netdev - Re: [PATCH V2] libceph: fix use after free

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Date:   Tue, 27 Nov 2018 11:17:26 +0100
From:   Ilya Dryomov <idryomov@...il.com>
To:     bianpan2016@....com
Cc:     "Yan, Zheng" <zyan@...hat.com>, Sage Weil <sage@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Ceph Development <ceph-devel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] libceph: fix use after free

On Tue, Nov 27, 2018 at 10:22 AM Pan Bian <bianpan2016@....com> wrote:
>
> The function ceph_monc_handle_map calls kfree(old) to free the old
> monitor map, old points to monc->monmap. However, after that, it reads
> monc->monmap->epoch and passes it to __ceph_monc_got_map. This will
> result in a use-after-free bug. The patch moves the free operation after
> the call to __ceph_monc_got_map.
>
> Fixes: 82dcabad750 ("libceph: revamp subs code, switch to SUBSCRIBE2 protocol")
>
> Signed-off-by: Pan Bian <bianpan2016@....com>
> ---
> V2: correct the format of the tag Fixes
> ---
>  net/ceph/mon_client.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/net/ceph/mon_client.c b/net/ceph/mon_client.c
> index 18deb3d..05ef5aa 100644
> --- a/net/ceph/mon_client.c
> +++ b/net/ceph/mon_client.c
> @@ -478,9 +478,10 @@ static void ceph_monc_handle_map(struct ceph_mon_client *monc,
>         }
>
>         client->monc.monmap = monmap;

monc->monmap is assigned here.  It's not obvious, but monc->monmap and
client->monc.monmap is the same pointer.

> -       kfree(old);
>
>         __ceph_monc_got_map(monc, CEPH_SUB_MONMAP, monc->monmap->epoch);

ceph_monmap_decode() returns the new map, kfree() frees the old map.
I don't see a use-after-free here.

Thanks,

                Ilya