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Date: Thu, 13 Jun 2024 12:44:20 +0100
From: Lee Jones <lee@kernel.org>
To: Pavel Machek <pavel@ucw.cz>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Michal Hocko <mhocko@suse.com>, cve@kernel.org,
	linux-kernel@vger.kernel.org,
	Felix Kuehling <felix.kuehling@amd.com>
Subject: Re: CVE-2024-26628: drm/amdkfd: Fix lock dependency warning
Message-ID: <20240613114420.GM1504919@google.com>
References: <2024030649-CVE-2024-26628-f6ce@gregkh>
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 <ZfsBpal_29lihveI@tiehlicka>
 <20240320154734.GU1522089@google.com>
 <Zmq8uSVv0X5f7xx+@duo.ucw.cz>
 <2024061335-wistful-brownnose-28ea@gregkh>
 <ZmrMo6qFbt9/3xoh@duo.ucw.cz>
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On Thu, 13 Jun 2024, Pavel Machek wrote:

> On Thu 2024-06-13 12:16:50, Greg Kroah-Hartman wrote:
> > On Thu, Jun 13, 2024 at 11:32:41AM +0200, Pavel Machek wrote:
> > > On Wed 2024-03-20 15:47:34, 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.
> > > 
> > > So... because it is possible to configure machine to reboot on
> > > warning, now every warning is a security issue?
> > > 
> > > Lockdep is for debugging, if someone uses it in production with panic
> > > on reboot, they are getting exactly what they are asking for.
> > > 
> > > Not a security problem.
> > 
> > And we agree, I don't know what you are arguing about here, please stop.
> 
> So you agree that WARN triggering randomly is not a security problem?
> 
> Following communication did not say so.
> 
> "The splat in the circular lockdep detection code appears to be generated
> using some stacked pr_warn() calls, rather than a WARN()."

We agree that the lockdep detection is a debugging feature AND that even
though the splat looks like a WARN(), it does not behave like one.
Therefore it does not constitute a security issue.

However, yes, we believe that if an attacker can trip a WARN() and
reboot a victim's machine on demand then this is equivalent to a local
DoS attack and merits CVE status.

-- 
Lee Jones [李琼斯]