netdev - Re: [PATCH net v4 1/1] can: can_skb_set_owner(): fix ref counting if socket was closed before setting skb ownership

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Message-ID: <2e9e82d1-7e52-0f5b-c1bf-848c2f162019@hartkopp.net>
Date:   Fri, 26 Feb 2021 18:15:04 +0100
From:   Oliver Hartkopp <socketcan@...tkopp.net>
To:     Oleksij Rempel <o.rempel@...gutronix.de>, mkl@...gutronix.de,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Robin van der Gracht <robin@...tonic.nl>
Cc:     Andre Naujoks <nautsch2@...il.com>,
        Eric Dumazet <edumazet@...gle.com>, kernel@...gutronix.de,
        linux-can@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net v4 1/1] can: can_skb_set_owner(): fix ref counting if
 socket was closed before setting skb ownership



On 26.02.21 10:24, Oleksij Rempel wrote:
> There are two ref count variables controlling the free()ing of a socket:
> - struct sock::sk_refcnt - which is changed by sock_hold()/sock_put()
> - struct sock::sk_wmem_alloc - which accounts the memory allocated by
>    the skbs in the send path.
> 
> In case there are still TX skbs on the fly and the socket() is closed,
> the struct sock::sk_refcnt reaches 0. In the TX-path the CAN stack
> clones an "echo" skb, calls sock_hold() on the original socket and
> references it. This produces the following back trace:
> 
> | WARNING: CPU: 0 PID: 280 at lib/refcount.c:25 refcount_warn_saturate+0x114/0x134
> | refcount_t: addition on 0; use-after-free.
> | Modules linked in: coda_vpu(E) v4l2_jpeg(E) videobuf2_vmalloc(E) imx_vdoa(E)
> | CPU: 0 PID: 280 Comm: test_can.sh Tainted: G            E     5.11.0-04577-gf8ff6603c617 #203
> | Hardware name: Freescale i.MX6 Quad/DualLite (Device Tree)
> | Backtrace:
> | [<80bafea4>] (dump_backtrace) from [<80bb0280>] (show_stack+0x20/0x24) r7:00000000 r6:600f0113 r5:00000000 r4:81441220
> | [<80bb0260>] (show_stack) from [<80bb593c>] (dump_stack+0xa0/0xc8)
> | [<80bb589c>] (dump_stack) from [<8012b268>] (__warn+0xd4/0x114) r9:00000019 r8:80f4a8c2 r7:83e4150c r6:00000000 r5:00000009 r4:80528f90
> | [<8012b194>] (__warn) from [<80bb09c4>] (warn_slowpath_fmt+0x88/0xc8) r9:83f26400 r8:80f4a8d1 r7:00000009 r6:80528f90 r5:00000019 r4:80f4a8c2
> | [<80bb0940>] (warn_slowpath_fmt) from [<80528f90>] (refcount_warn_saturate+0x114/0x134) r8:00000000 r7:00000000 r6:82b44000 r5:834e5600 r4:83f4d540
> | [<80528e7c>] (refcount_warn_saturate) from [<8079a4c8>] (__refcount_add.constprop.0+0x4c/0x50)
> | [<8079a47c>] (__refcount_add.constprop.0) from [<8079a57c>] (can_put_echo_skb+0xb0/0x13c)
> | [<8079a4cc>] (can_put_echo_skb) from [<8079ba98>] (flexcan_start_xmit+0x1c4/0x230) r9:00000010 r8:83f48610 r7:0fdc0000 r6:0c080000 r5:82b44000 r4:834e5600
> | [<8079b8d4>] (flexcan_start_xmit) from [<80969078>] (netdev_start_xmit+0x44/0x70) r9:814c0ba0 r8:80c8790c r7:00000000 r6:834e5600 r5:82b44000 r4:82ab1f00
> | [<80969034>] (netdev_start_xmit) from [<809725a4>] (dev_hard_start_xmit+0x19c/0x318) r9:814c0ba0 r8:00000000 r7:82ab1f00 r6:82b44000 r5:00000000 r4:834e5600
> | [<80972408>] (dev_hard_start_xmit) from [<809c6584>] (sch_direct_xmit+0xcc/0x264) r10:834e5600 r9:00000000 r8:00000000 r7:82b44000 r6:82ab1f00 r5:834e5600 r4:83f27400
> | [<809c64b8>] (sch_direct_xmit) from [<809c6c0c>] (__qdisc_run+0x4f0/0x534)
> 
> To fix this problem, only set skb ownership to sockets which have still
> a ref count > 0.
> 
> Cc: Oliver Hartkopp <socketcan@...tkopp.net>
> Cc: Andre Naujoks <nautsch2@...il.com>
> Suggested-by: Eric Dumazet <edumazet@...gle.com>
> Fixes: 0ae89beb283a ("can: add destructor for self generated skbs")
> Signed-off-by: Oleksij Rempel <o.rempel@...gutronix.de>
> ---
>   include/linux/can/skb.h | 8 ++++++--
>   1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/can/skb.h b/include/linux/can/skb.h
> index 685f34cfba20..d82018cc0d0b 100644
> --- a/include/linux/can/skb.h
> +++ b/include/linux/can/skb.h
> @@ -65,8 +65,12 @@ static inline void can_skb_reserve(struct sk_buff *skb)
>   
>   static inline void can_skb_set_owner(struct sk_buff *skb, struct sock *sk)
>   {
> -	if (sk) {
> -		sock_hold(sk);
> +	/*
> +	 * If the socket has already been closed by user space, the refcount may
> +	 * already be 0 (and the socket will be freed after the last TX skb has
> +	 * been freed). So only increase socket refcount if the refcount is > 0.
> +	 */
> +	if (sk && refcount_inc_not_zero(&sk->sk_refcnt)) {
>   		skb->destructor = sock_efree;
>   		skb->sk = sk;
>   	}

Ah, now I got the problem. You are replacing sock_hold(). Maybe I was a 
bit slow-witted here - but with the comment it is ok.

Reviewed-by: Oliver Hartkopp <socketcan@...tkopp.net>

Many thanks!

Oliver