Re: KASAN: use-after-free Read in tcf_block_find

[Eric Dumazet]

On 09/26/2018 02:44 PM, Cong Wang wrote:
> On Wed, Sep 26, 2018 at 8:44 AM syzbot
> <syzbot+37b8770e6d5a8220a039@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>>
>> Hello,
>>
>> syzbot found the following crash on:
>>
>> HEAD commit:    4b1bd6976945 net: phy: marvell: Fix build.
>> git tree:       net-next
>> console output: https://syzkaller.appspot.com/x/log.txt?x=16f763fa400000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=443816db871edd66
>> dashboard link: https://syzkaller.appspot.com/bug?extid=37b8770e6d5a8220a039
>> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=17a5614e400000
>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=141a532a400000
>>
>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>> Reported-by: syzbot+37b8770e6d5a8220a039@xxxxxxxxxxxxxxxxxxxxxxxxx
>>
>> IPv6: ADDRCONF(NETDEV_CHANGE): veth0: link becomes ready
>> 8021q: adding VLAN 0 to HW filter on device team0
>> ==================================================================
>> BUG: KASAN: use-after-free in tcf_block_find+0x9d1/0xb90
>> net/sched/cls_api.c:646
>> Read of size 4 at addr ffff8801cc126978 by task syz-executor002/5646
>>
>> CPU: 1 PID: 5646 Comm: syz-executor002 Not tainted 4.19.0-rc5+ #232
>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
>> Google 01/01/2011
>> Call Trace:
>>   __dump_stack lib/dump_stack.c:77 [inline]
>>   dump_stack+0x1c4/0x2b4 lib/dump_stack.c:113
>>   print_address_description.cold.8+0x9/0x1ff mm/kasan/report.c:256
>>   kasan_report_error mm/kasan/report.c:354 [inline]
>>   kasan_report.cold.9+0x242/0x309 mm/kasan/report.c:412
>>   __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:432
>>   tcf_block_find+0x9d1/0xb90 net/sched/cls_api.c:646
> 
> Hmm. looks like missing a rcu_dereference() here:
> 
>                 if (!*parent) {
>                         *q = dev->qdisc;
>                         *parent = (*q)->handle;
> 

We hold RTNL here.

Have we already done the changes allowing dev->qdisc being changed
without RTNL being required ?