Re: KMSAN: uninit-value in br_dev_xmit

[Nikolay Aleksandrov <nikolay@cumulusnetworks.com>]

On 24/02/2020 10:38, syzbot wrote:
> Hello,
> 
> syzbot found the following crash on:
> 
> HEAD commit:    8bbbc5cf kmsan: don't compile memmove
> git tree:       https://github.com/google/kmsan.git master
> console output: https://syzkaller.appspot.com/x/log.txt?x=14d9a3d9e00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=cd0e9a6b0e555cc3
> dashboard link: https://syzkaller.appspot.com/bug?extid=18c8b623c66fc198c493
> compiler:       clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
> userspace arch: i386
> 
> Unfortunately, I don't have any reproducer for this crash yet.
> 
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+18c8b623c66fc198c493@syzkaller.appspotmail.com
> 

br_allowed_ingress() can call skb_vlan_untag() which does pskb_may_pull() and so
the cached eth header can become invalid.
I'll prepare a fix.

Cheers,
 Nik

> =====================================================
> BUG: KMSAN: uninit-value in br_dev_xmit+0x99a/0x1730 net/bridge/br_device.c:64
> CPU: 0 PID: 14704 Comm: syz-executor.1 Not tainted 5.6.0-rc2-syzkaller #0
> 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+0x1c9/0x220 lib/dump_stack.c:118
>  kmsan_report+0xf7/0x1e0 mm/kmsan/kmsan_report.c:118
>  __msan_warning+0x58/0xa0 mm/kmsan/kmsan_instr.c:215
>  br_dev_xmit+0x99a/0x1730 net/bridge/br_device.c:64
>  __netdev_start_xmit include/linux/netdevice.h:4524 [inline]
>  netdev_start_xmit include/linux/netdevice.h:4538 [inline]
>  xmit_one net/core/dev.c:3470 [inline]
>  dev_hard_start_xmit+0x531/0xab0 net/core/dev.c:3486
>  __dev_queue_xmit+0x37de/0x4220 net/core/dev.c:4063
>  dev_queue_xmit+0x4b/0x60 net/core/dev.c:4096
>  __bpf_tx_skb net/core/filter.c:2061 [inline]
>  __bpf_redirect_common net/core/filter.c:2100 [inline]
>  __bpf_redirect+0x11d5/0x1440 net/core/filter.c:2107
>  ____bpf_clone_redirect net/core/filter.c:2140 [inline]
>  bpf_clone_redirect+0x466/0x620 net/core/filter.c:2112
>  bpf_prog_a481c1313990ee2c+0x5e0/0x1000
>  bpf_dispatcher_nopfunc include/linux/bpf.h:521 [inline]
>  bpf_test_run+0x60c/0xe50 net/bpf/test_run.c:48
>  bpf_prog_test_run_skb+0xcab/0x24a0 net/bpf/test_run.c:388
>  bpf_prog_test_run kernel/bpf/syscall.c:2572 [inline]
>  __do_sys_bpf+0xa684/0x13510 kernel/bpf/syscall.c:3414
>  __se_sys_bpf kernel/bpf/syscall.c:3355 [inline]
>  __ia32_sys_bpf+0xdb/0x120 kernel/bpf/syscall.c:3355
>  do_syscall_32_irqs_on arch/x86/entry/common.c:339 [inline]
>  do_fast_syscall_32+0x3c7/0x6e0 arch/x86/entry/common.c:410
>  entry_SYSENTER_compat+0x68/0x77 arch/x86/entry/entry_64_compat.S:139
> RIP: 0023:0xf7f79d99
> Code: 90 e8 0b 00 00 00 f3 90 0f ae e8 eb f9 8d 74 26 00 89 3c 24 c3 90 90 90 90 90 90 90 90 90 90 90 90 51 52 55 89 e5 0f 34 cd 80 <5d> 5a 59 c3 90 90 90 90 eb 0d 90 90 90 90 90 90 90 90 90 90 90 90
> RSP: 002b:00000000f5d740cc EFLAGS: 00000296 ORIG_RAX: 0000000000000165
> RAX: ffffffffffffffda RBX: 000000000000000a RCX: 0000000020000140
> RDX: 0000000000000040 RSI: 0000000000000000 RDI: 0000000000000000
> RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
> 
> Uninit was created at:
>  kmsan_save_stack_with_flags mm/kmsan/kmsan.c:144 [inline]
>  kmsan_internal_poison_shadow+0x66/0xd0 mm/kmsan/kmsan.c:127
>  kmsan_slab_alloc+0x8a/0xe0 mm/kmsan/kmsan_hooks.c:82
>  slab_alloc_node mm/slub.c:2793 [inline]
>  __kmalloc_node_track_caller+0xb40/0x1200 mm/slub.c:4401
>  __kmalloc_reserve net/core/skbuff.c:142 [inline]
>  pskb_expand_head+0x20b/0x1b00 net/core/skbuff.c:1629
>  skb_ensure_writable+0x3ea/0x490 net/core/skbuff.c:5453
>  __bpf_try_make_writable net/core/filter.c:1635 [inline]
>  bpf_try_make_writable net/core/filter.c:1641 [inline]
>  bpf_try_make_head_writable net/core/filter.c:1649 [inline]
>  ____bpf_clone_redirect net/core/filter.c:2134 [inline]
>  bpf_clone_redirect+0x251/0x620 net/core/filter.c:2112
>  bpf_prog_a481c1313990ee2c+0x5e0/0x1000
>  bpf_dispatcher_nopfunc include/linux/bpf.h:521 [inline]
>  bpf_test_run+0x60c/0xe50 net/bpf/test_run.c:48
>  bpf_prog_test_run_skb+0xcab/0x24a0 net/bpf/test_run.c:388
>  bpf_prog_test_run kernel/bpf/syscall.c:2572 [inline]
>  __do_sys_bpf+0xa684/0x13510 kernel/bpf/syscall.c:3414
>  __se_sys_bpf kernel/bpf/syscall.c:3355 [inline]
>  __ia32_sys_bpf+0xdb/0x120 kernel/bpf/syscall.c:3355
>  do_syscall_32_irqs_on arch/x86/entry/common.c:339 [inline]
>  do_fast_syscall_32+0x3c7/0x6e0 arch/x86/entry/common.c:410
>  entry_SYSENTER_compat+0x68/0x77 arch/x86/entry/entry_64_compat.S:139
> =====================================================
> 
> 
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