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

[Eric Dumazet <eric.dumazet@gmail.com>]

On 11/29/2018 02:55 PM, Christoph Paasch wrote:
> On 29/11/18 - 14:44:44, Eric Dumazet wrote:
>>
>>
>> On 11/29/2018 02:27 PM, Christoph Paasch wrote:
>>> There are places in the stack, where we access skb->prev directly and
>>> modify it. Namely, __qdisc_drop_all().
>>>
>>> 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 also set to NULL when removing
>>> it from the list.
>>>
>>> The bug is in v4.19.x as well, but the patch can't be backported easily.
>>> I can post a follow-up for that.
>>>
>>> Cc: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
>>> Cc: Tyler Hicks <tyhicks@canonical.com>
>>> Fixes: 68d2f84a1368 ("net: gro: properly remove skb from list")
>>> Signed-off-by: Christoph Paasch <cpaasch@apple.com>
>>> ---
>>>  include/linux/skbuff.h | 1 +
>>>  1 file changed, 1 insertion(+)
>>>
>>> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
>>> index 0d1b2c3f127b..3bb3bfd390eb 100644
>>> --- a/include/linux/skbuff.h
>>> +++ b/include/linux/skbuff.h
>>> @@ -1373,6 +1373,7 @@ static inline void skb_zcopy_abort(struct sk_buff *skb)
>>>  static inline void skb_mark_not_on_list(struct sk_buff *skb)
>>>  {
>>>  	skb->next = NULL;
>>> +	skb->prev = NULL;
>>>  }
>>>  
>>>  static inline void skb_list_del_init(struct sk_buff *skb)
>>>
>>
>> skb_mark_not_on_list() is used in many place where we do not care of skb->prev
>>
>> What about fixing netem instead ?
> 
> Yes, I have been looking at that and Alexey's patch which introduced the
> access to skb->prev (cfr.: https://patchwork.ozlabs.org/patch/880717/).
> 
> But then I thought that setting skb->prev to NULL is a less risky approach for
> -stable.
> 
> 
> How would you go about fixing netem instead?
> 
> Because, from what I see we basically can enter netem_enqueue here with two
> different "types" of skb's. The ones where skb->prev points to the tail of
> the list of the segment and the ones where skb->prev points to the
> list-head.
> 
> Could I match on skb_is_gso() to see if skb->prev is something valid?
>


I was simply thinking of something like :

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index 2c38e3d0792468162ee0dc4137f1400160ab9276..22cd46a600576f286803536d45875cd9d537cdca 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -431,6 +431,9 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch,
        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;