Re: [PATCH v3 0/3] namei: implement various lookup restriction AT_* flags

On 2018-10-09, Aleksa Sarai [off-list ref] wrote:

The need for some sort of control over VFS's path resolution (to avoid
malicious paths resulting in inadvertent breakouts) has been a very
long-standing desire of many userspace applications. This patchset is a
revival of Al Viro's old AT_NO_JUMPS[1,2] patchset (which was a variant
of David Drysdale's O_BENEATH patchset[3] which was a spin-off of the
Capsicum project[4]) with a few additions and changes made based on the
previous discussion within [5] as well as others I felt were useful.

I've been thinking about this problem a little more (from the UX side of
things) and I have a feeling that adding 5 different O_* flags related
to resolution -- rather than properties related to opening the file --
might be less than ideal (even though, as discussed in previous threads,
there is a need for these flags and for them to be separated).

There is *some* precedence for this with O_PATH[**] changing fairly
large semantics of openat(2) but there are some things about O_PATH
which I think could be improved.

What if we had a resolveat(2) which acted like openat(..., O_PATH) *but*
it allowed us to have new flags and to separate the scoping flags from
the (fairly limited) space of O_* flags. Then O_PATH could effectively
just be a legacy way of doing resolveat(2) -- with only O_CLOEXEC,
O_DIRECTORY, and O_NOFOLLOW support.

And the main things we could add would be:

  * These resolution flags, with only support available from
	resolveat(2) for the moment. The idea would be that AT_EMPTY_PATH
	would be the recommended way to make use of this.

  * Support for RESOLVE_{NOPERM,RDONLY,WRONLY,RDWR} (which after some
	discussions with Eric last year might be necessary in order to make
	/proc/$pid/fd/$fd re-opening of O_PATH descriptors safer -- which is
	something that we use in both runc and LXC).

Is this idea palatable, or was this something considered during the
development of O_PATH and someone had an argument why augmenting O_PATH
is better than a new syscall?

[**] And while writing this paragraph I noticed that I didn't update the
     O_PATH "flag whitelist" to allow the scoping flags to affect it. I
	 will include a fix for this in v4 (I must've lost it in an early
	 rebase before I sent v1).

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>