Re: KMSAN: uninit-value in br_dev_xmit

[syzbot <syzbot+18c8b623c66fc198c493@syzkaller.appspotmail.com>]

syzbot has found a reproducer for 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=1508df95e00000
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
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=166ecc81e00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16b103ede00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+18c8b623c66fc198c493@syzkaller.appspotmail.com

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BUG: KMSAN: uninit-value in br_dev_xmit+0x99a/0x1730 net/bridge/br_device.c:64
CPU: 0 PID: 11855 Comm: syz-executor414 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+0x554/0x1000
 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: 0023:0xf7f39d99
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:00000000ff88460c EFLAGS: 00000246 ORIG_RAX: 0000000000000165
RAX: ffffffffffffffda RBX: 000000000000000a RCX: 0000000020000040
RDX: 0000000000000040 RSI: 0000000000000000 RDI: 000000000000005b
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+0x554/0x1000
 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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