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Date: Thu, 13 Jun 2024 12:40:35 +0200
From: Pavel Machek <pavel@ucw.cz>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Lee Jones <lee@kernel.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: <ZmrMo6qFbt9/3xoh@duo.ucw.cz>
References: <2024030649-CVE-2024-26628-f6ce@gregkh>
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 <20240314110938.GM1522089@google.com>
 <ZfsBpal_29lihveI@tiehlicka>
 <20240320154734.GU1522089@google.com>
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 <2024061335-wistful-brownnose-28ea@gregkh>
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In-Reply-To: <2024061335-wistful-brownnose-28ea@gregkh>


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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:
> > >=20
> > > > On Thu 14-03-24 11:09:38, Lee Jones wrote:
> > > > > On Fri, 08 Mar 2024, Michal Hocko wrote:
> > > > >=20
> > > > > > On Wed 06-03-24 06:46:11, Greg KH wrote:
> > > > > > [...]
> > > > > > >  Possible unsafe locking scenario:
> > > > > > >=20
> > > > > > >        CPU0                    CPU1
> > > > > > >        ----                    ----
> > > > > > >   lock(&svms->lock);
> > > > > > >                                lock(&mm->mmap_lock);
> > > > > > >                                lock(&svms->lock);
> > > > > > >   lock((work_completion)(&svm_bo->eviction_work));
> > > > > > >=20
> > > > > > > I believe this cannot really lead to a deadlock in practice, =
because
> > > > > > > svm_range_evict_svm_bo_worker only takes the mmap_read_lock i=
f 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 ann=
otate this.
> > > > > >=20
> > > > > > OK, so is this even a bug (not to mention a security/weakness)?
> > > > >=20
> > > > > Looks like the patch fixes a warning which can crash some kernels=
=2E  So
> > > > > the CVE appears to be fixing that, rather than the impossible dea=
dlock.
> > > >=20
> > > > Are you talking about lockdep warning or anything else?
> > >=20
> > > 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 loc=
al
> > > DoS and attract CVE status.
> >=20
> > So... because it is possible to configure machine to reboot on
> > warning, now every warning is a security issue?
> >=20
> > Lockdep is for debugging, if someone uses it in production with panic
> > on reboot, they are getting exactly what they are asking for.
> >=20
> > Not a security problem.
>=20
> 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()."
									Pavel
--=20
People of Russia, stop Putin before his war on Ukraine escalates.

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