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Date:   Sat, 8 Oct 2022 07:51:33 +0200
From:   Christian Brauner <brauner@kernel.org>
To:     syzbot <syzbot+534ee3d24c37c411f37f@syzkaller.appspotmail.com>
Cc:     gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org,
        syzkaller-bugs@googlegroups.com, Tejun Heo <tj@kernel.org>
Subject: Re: [syzbot] general protection fault in kernfs_get_inode
Message-ID: <20221008055133.jqreizhg5k23umgp@wittgenstein>
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On Sat, Oct 08, 2022 at 07:46:12AM +0200, Christian Brauner wrote:
> On Fri, Oct 07, 2022 at 11:35:49AM -1000, Tejun Heo wrote:
> > (cc'ing Christian and quoting whole body)
> > 
> > Christan, I can't repro it here but think what we need is sth like the
> > following. The problem is that cgroup_is_dead() check in the fork path isn't
> > enough as the perm check depends on cgrp->procs_file being available but
> > that is cleared while DYING before DEAD. So, depending on the timing, we can
> > end up trying to deref NULL pointer in may_write.
> > 
> > diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> > index 8ad2c267ff471..603b7167450a1 100644
> > --- a/kernel/cgroup/cgroup.c
> > +++ b/kernel/cgroup/cgroup.c
> > @@ -4934,6 +4934,10 @@ static int cgroup_may_write(const struct cgroup *cgrp, struct super_block *sb)
> >  
> >  	lockdep_assert_held(&cgroup_mutex);
> >  
> > +	/*if @cgrp is being removed, procs_file may already be gone */
> > +	if (!cgrp->procs_file.kn)
> > +		return -ENODEV;
> > +
> >  	inode = kernfs_get_inode(sb, cgrp->procs_file.kn);
> >  	if (!inode)
> >  		return -ENOMEM;
> 
> Tejun, thanks for the Cc.
> Yep, that seems to be the correct analysis.
> I assume you're turning this into a proper patch, so:
> 
> Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org>

#syz test: git@gitlab.com:brauner/linux.git kernel.cgroup_may_write.fix

From f2517f35b571ee80ab046f205ef8b3143d039d57 Mon Sep 17 00:00:00 2001
From: Not My Commit <not@my.commit>
Date: Sat, 8 Oct 2022 07:44:57 +0200
Subject: [PATCH] NOT A REAL COMMIT

THIS IS JUST FOR SYZBOT.
---
 kernel/cgroup/cgroup.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 8ad2c267ff47..f8386a066e0e 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -4934,6 +4934,9 @@ static int cgroup_may_write(const struct cgroup *cgrp, struct super_block *sb)
 
 	lockdep_assert_held(&cgroup_mutex);
 
+	if (!cgrp->procs_file.kn)
+		return -ENODEV;
+
 	inode = kernfs_get_inode(sb, cgrp->procs_file.kn);
 	if (!inode)
 		return -ENOMEM;
-- 
2.34.1