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Date:   Wed, 6 Nov 2019 07:24:07 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Thibaut Sautereau <thibaut@sautereau.fr>
Cc:     Christian Brauner <christian.brauner@ubuntu.com>,
        dhowells@redhat.com, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Christian Brauner <christian@brauner.io>,
        cgroups@vger.kernel.org
Subject: Re: NULL pointer deref in put_fs_context with unprivileged LXC
Message-ID: <20191106072407.GU26530@ZenIV.linux.org.uk>
References: <20191010213512.GA875@gandi.net>
 <20191011141403.ghjptf4nrttgg7jd@wittgenstein>
 <20191105205830.GA871@gandi.net>
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On Tue, Nov 05, 2019 at 09:58:30PM +0100, Thibaut Sautereau wrote:

> > > 	BUG: kernel NULL pointer dereference, address: 0000000000000043

ERR_PTR(something)->d_sb, most likely.

> > > 	493		if (fc->root) {
> > > 	494			sb = fc->root->d_sb;
> > > 	495			dput(fc->root);
> > > 	496			fc->root = NULL;
> > > 	497			deactivate_super(sb);
> > > 	498		}

> 	fs_context: DEBUG: fc->root = fffffffffffffff3
> 	fs_context: DEBUG: fc->source = cgroup2

Yup.  That'd be ERR_PTR(-13), i.e. ERR_PTR(-EACCES).  Most likely
from
                nsdentry = kernfs_node_dentry(cgrp->kn, sb);
                dput(fc->root);
                fc->root = nsdentry;
                if (IS_ERR(nsdentry)) {
                        ret = PTR_ERR(nsdentry);
                        deactivate_locked_super(sb);
                }

in cgroup_do_get_tree().  As a quick test, try to add fc->root = NULL;
next to that deactivate_locked_super(sb); inside the if (IS_ERR(...))
body and see if it helps; it's not the best way to fix it (I'd rather
go for
                if (IS_ERR(nsdentry)) {
                        ret = PTR_ERR(nsdentry);
                        deactivate_locked_super(sb);
			nsdentry = NULL;
                }
                fc->root = nsdentry;
), but it would serve to verify that this is the source of that crap.