Re: [PATCH 28/32] selinux: Use mem_to_flex_dup() with xfrm and sidtab

Kees Cook
2022-05-05T18:44:46Z

On Wed, May 04, 2022 at 11:14:42PM -0400, Paul Moore wrote:
> On Wed, May 4, 2022 at 7:34 PM Gustavo A. R. Silva
> <gustavoars@kernel.org> wrote:
> >
> > Hi Paul,
> >
> > On Wed, May 04, 2022 at 06:57:28PM -0400, Paul Moore wrote:
> > > On Tue, May 3, 2022 at 9:57 PM Kees Cook <keescook@chromium.org> wrote:
> >
> > [..]
> >
> > > > +++ b/include/uapi/linux/xfrm.h
> > > > @@ -31,9 +31,9 @@ struct xfrm_id {
> > > > struct xfrm_sec_ctx {
> > > > __u8 ctx_doi;
> > > > __u8 ctx_alg;
> > > > - __u16 ctx_len;
> > > > + __DECLARE_FLEX_ARRAY_ELEMENTS_COUNT(__u16, ctx_len);
> > > > __u32 ctx_sid;
> > > > - char ctx_str[0];
> > > > + __DECLARE_FLEX_ARRAY_ELEMENTS(char, ctx_str);
> > > > };
> > >
> > > While I like the idea of this in principle, I'd like to hear about the
> > > testing you've done on these patches. A previous flex array
> > > conversion in the audit uapi headers ended up causing a problem with
> >
> > I'm curious about which commit caused those problems...?
>
> Commit ed98ea2128b6 ("audit: replace zero-length array with
> flexible-array member"), however, as I said earlier, the problem was
> actually with SWIG, it just happened to be triggered by the kernel
> commit. There was a brief fedora-devel mail thread about the problem,
> see the link below:
>
> * https://www.spinics.net/lists/fedora-devel/msg297991.html

Wow, that's pretty weird -- it looks like SWIG was scraping the headers
to build its conversions? I assume SWIG has been fixed now?

> To reiterate, I'm supportive of changes like this, but I would like to
> hear how it was tested to ensure there are no unexpected problems with
> userspace. If there are userspace problems it doesn't mean we can't
> make changes like this, it just means we need to ensure that the
> userspace issues are resolved first.

Well, as this is the first and only report of any problems with [0] -> []
conversions (in UAPI or anywhere) that I remember seeing, and they've
been underway since at least v5.9, I hadn't been doing any new testing.

So, for this case, I guess I should ask what tests you think would be
meaningful here? Anything using #include should be fine:
https://codesearch.debian.net/search?q=linux%2Fxfrm.h&literal=1&perpkg=1
Which leaves just this, which may be doing something weird:

libabigail_2.0-1/tests/data/test-diff-filter/test-PR27569-v0.abi
</data-member>
<data-member access="public" layout-offset-in-bits="128">
<var-decl name="seq_hi" type-id="3f1a6b60" visibility="default" filepath="include/uapi/linux/xfrm.h" line="97" column="1"/>
</data-member>
<data-member access="public" layout-offset-in-bits="160">

But I see that SWIG doesn't show up in a search for linux/audit.h:
https://codesearch.debian.net/search?q=linux%2Faudit.h&literal=1&perpkg=1

So this may not be a sufficient analysis...

--
Kees Cook