>> 
>> There is a small bug in the code that recomputes parts of bprm->cred
>> for every bprm->file.  The code never recomputes the part of
>> clear_dangerous_personality_flags it is responsible for.
>> 
>> Which means that in practice if someone creates a sgid script
>> the interpreter will not be able to use any of:
>> 	READ_IMPLIES_EXEC
>> 	ADDR_NO_RANDOMIZE
>> 	ADDR_COMPAT_LAYOUT
>> 	MMAP_PAGE_ZERO.
>> 
>> This accentially clearing of personality flags probably does
>> not matter in practice because no one has complained
>> but it does make the code more difficult to understand.
>> 
>> Further remaining bug compatible prevents the recomputation from being
>> removed and replaced by simply computing bprm->cred once from the
>> final bprm->file.
>> 
>> Making this change removes the last behavior difference between
>> computing bprm->creds from the final file and recomputing
>> bprm->cred several times.  Which allows this behavior change
>> to be justified for it's own reasons, and for any but hunts
>> looking into why the behavior changed to wind up here instead
>> of in the code that will follow that computes bprm->cred
>> from the final bprm->file.
>> 
>> This small logic bug appears to have existed since the code
>> started clearing dangerous personality bits.
>> 
>> History Tree: git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
>> Fixes: 1bb0fa189c6a ("[PATCH] NX: clean up legacy binary support")
>> Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>
>
> Yup, this looks good. Pointless nit because it's removed in the next
> patch, but pf_per_clear is following the same behavioral pattern as
> active_secureexec, it could be named active_per_clear, but since this
> already been bikeshed in v1, it's fine! :)

That plus it is very much true that active_ isn't a particularly good
prefix.  pf_ for per_file seems slightly better.

The only time I can imagine this patch seeing the light of day is if
someone happens to discover that this fixes a bug for them and just this
patch is backported.  At which point pf_per_clear pairs with
cap_elevated.  So I don't think it hurts.

*Shrug*

The next patch is my long term solution to the mess.

> Reviewed-by: Kees Cook <keescook at chromium.org>
>
> I wish we had more robust execve tests. :(

I think you have more skill at writing automated tests than I do.  So
feel free to write some.

Eric