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Date:   Sun, 7 Jun 2020 06:23:40 +1000
From:   Stephen Rothwell <sfr@canb.auug.org.au>
To:     Amol Grover <frextrite@gmail.com>
Cc:     James Morris <jmorris@namei.org>,
        "Serge E . Hallyn" <serge@hallyn.com>,
        linux-kernel@vger.kernel.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        Joel Fernandes <joel@joelfernandes.org>,
        Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        linux-security-module@vger.kernel.org
Subject: Re: [PATCH RESEND] device_cgroup: Fix RCU list debugging warning
Message-ID: <20200607062340.7be7e8d5@canb.auug.org.au>
In-Reply-To: <20200406105950.GA2285@workstation-kernel-dev>
References: <20200406105950.GA2285@workstation-kernel-dev>
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Hi all,

On Mon, 6 Apr 2020 16:29:50 +0530 Amol Grover <frextrite@gmail.com> wrote:
>
> exceptions may be traversed using list_for_each_entry_rcu()
> outside of an RCU read side critical section BUT under the
> protection of decgroup_mutex. Hence add the corresponding
> lockdep expression to fix the following false-positive
> warning:
>=20
> [    2.304417] =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> [    2.304418] WARNING: suspicious RCU usage
> [    2.304420] 5.5.4-stable #17 Tainted: G            E
> [    2.304422] -----------------------------
> [    2.304424] security/device_cgroup.c:355 RCU-list traversed in non-rea=
der section!!
>=20
> Signed-off-by: Amol Grover <frextrite@gmail.com>
> ---
>  security/device_cgroup.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>=20
> diff --git a/security/device_cgroup.c b/security/device_cgroup.c
> index 7d0f8f7431ff..b7da9e0970d9 100644
> --- a/security/device_cgroup.c
> +++ b/security/device_cgroup.c
> @@ -352,7 +352,8 @@ static bool match_exception_partial(struct list_head =
*exceptions, short type,
>  {
>  	struct dev_exception_item *ex;
> =20
> -	list_for_each_entry_rcu(ex, exceptions, list) {
> +	list_for_each_entry_rcu(ex, exceptions, list,
> +				lockdep_is_held(&devcgroup_mutex)) {
>  		if ((type & DEVCG_DEV_BLOCK) && !(ex->type & DEVCG_DEV_BLOCK))
>  			continue;
>  		if ((type & DEVCG_DEV_CHAR) && !(ex->type & DEVCG_DEV_CHAR))
> --=20
> 2.24.1
>=20

I have been carrying the above patch in linux-next for some time now.
I have been carrying it because it fixes problems for syzbot (see the
third warning in
https://lore.kernel.org/linux-next/CACT4Y+YnjK+kq0pfb5fe-q1bqe2T1jq_mvKHf--=
Z80Z3wkyK1Q@mail.gmail.com/).
Is there some reason it has not been applied to some tree?

--=20
Cheers,
Stephen Rothwell

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