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Date:   Thu, 4 Apr 2019 10:47:56 +0200
From:   Jesper Dangaard Brouer <brouer@redhat.com>
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc:     Eric Dumazet <eric.dumazet@gmail.com>,
        syzbot <syzbot+457d3e2ffbcf31aee5c0@syzkaller.appspotmail.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        LKML <linux-kernel@vger.kernel.org>,
        Network Development <netdev@vger.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        brouer@redhat.com, John Fastabend <john.fastabend@gmail.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: KASAN: use-after-free Read in __dev_map_entry_free
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On Wed, 3 Apr 2019 20:59:24 -0700
Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:

> On Tue, Apr 2, 2019 at 1:03 PM Eric Dumazet <eric.dumazet@gmail.com> wrote:
> >
> >
> >
> > On 06/20/2018 08:19 AM, syzbot wrote:  
> > > Hello,
> > >
> > > syzbot found the following crash on:
> > >
> > > HEAD commit:    f0dc7f9c6dd9 Merge git://git.kernel.org/pub/scm/linux/kern..
> > > git tree:       bpf-next
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=15ad7d10400000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=fa9c20c48788d1c1
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=457d3e2ffbcf31aee5c0
> > > compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
> > > syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=1195225f800000
> > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=171a7ce4400000
> > >
> > > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > > Reported-by: syzbot+457d3e2ffbcf31aee5c0@syzkaller.appspotmail.com
> > >
> > > ==================================================================
> > > BUG: KASAN: use-after-free in dev_map_flush_old kernel/bpf/devmap.c:365 [inline]
> > > BUG: KASAN: use-after-free in __dev_map_entry_free+0x2a8/0x300 kernel/bpf/devmap.c:379
> > > Read of size 8 at addr ffff8801b8da38c8 by task ksoftirqd/1/18
> > >
> > > CPU: 1 PID: 18 Comm: ksoftirqd/1 Not tainted 4.17.0+ #39
> > > 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+0x1b9/0x294 lib/dump_stack.c:113
> > >  print_address_description+0x6c/0x20b mm/kasan/report.c:256
> > >  kasan_report_error mm/kasan/report.c:354 [inline]
> > >  kasan_report.cold.7+0x242/0x2fe mm/kasan/report.c:412
> > >  __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:433
> > >  dev_map_flush_old kernel/bpf/devmap.c:365 [inline]
> > >  __dev_map_entry_free+0x2a8/0x300 kernel/bpf/devmap.c:379
> > >  __rcu_reclaim kernel/rcu/rcu.h:178 [inline]
> > >  rcu_do_batch kernel/rcu/tree.c:2558 [inline]
> > >  invoke_rcu_callbacks kernel/rcu/tree.c:2818 [inline]
> > >  __rcu_process_callbacks kernel/rcu/tree.c:2785 [inline]
> > >  rcu_process_callbacks+0xe9d/0x1760 kernel/rcu/tree.c:2802
> > >  __do_softirq+0x2e0/0xaf5 kernel/softirq.c:284
> > >  run_ksoftirqd+0x86/0x100 kernel/softirq.c:645
> > >  smpboot_thread_fn+0x417/0x870 kernel/smpboot.c:164
> > >  kthread+0x345/0x410 kernel/kthread.c:240
> > >  ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:412
> > >
> > > Allocated by task 6675:
> > >  save_stack+0x43/0xd0 mm/kasan/kasan.c:448
> > >  set_track mm/kasan/kasan.c:460 [inline]
> > >  kasan_kmalloc+0xc4/0xe0 mm/kasan/kasan.c:553
> > >  kmem_cache_alloc_trace+0x152/0x780 mm/slab.c:3620
> > >  kmalloc include/linux/slab.h:513 [inline]
> > >  kzalloc include/linux/slab.h:706 [inline]
> > >  dev_map_alloc+0x208/0x7f0 kernel/bpf/devmap.c:102
> > >  find_and_alloc_map kernel/bpf/syscall.c:129 [inline]
> > >  map_create+0x393/0x1010 kernel/bpf/syscall.c:453
> > >  __do_sys_bpf kernel/bpf/syscall.c:2351 [inline]
