Re: CVE-2024-26628: drm/amdkfd: Fix lock dependency warning

[Michal Hocko]

On Wed 20-03-24 16:51:27, Lee Jones wrote:
> On Wed, 20 Mar 2024, Lee Jones wrote:
> 
> > On Wed, 20 Mar 2024, Michal Hocko wrote:
> > 
> > > On Thu 14-03-24 11:09:38, Lee Jones wrote:
> > > > On Fri, 08 Mar 2024, Michal Hocko wrote:
> > > > 
> > > > > On Wed 06-03-24 06:46:11, Greg KH wrote:
> > > > > [...]
> > > > > >  Possible unsafe locking scenario:
> > > > > > 
> > > > > >        CPU0                    CPU1
> > > > > >        ----                    ----
> > > > > >   lock(&svms->lock);
> > > > > >                                lock(&mm->mmap_lock);
> > > > > >                                lock(&svms->lock);
> > > > > >   lock((work_completion)(&svm_bo->eviction_work));
> > > > > > 
> > > > > > I believe this cannot really lead to a deadlock in practice, because
> > > > > > svm_range_evict_svm_bo_worker only takes the mmap_read_lock if the BO
> > > > > > refcount is non-0. That means it's impossible that svm_range_bo_release
> > > > > > is running concurrently. However, there is no good way to annotate this.
> > > > > 
> > > > > OK, so is this even a bug (not to mention a security/weakness)?
> > > > 
> > > > Looks like the patch fixes a warning which can crash some kernels.  So
> > > > the CVE appears to be fixing that, rather than the impossible deadlock.
> > > 
> > > Are you talking about lockdep warning or anything else?
> > 
> > Anything that triggers a BUG() or a WARN() (as per the splat in the
> > commit message).  Many in-field kernels are configured to panic on
> > BUG()s and WARN()s, thus triggering them are presently considered local
> > DoS and attract CVE status.

yes I do agree that WARN() should be treated same as BUG() if
triggerable by an user (for reasons you have mentioned). Lockdep is a
different thing as you follow up below.

> We have discussed this internally and agree with your thinking.
> 
> The splat in the circular lockdep detection code appears to be generated
> using some stacked pr_warn() calls, rather than a WARN().
> 
> Thus, CVE-2024-26628 has now been rejected.
> 
>   https://lore.kernel.org/all/20240320164818.3778843-2-lee@xxxxxxxxxx/
> 
> Thank you for your input Michal.

Thanks!
-- 
Michal Hocko
SUSE Labs