Re: KASAN: slab-use-after-free Read in radix_tree_lookup in&after Linux Kernel 6.4-rc6

[Qi Zheng]

Hi Matthew,

On 2023/10/20 22:58, Matthew Wilcox wrote:
> On Fri, Oct 20, 2023 at 09:51:18PM +0800, Qi Zheng wrote:
> > Hi all,
> >
> > On 2023/10/20 20:34, Matthew Wilcox wrote:
> > > On Fri, Oct 20, 2023 at 10:26:31AM +0200, Petr Mladek wrote:
> > > > Adding Matthew into Cc in the hope that he is still familiar with the
> > > > code. Also adding Andrew who accepts patches.
> > >
> > > oh joy.  i love dealing with cves.
> > >
> > > > > > I agree, this issue looks to be in kernel-core radix tree code in ./lib/radix-tree.c in two of any places.
> > >
> > > the radix tree code is the victim here.  maybe also the perpetrator, but
> > > it's rather hard to say.
> > >
> > > shrink_slab_memcg()
> > > 	down_read_trylock(&shrinker_rwsem)
> > > 	shrinker = idr_find(&shrinker_idr, i);
> > >
> > > i assume is the path to this bug.  the reporter didn't run the
> > > stacktrace through scripts/decode_stacktrace.sh so it's less useful than
> > > we might want.
> > >
> > > prealloc_memcg_shrinker() calls idr_alloc and idr_remove under
> > > shrinker_rwsem in write mode, so that should be fine.
> > >
> > > unregister_memcg_shrinker() calls idr_remove after asserting &shrinker_rwsem
> > > is held (although not asserting that it's held for write ... hmm ... but
> > > both callers appear to hold it for write anyway)
> > >
> > > so i don't see why we'd get a UAF here.
> > >
> > > anyway, adding Qi Zheng to the cc since they're responsible for the
> > > shrinker code.
> >
> > Thanks for CC'ing me, I'd be happy to troubleshoot any issues that may
> > be shrinker related.
> >
> > Between v6.4-rc1 and v6.4 versions, we briefly implemented lockless slab
> > shrink using the SRCU method. In these versions, we call idr_alloc and
> > idr_remove under shrinker_mutex, and idr_find under srcu_read_lock.
> >
> > These are all legitimate uses of the IDR APIs and the shrinker_idr
> > will never be destroyed, so at a quick glance I didn't see why it would
> > cause UAF here.
>
> I'm not an expert on how all the RCU flavours interact, but I don't
> think that's safe.  The IDR (radix tree) will RCU-free nodes, but I
> don't think holding the srcu_read_lock is enough to prevent the nodes
> being freed.  

Oh, Indeed, I just saw the Documeentation/RCU/checklist.rst:

```
	If the updater uses call_rcu() or synchronize_rcu(), then
	the corresponding readers may use:  (1) rcu_read_lock() and
	rcu_read_unlock(), (2) any pair of primitives that disables
	and re-enables softirq, for example, rcu_read_lock_bh() and
	rcu_read_unlock_bh(), or (3) any pair of primitives that disables
	and re-enables preemption, for example, rcu_read_lock_sched() and
	rcu_read_unlock_sched().  If the updater uses synchronize_srcu()
	or call_srcu(), then the corresponding readers must use
	srcu_read_lock() and srcu_read_unlock(), and with the same
	srcu_struct.
```

> I think you'd need to take the rcu_read_lock() around
> the call to idr_find().
For latest RCU+refcount method, we call idr_find() under
rcu_read_lock(), so it's safe.

Thanks,
Qi

> > Anyway I will keep working on this issue, and it would be nice if
> > there was a way to reproduce it.
>
> So I think the CVE is inappropriately issued.  The SRCU code was added in
> v6.4-rc1 and removed before v6.4.  I don't think CVEs are appropriate for
> bugs which only existed in development kernels.  How do we revoke CVEs?