Re: [PATCH 4.14 09/89] net: Prevent invalid access to skb->prev in __qdisc_drop_all

[Christoph Paasch]

Hello Greg,

On 14/12/18 - 12:59:22, Greg Kroah-Hartman wrote:
> 4.14-stable review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Christoph Paasch <cpaasch@xxxxxxxxx>
> 
> [ Upstream commit 9410d386d0a829ace9558336263086c2fbbe8aed ]
> 
> __qdisc_drop_all() accesses skb->prev to get to the tail of the
> segment-list.
> 
> With commit 68d2f84a1368 ("net: gro: properly remove skb from list")
> the skb-list handling has been changed to set skb->next to NULL and set
> the list-poison on skb->prev.
> 
> With that change, __qdisc_drop_all() will panic when it tries to
> dereference skb->prev.
> 
> Since commit 992cba7e276d ("net: Add and use skb_list_del_init().")
> __list_del_entry is used, leaving skb->prev unchanged (thus,
> pointing to the list-head if it's the first skb of the list).
> This will make __qdisc_drop_all modify the next-pointer of the list-head
> and result in a panic later on:
> 
> [   34.501053] general protection fault: 0000 [#1] SMP KASAN PTI
> [   34.501968] CPU: 2 PID: 0 Comm: swapper/2 Not tainted 4.20.0-rc2.mptcp #108
> [   34.502887] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 0.5.1 01/01/2011
> [   34.504074] RIP: 0010:dev_gro_receive+0x343/0x1f90
> [   34.504751] Code: e0 48 c1 e8 03 42 80 3c 30 00 0f 85 4a 1c 00 00 4d 8b 24 24 4c 39 65 d0 0f 84 0a 04 00 00 49 8d 7c 24 38 48 89 f8 48 c1 e8 03 <42> 0f b6 04 30 84 c0 74 08 3c 04
> [   34.507060] RSP: 0018:ffff8883af507930 EFLAGS: 00010202
> [   34.507761] RAX: 0000000000000007 RBX: ffff8883970b2c80 RCX: 1ffff11072e165a6
> [   34.508640] RDX: 1ffff11075867008 RSI: ffff8883ac338040 RDI: 0000000000000038
> [   34.509493] RBP: ffff8883af5079d0 R08: ffff8883970b2d40 R09: 0000000000000062
> [   34.510346] R10: 0000000000000034 R11: 0000000000000000 R12: 0000000000000000
> [   34.511215] R13: 0000000000000000 R14: dffffc0000000000 R15: ffff8883ac338008
> [   34.512082] FS:  0000000000000000(0000) GS:ffff8883af500000(0000) knlGS:0000000000000000
> [   34.513036] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [   34.513741] CR2: 000055ccc3e9d020 CR3: 00000003abf32000 CR4: 00000000000006e0
> [   34.514593] Call Trace:
> [   34.514893]  <IRQ>
> [   34.515157]  napi_gro_receive+0x93/0x150
> [   34.515632]  receive_buf+0x893/0x3700
> [   34.516094]  ? __netif_receive_skb+0x1f/0x1a0
> [   34.516629]  ? virtnet_probe+0x1b40/0x1b40
> [   34.517153]  ? __stable_node_chain+0x4d0/0x850
> [   34.517684]  ? kfree+0x9a/0x180
> [   34.518067]  ? __kasan_slab_free+0x171/0x190
> [   34.518582]  ? detach_buf+0x1df/0x650
> [   34.519061]  ? lapic_next_event+0x5a/0x90
> [   34.519539]  ? virtqueue_get_buf_ctx+0x280/0x7f0
> [   34.520093]  virtnet_poll+0x2df/0xd60
> [   34.520533]  ? receive_buf+0x3700/0x3700
> [   34.521027]  ? qdisc_watchdog_schedule_ns+0xd5/0x140
> [   34.521631]  ? htb_dequeue+0x1817/0x25f0
> [   34.522107]  ? sch_direct_xmit+0x142/0xf30
> [   34.522595]  ? virtqueue_napi_schedule+0x26/0x30
> [   34.523155]  net_rx_action+0x2f6/0xc50
> [   34.523601]  ? napi_complete_done+0x2f0/0x2f0
> [   34.524126]  ? kasan_check_read+0x11/0x20
> [   34.524608]  ? _raw_spin_lock+0x7d/0xd0
> [   34.525070]  ? _raw_spin_lock_bh+0xd0/0xd0
> [   34.525563]  ? kvm_guest_apic_eoi_write+0x6b/0x80
> [   34.526130]  ? apic_ack_irq+0x9e/0xe0
> [   34.526567]  __do_softirq+0x188/0x4b5
> [   34.527015]  irq_exit+0x151/0x180
> [   34.527417]  do_IRQ+0xdb/0x150
> [   34.527783]  common_interrupt+0xf/0xf
> [   34.528223]  </IRQ>
> 
> This patch makes sure that skb->prev is set to NULL when entering
> netem_enqueue.
> 
> Cc: Prashant Bhole <bhole_prashant_q7@xxxxxxxxxxxxx>
> Cc: Tyler Hicks <tyhicks@xxxxxxxxxxxxx>
> Cc: Eric Dumazet <eric.dumazet@xxxxxxxxx>
> Fixes: 68d2f84a1368 ("net: gro: properly remove skb from list")
> Suggested-by: Eric Dumazet <eric.dumazet@xxxxxxxxx>
> Signed-off-by: Christoph Paasch <cpaasch@xxxxxxxxx>
> Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
> Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

I don't think this patch needs to be backported. I haven't hit this panic on
v4.14.x (although, I only have been using v4.14.79 for now - not yet the
latest one). And the commits that seem to have introduced the issue haven't
been backported either AFAICS. That being said, I don't think a backport
would cause any side-effects.

Or have you seen reports of this panic in the stable branches?

(same for the other stable branches v4.4,... - cfr., the other mails
regarding this patch here)


Cheers,
Christoph




> ---
>  net/sched/sch_netem.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> --- a/net/sched/sch_netem.c
> +++ b/net/sched/sch_netem.c
> @@ -436,6 +436,9 @@ static int netem_enqueue(struct sk_buff
>  	int count = 1;
>  	int rc = NET_XMIT_SUCCESS;
>  
> +	/* Do not fool qdisc_drop_all() */
> +	skb->prev = NULL;
> +
>  	/* Random duplication */
>  	if (q->duplicate && q->duplicate >= get_crandom(&q->dup_cor))
>  		++count;
> 
>