Re: KASAN: slab-out-of-bounds Read in _decode_session6

[Alexei Starovoitov]

On Thu, Sep 06, 2018 at 12:00:26AM -0700, Eric Dumazet wrote:
> 
> 
> On 09/05/2018 08:17 PM, syzbot wrote:
> > syzbot has found a reproducer for the following crash on:
> > 
> > HEAD commit:    b36fdc6853a3 Merge tag 'gpio-v4.19-2' of git://git.kernel...
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=164938d1400000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=4c7e83258d6e0156
> > dashboard link: https://syzkaller.appspot.com/bug?extid=acffccec848dc13fe459
> > compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=115f172e400000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16399be1400000
> > 
> > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > Reported-by: syzbot+acffccec848dc13fe459@xxxxxxxxxxxxxxxxxxxxxxxxx
> > 
> > IPv6: ADDRCONF(NETDEV_UP): veth1: link is not ready
> > IPv6: ADDRCONF(NETDEV_CHANGE): veth1: link becomes ready
> > IPv6: ADDRCONF(NETDEV_CHANGE): veth0: link becomes ready
> > 8021q: adding VLAN 0 to HW filter on device team0
> > ==================================================================
> > BUG: KASAN: slab-out-of-bounds in _decode_session6+0x1331/0x14e0 net/ipv6/xfrm6_policy.c:161
> > Read of size 1 at addr ffff8801d4a67f07 by task syz-executor092/4673
> > 
> > CPU: 1 PID: 4673 Comm: syz-executor092 Not tainted 4.19.0-rc2+ #223
> > 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+0x1c9/0x2b4 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+0x242/0x30d mm/kasan/report.c:412
> >  __asan_report_load1_noabort+0x14/0x20 mm/kasan/report.c:430
> >  _decode_session6+0x1331/0x14e0 net/ipv6/xfrm6_policy.c:161
> >  __xfrm_decode_session+0x71/0x140 net/xfrm/xfrm_policy.c:2299
> >  xfrm_decode_session include/net/xfrm.h:1232 [inline]
> >  vti6_tnl_xmit+0x3fc/0x1bb1 net/ipv6/ip6_vti.c:542
> >  __netdev_start_xmit include/linux/netdevice.h:4287 [inline]
> >  netdev_start_xmit include/linux/netdevice.h:4296 [inline]
> >  xmit_one net/core/dev.c:3216 [inline]
> >  dev_hard_start_xmit+0x272/0xc10 net/core/dev.c:3232
> >  __dev_queue_xmit+0x2ab2/0x3870 net/core/dev.c:3802
> >  dev_queue_xmit+0x17/0x20 net/core/dev.c:3835
> >  __bpf_tx_skb net/core/filter.c:2012 [inline]
> >  __bpf_redirect_common net/core/filter.c:2050 [inline]
> >  __bpf_redirect+0x5b7/0xae0 net/core/filter.c:2057
> >  ____bpf_clone_redirect net/core/filter.c:2090 [inline]
> >  bpf_clone_redirect+0x2f6/0x490 net/core/filter.c:2062
> >  bpf_prog_c39d1ba309a769f7+0xe9e/0x1000
> > 
> > Allocated by task 4673:
> >  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
> >  __do_kmalloc_node mm/slab.c:3682 [inline]
> >  __kmalloc_node_track_caller+0x47/0x70 mm/slab.c:3696
> >  __kmalloc_reserve.isra.41+0x3a/0xe0 net/core/skbuff.c:137
> >  pskb_expand_head+0x230/0x10e0 net/core/skbuff.c:1463
> >  skb_ensure_writable+0x3dd/0x640 net/core/skbuff.c:5129
> >  __bpf_try_make_writable net/core/filter.c:1633 [inline]
> >  bpf_try_make_writable net/core/filter.c:1639 [inline]
> >  bpf_try_make_head_writable net/core/filter.c:1647 [inline]
> >  ____bpf_clone_redirect net/core/filter.c:2084 [inline]
> >  bpf_clone_redirect+0x14a/0x490 net/core/filter.c:2062
> >  bpf_prog_c39d1ba309a769f7+0xe9e/0x1000
> > 
> > Freed by task 3286:
> >  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+0xd9/0x210 mm/slab.c:3813
> >  load_elf_binary+0x2569/0x5610 fs/binfmt_elf.c:1118
> >  search_binary_handler+0x17d/0x570 fs/exec.c:1653
> >  exec_binprm fs/exec.c:1695 [inline]
> >  __do_execve_file.isra.35+0x15ff/0x2460 fs/exec.c:1819
> >  do_execveat_common fs/exec.c:1866 [inline]
> >  do_execve fs/exec.c:1883 [inline]
> >  __do_sys_execve fs/exec.c:1964 [inline]
> >  __se_sys_execve fs/exec.c:1959 [inline]
> >  __x64_sys_execve+0x8f/0xc0 fs/exec.c:1959
> >  do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
> >  entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > 
> > The buggy address belongs to the object at ffff8801d4a67d00
> >  which belongs to the cache kmalloc-512 of size 512
> > The buggy address is located 7 bytes to the right of
> >  512-byte region [ffff8801d4a67d00, ffff8801d4a67f00)
> > The buggy address belongs to the page:
> > page:ffffea00075299c0 count:1 mapcount:0 mapping:ffff8801dac00940 index:0x0
> > flags: 0x2fffc0000000100(slab)
> > raw: 02fffc0000000100 ffffea0007529988 ffffea0007529a48 ffff8801dac00940
> > raw: 0000000000000000 ffff8801d4a67080 0000000100000006 0000000000000000
> > page dumped because: kasan: bad access detected
> > 
> > Memory state around the buggy address:
> >  ffff8801d4a67e00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> >  ffff8801d4a67e80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> >> ffff8801d4a67f00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> >                    ^
> >  ffff8801d4a67f80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> >  ffff8801d4a68000: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> > ==================================================================
> > 
> 
> 
> What about :
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index aecdeba052d3f0ff3d4f0a33ec36891f9738052c..a662f59786bd0677850c1c60a2c92faa6fb6c5bb 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -2081,7 +2081,7 @@ BPF_CALL_3(bpf_clone_redirect, struct sk_buff *, skb, u32, ifindex, u64, flags)
>          * here, we need to free the just generated clone to unclone once
>          * again.
>          */
> -       ret = bpf_try_make_head_writable(skb);
> +       ret = bpf_try_make_head_writable(clone);

This part is fine. I think the bug is in _decode_session6,
but I have a hard time reproducing the issue, so will appreciate
if somebody can test the following patch: