Re: [PATCH v13 3/5] security: Replace indirect LSM hook calls with static calls

Paul Moore
2024-07-03T22:52:35Z

On Wed, Jul 3, 2024 at 6:22 PM KP Singh <kpsingh@kernel.org> wrote:
> On Wed, Jul 3, 2024 at 10:56 PM Paul Moore <paul@paul-moore.com> wrote:
> > On Wed, Jul 3, 2024 at 12:55 PM KP Singh <kpsingh@kernel.org> wrote:
> > > On Wed, Jul 3, 2024 at 2:07 AM Paul Moore <paul@paul-moore.com> wrote:
> > > >
> > > > On Jun 29, 2024 KP Singh <kpsingh@kernel.org> wrote:
> > > > >
> > > > > LSM hooks are currently invoked from a linked list as indirect calls
> > > > > which are invoked using retpolines as a mitigation for speculative
> > > > > attacks (Branch History / Target injection) and add extra overhead which
> > > > > is especially bad in kernel hot paths:
> > >
> > > [...]
> > >
> > > > should fix the more obvious problems. I'd like to know if you are
> > > > aware of any others? If not, the text above should be adjusted and
> > > > we should reconsider patch 5/5. If there are other problems I'd
> > > > like to better understand them as there may be an independent
> > > > solution for those particular problems.
> > >
> > > We did have problems with some other hooks but I was unable to dig up
> > > specific examples though, it's been a while. More broadly speaking, a
> > > default decision is still a decision. Whereas the intent from the BPF
> > > LSM is not to make a default decision unless a BPF program is loaded.
> > > I am quite worried about the holes this leaves open, subtle bugs
> > > (security or crashes) we have not caught yet and PATCH 5/5 engineers away
> > > the problem of the "default decision".
> >
> > The inode/xattr problem you originally mentioned wasn't really rooted
> > in a "bad" default return value, it was really an issue with how the
> > LSM hook was structured due to some legacy design assumptions made
> > well before the initial stacking patches were merged. That should be
> > fixed now[1] and given that the inode/xattr set/remove hooks were
> > unique in this regard (individual LSMs were responsible for performing
> > the capabilities checks) I don't expect this to be a general problem.
> >
> > There were also some issues caused by the fact that we were defining
> > the default return value in multiple places and these values had gone
> > out of sync in a number of hooks. We've also fixed this problem by
> > only defining the default return value once for each hook, solving all
> > of those problems.
>
> I don't see how this solves problems or prevents any future problems
> with side-effects. I have always been uncomfortable with an extraneous
> function being called with a side effect ever since we merged BPF LSM
> with default callback. We have found one bug due to this, not all the
> bugs.

You've got to give me something more concrete than that. If you can't
provide any concrete examples, start with providing a basic concept
with far more detail than just "side-effects".

> > I'm not aware of any other existing problems relating to the LSM hook
> > default values, if there are any, we need to fix them independent of
> > this patchset. The LSM framework should function properly if the
> > "default" values are used.
>
> Patch 5 eliminates the possibilities of errors and subtle bugs all
> together. The problem with subtle bugs is, well, they are subtle, if
> you and I knew of the bugs, we would fix all of them, but we don't. I
> really feel we ought to eliminate the class of issues and not just
> whack-a-mole when we see the bugs.

Here's the thing, I don't really like patch 5/5. To be honest, I
don't really like a lot of this patchset. From my perspective, the
complexity of the code is likely going to mean more maintenance
headaches down the road, but Linus hath spoken so we're doing this
(although "this" is still a bit undefined as far as I'm concerned).
If you want me to merge patch 5/5 you've got to give me something real
and convincing that can't be fixed by any other means. My current
opinion is that you're trying to use a previously fixed bug to scare
and/or coerce the merging of some changes I don't really want to
merge. If you want me to take patch 5/5, you've got to give me a
reason that is far more compelling that what you've written thus far.

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