Subject: Re: general protection fault in dev_map_hash_update_elem

From: Jesper Dangaard Brouer <[email-masked]>
Date: Fri, 6 Sep 2019 14:54:08 +0200

On Thu, 5 Sep 2019 14:44:37 -0700
Alexei Starovoitov <[email-masked]> wrote:

> On Thu, Sep 5, 2019 at 1:08 PM syzbot
> <[email-masked]> wrote:
> >
> > Hello,
> >
> > syzbot found the following crash on:
> >
> > HEAD commit:    6d028043 Add linux-next specific files for 20190830
> > git tree:       linux-next
> > console output: https://syzkaller.appspot.com/x/log.txt?x=135c1a92600000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=82a6bec43ab0cb69
> > dashboard link: https://syzkaller.appspot.com/bug?extid=4e7a85b1432052e8d6f8
> > compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=109124e1600000
> >
> > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > Reported-by: [email-masked]
> >
> > kasan: CONFIG_KASAN_INLINE enabled
> > kasan: GPF could be caused by NULL-ptr deref or user memory access
> > general protection fault: 0000 [#1] PREEMPT SMP KASAN
> > CPU: 1 PID: 10235 Comm: syz-executor.0 Not tainted 5.3.0-rc6-next-20190830
> > #75
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> > Google 01/01/2011
> > RIP: 0010:__write_once_size include/linux/compiler.h:203 [inline]
> > RIP: 0010:__hlist_del include/linux/list.h:795 [inline]
> > RIP: 0010:hlist_del_rcu include/linux/rculist.h:475 [inline]
> > RIP: 0010:__dev_map_hash_update_elem kernel/bpf/devmap.c:668 [inline]
> > RIP: 0010:dev_map_hash_update_elem+0x3c8/0x6e0 kernel/bpf/devmap.c:691
> > Code: 48 89 f1 48 89 75 c8 48 c1 e9 03 80 3c 11 00 0f 85 d3 02 00 00 48 b9
> > 00 00 00 00 00 fc ff df 48 8b 53 10 48 89 d6 48 c1 ee 03 <80> 3c 0e 00 0f
> > 85 97 02 00 00 48 85 c0 48 89 02 74 38 48 89 55 b8
> > RSP: 0018:ffff88808d607c30 EFLAGS: 00010046
> > RAX: 0000000000000000 RBX: ffff8880a7f14580 RCX: dffffc0000000000
> > RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff8880a7f14588
> > RBP: ffff88808d607c78 R08: 0000000000000004 R09: ffffed1011ac0f73
> > R10: ffffed1011ac0f72 R11: 0000000000000003 R12: ffff88809f4e9400
> > R13: ffff88809b06ba00 R14: 0000000000000000 R15: ffff88809f4e9528
> > FS:  00007f3a3d50c700(0000) GS:ffff8880ae900000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 00007feb3fcd0000 CR3: 00000000986b9000 CR4: 00000000001406e0
> > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > Call Trace:
> >   map_update_elem+0xc82/0x10b0 kernel/bpf/syscall.c:966
> >   __do_sys_bpf+0x8b5/0x3350 kernel/bpf/syscall.c:2854
> >   __se_sys_bpf kernel/bpf/syscall.c:2825 [inline]
> >   __x64_sys_bpf+0x73/0xb0 kernel/bpf/syscall.c:2825
> >   do_syscall_64+0xfa/0x760 arch/x86/entry/common.c:290
> >   entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > RIP: 0033:0x459879
> > Code: fd b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7
> > 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff
> > ff 0f 83 cb b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00
> > RSP: 002b:00007f3a3d50bc78 EFLAGS: 00000246 ORIG_RAX: 0000000000000141
> > RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000000459879
> > RDX: 0000000000000020 RSI: 0000000020000040 RDI: 0000000000000002
> > RBP: 000000000075bf20 R08: 0000000000000000 R09: 0000000000000000
> > R10: 0000000000000000 R11: 0000000000000246 R12: 00007f3a3d50c6d4
> > R13: 00000000004bfc86 R14: 00000000004d1960 R15: 00000000ffffffff
> > Modules linked in:
> > ---[ end trace 083223e21dbd0ae5 ]---
> > RIP: 0010:__write_once_size include/linux/compiler.h:203 [inline]
> > RIP: 0010:__hlist_del include/linux/list.h:795 [inline]
> > RIP: 0010:hlist_del_rcu include/linux/rculist.h:475 [inline]
> > RIP: 0010:__dev_map_hash_update_elem kernel/bpf/devmap.c:668 [inline]
> > RIP: 0010:dev_map_hash_update_elem+0x3c8/0x6e0 kernel/bpf/devmap.c:691  
> 
> Toke,
> please take a look.
> Thanks!

Hi Toke,

I think the problem is that you read:
 old_dev = __dev_map_hash_lookup_elem(map, idx);

Before holding the lock dtab->index_lock... 

I'm not sure this is the correct fix, but I think below change should
solve the issue (not even compile tested):

[bpf-next]$ git diff

diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
index 9af048a932b5..c41854a68e9e 100644
--- a/kernel/bpf/devmap.c
+++ b/kernel/bpf/devmap.c
@@ -664,6 +664,9 @@ static int __dev_map_hash_update_elem(struct net *net, struct bpf_map *map,
 
        spin_lock_irqsave(&dtab->index_lock, flags);
 
+       /* Re-read old_dev while holding lock*/
+       old_dev = __dev_map_hash_lookup_elem(map, idx);
+
        if (old_dev) {
                hlist_del_rcu(&old_dev->index_hlist);
        } else {

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