From:   Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
To:     Yu Kuai <yukuai1@huaweicloud.com>
CC:     "jack@suse.cz" <jack@suse.cz>,
        "paolo.valente@linaro.org" <paolo.valente@linaro.org>,
        "axboe@kernel.dk" <axboe@kernel.dk>,
        "glusvardi@posteo.net" <glusvardi@posteo.net>,
        "damien.lemoal@opensource.wdc.com" <damien.lemoal@opensource.wdc.com>,
        "felicigb@gmail.com" <felicigb@gmail.com>,
        "inbox@emilianomaccaferri.com" <inbox@emilianomaccaferri.com>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "yukuai3@huawei.com" <yukuai3@huawei.com>,
        "yi.zhang@huawei.com" <yi.zhang@huawei.com>,
        "yangerkun@huawei.com" <yangerkun@huawei.com>
Subject: Re: [PATCH] block, bfq: fix uaf for 'stable_merge_bfqq'
Thread-Topic: [PATCH] block, bfq: fix uaf for 'stable_merge_bfqq'
Thread-Index: AQHZUWL5J4Loq78SFkKpVSpg5Wk4Qq7wYskA
Date:   Wed, 8 Mar 2023 05:56:52 +0000
Message-ID: <20230308055652.l3redb6gbb3pz3xp@shindev>
References: <4e6e1606-1d9e-9903-8a44-ccac58a1fe06@kernel.dk>
 <20230308023208.379465-1-yukuai1@huaweicloud.com>
In-Reply-To: <20230308023208.379465-1-yukuai1@huaweicloud.com>
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On Mar 08, 2023 / 10:32, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@huawei.com>
>=20
> Before commit fd571df0ac5b ("block, bfq: turn bfqq_data into an array
> in bfq_io_cq"), process reference is read before bfq_put_stable_ref(),
> and it's safe if bfq_put_stable_ref() put the last reference, because
> process reference will be 0 and 'stable_merge_bfqq' won't be accessed
> in this case. However, the commit changed the order and  will cause
> uaf for 'stable_merge_bfqq'.
>=20
> In order to emphasize that bfq_put_stable_ref() can drop the last
> reference, fix the problem by moving bfq_put_stable_ref() to the end of
> bfq_setup_stable_merge().
>=20
> Fixes: fd571df0ac5b ("block, bfq: turn bfqq_data into an array in bfq_io_=
cq")
> Reported-and-tested-by: Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
> Link: https://lore.kernel.org/linux-block/20230307071448.rzihxbm4jhbf5krj=
@shindev/
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> ---
>  block/bfq-iosched.c | 18 +++++++++---------
>  1 file changed, 9 insertions(+), 9 deletions(-)
>=20
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index 8a8d4441519c..d9ed3108c17a 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -2854,11 +2854,11 @@ bfq_setup_stable_merge(struct bfq_data *bfqd, str=
uct bfq_queue *bfqq,
>  {
>  	int proc_ref =3D min(bfqq_process_refs(bfqq),
>  			   bfqq_process_refs(stable_merge_bfqq));
> -	struct bfq_queue *new_bfqq;
> +	struct bfq_queue *new_bfqq =3D NULL;
> =20
> -	if (idling_boosts_thr_without_issues(bfqd, bfqq) ||
> -	    proc_ref =3D=3D 0)
> -		return NULL;
> +	bfqq_data->stable_merge_bfqq =3D NULL;
> +	if (idling_boosts_thr_without_issues(bfqd, bfqq) || proc_ref =3D=3D 0)
> +		goto out;
> =20
>  	/* next function will take at least one ref */
>  	new_bfqq =3D bfq_setup_merge(bfqq, stable_merge_bfqq);
> @@ -2873,6 +2873,11 @@ bfq_setup_stable_merge(struct bfq_data *bfqd, stru=
ct bfq_queue *bfqq,
>  			new_bfqq_data->stably_merged =3D true;
>  		}
>  	}
> +
> +out:
> +	/* deschedule stable merge, because done or aborted here */
> +	bfq_put_stable_ref(stable_merge_bfqq);
> +
>  	return new_bfqq;
>  }
> =20
> @@ -2933,11 +2938,6 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct=
 bfq_queue *bfqq,
>  			struct bfq_queue *stable_merge_bfqq =3D
>  				bfqq_data->stable_merge_bfqq;

Nit: I suggest to remove the local variable above. The two references to it
below are removed, and the argument of bfq_setup_stable_merge() can be repl=
aced
with bfqq_data->stable_merge_bfqq.

> =20
> -			/* deschedule stable merge, because done or aborted here */
> -			bfq_put_stable_ref(stable_merge_bfqq);
> -
> -			bfqq_data->stable_merge_bfqq =3D NULL;
> -
>  			return bfq_setup_stable_merge(bfqd, bfqq,
>  						      stable_merge_bfqq,
>  						      bfqq_data);
> --=20
> 2.31.1
>=20

--=20
Shin'ichiro Kawasaki=