> > >  __se_sys_bpf kernel/bpf/syscall.c:2328 [inline]
> > >  __x64_sys_bpf+0x303/0x510 kernel/bpf/syscall.c:2328
> > >  do_syscall_64+0x1b1/0x800 arch/x86/entry/common.c:290
> > >  entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > >
> > > Freed by task 26:
> > >  save_stack+0x43/0xd0 mm/kasan/kasan.c:448
> > >  set_track mm/kasan/kasan.c:460 [inline]
> > >  __kasan_slab_free+0x11a/0x170 mm/kasan/kasan.c:521
> > >  kasan_slab_free+0xe/0x10 mm/kasan/kasan.c:528
> > >  __cache_free mm/slab.c:3498 [inline]
> > >  kfree+0xd9/0x260 mm/slab.c:3813
> > >  dev_map_free+0x4fa/0x670 kernel/bpf/devmap.c:191
> > >  bpf_map_free_deferred+0xba/0xf0 kernel/bpf/syscall.c:262
> > >  process_one_work+0xc64/0x1b70 kernel/workqueue.c:2153
> > >  worker_thread+0x181/0x13a0 kernel/workqueue.c:2296
> > >  kthread+0x345/0x410 kernel/kthread.c:240
> > >  ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:412
> > >
> > > The buggy address belongs to the object at ffff8801b8da37c0
> > >  which belongs to the cache kmalloc-512 of size 512
> > > The buggy address is located 264 bytes inside of
> > >  512-byte region [ffff8801b8da37c0, ffff8801b8da39c0)
> > > The buggy address belongs to the page:
> > > page:ffffea0006e368c0 count:1 mapcount:0 mapping:ffff8801da800940 index:0xffff8801b8da3540
> > > flags: 0x2fffc0000000100(slab)
> > > raw: 02fffc0000000100 ffffea0007217b88 ffffea0006e30cc8 ffff8801da800940
> > > raw: ffff8801b8da3540 ffff8801b8da3040 0000000100000004 0000000000000000
> > > page dumped because: kasan: bad access detected
> > >
> > > Memory state around the buggy address:
> > >  ffff8801b8da3780: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
> > >  ffff8801b8da3800: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb  
> > >> ffff8801b8da3880: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb  
> > >                                               ^
> > >  ffff8801b8da3900: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> > >  ffff8801b8da3980: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
> > > ==================================================================
> > >
> > >
> > > ---
> > > This bug is generated by a bot. It may contain errors.
> > > See https://goo.gl/tpsmEJ for more information about syzbot.
> > > syzbot engineers can be reached at syzkaller@googlegroups.com.
> > >
> > > syzbot will keep track of this bug report. See:
> > > https://goo.gl/tpsmEJ#bug-status-tracking for how to communicate with syzbot.
> > > syzbot can test patches for this bug, for details see:
> > > https://goo.gl/tpsmEJ#testing-patches  
> >
> >
> > What about something like :
> >
> >
> >
> > diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
> > index 191b79948424f4b21b7aa120abc03801264bf0a6..1e525d70f83354e451b738ffb8e42d83b5fa932f 100644
> > --- a/kernel/bpf/devmap.c
> > +++ b/kernel/bpf/devmap.c
> > @@ -164,6 +164,9 @@ static void dev_map_free(struct bpf_map *map)
> >         bpf_clear_redirect_map(map);
> >         synchronize_rcu();
> >
> > +       /* Make sure prior __dev_map_entry_free() have completed. */
> > +       rcu_barrier();
> > +  
> 
> Eric, Thank you for looking at it. The fix makes sense to me.
> 
> Jesper, thoughts?

First I though it looked strange to have a synchronize_rcu() followed
by a rcu_barrier().  But I think the fix is actually correct.  We do
need the rcu_barrier() call as it states "Wait until all in-flight
call_rcu() callbacks complete".   I wonder if we also/still need the
synchronize_rcu(), but I think so as it functions as a memory-barrier
across all CPUs (as in bpf_clear_redirect_map we visit all CPUs and
clear any left-over per_cpu_ptr redirect_info).

Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer