Re: KMSAN: uninit-value in pppoe_rcv

Alexander Potapenko
2018-09-12T10:38:42Z

On Wed, Sep 12, 2018 at 12:24 PM syzbot
<syzbot+f5f6080811c849739212@syzkaller.appspotmail.com> wrote:
>
> Hello,
>
> syzbot found the following crash on:
>
> HEAD commit: d2d741e5d189 kmsan: add initialization for shmem pages
> git tree: https://github.com/google/kmsan.git/master
> console output: https://syzkaller.appspot.com/x/log.txt?x=1465fc37800000
> kernel config: https://syzkaller.appspot.com/x/.config?x=48f9de3384bcd0f
> dashboard link: https://syzkaller.appspot.com/bug?extid=f5f6080811c849739212
> compiler: clang version 7.0.0 (trunk 329391)
> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=14d6e607800000
> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=10a15b5b800000
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+f5f6080811c849739212@syzkaller.appspotmail.com
>
> IPVS: ftp: loaded support on port[0] = 21
> ==================================================================
> BUG: KMSAN: uninit-value in __get_item drivers/net/ppp/pppoe.c:172 [inline]
> BUG: KMSAN: uninit-value in get_item drivers/net/ppp/pppoe.c:236 [inline]
> BUG: KMSAN: uninit-value in pppoe_rcv+0xcef/0x10e0
> drivers/net/ppp/pppoe.c:450
> CPU: 0 PID: 4543 Comm: syz-executor355 Not tainted 4.16.0+ #87
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> Google 01/01/2011
> Call Trace:
> __dump_stack lib/dump_stack.c:17 [inline]
> dump_stack+0x185/0x1d0 lib/dump_stack.c:53
> kmsan_report+0x142/0x240 mm/kmsan/kmsan.c:1067
> __msan_warning_32+0x6c/0xb0 mm/kmsan/kmsan_instr.c:683
> __get_item drivers/net/ppp/pppoe.c:172 [inline]
> get_item drivers/net/ppp/pppoe.c:236 [inline]
> pppoe_rcv+0xcef/0x10e0 drivers/net/ppp/pppoe.c:450
> __netif_receive_skb_core+0x47df/0x4a90 net/core/dev.c:4562
> __netif_receive_skb net/core/dev.c:4627 [inline]
> netif_receive_skb_internal+0x49d/0x630 net/core/dev.c:4701
> netif_receive_skb+0x230/0x240 net/core/dev.c:4725
> tun_rx_batched drivers/net/tun.c:1555 [inline]
> tun_get_user+0x740f/0x7c60 drivers/net/tun.c:1962
> tun_chr_write_iter+0x1d4/0x330 drivers/net/tun.c:1990
> call_write_iter include/linux/fs.h:1782 [inline]
> new_sync_write fs/read_write.c:469 [inline]
> __vfs_write+0x7fb/0x9f0 fs/read_write.c:482
> vfs_write+0x463/0x8d0 fs/read_write.c:544
> SYSC_write+0x172/0x360 fs/read_write.c:589
> SyS_write+0x55/0x80 fs/read_write.c:581
> do_syscall_64+0x309/0x430 arch/x86/entry/common.c:287
> entry_SYSCALL_64_after_hwframe+0x3d/0xa2
> RIP: 0033:0x4447c9
> RSP: 002b:00007fff64c8fc28 EFLAGS: 00000297 ORIG_RAX: 0000000000000001
> RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00000000004447c9
> RDX: 000000000000fd87 RSI: 0000000020000600 RDI: 0000000000000004
> RBP: 00000000006cf018 R08: 00007fff64c8fda8 R09: 00007fff00006bda
> R10: 0000000000005fe7 R11: 0000000000000297 R12: 00000000004020d0
> R13: 0000000000402160 R14: 0000000000000000 R15: 0000000000000000
>
> Uninit was created at:
> kmsan_save_stack_with_flags mm/kmsan/kmsan.c:278 [inline]
> kmsan_internal_poison_shadow+0xb8/0x1b0 mm/kmsan/kmsan.c:188
> kmsan_kmalloc+0x94/0x100 mm/kmsan/kmsan.c:314
> kmsan_slab_alloc+0x11/0x20 mm/kmsan/kmsan.c:321
> slab_post_alloc_hook mm/slab.h:445 [inline]
> slab_alloc_node mm/slub.c:2737 [inline]
> __kmalloc_node_track_caller+0xaed/0x11c0 mm/slub.c:4369
> __kmalloc_reserve net/core/skbuff.c:138 [inline]
> __alloc_skb+0x2cf/0x9f0 net/core/skbuff.c:206
> alloc_skb include/linux/skbuff.h:984 [inline]
> alloc_skb_with_frags+0x1d4/0xb20 net/core/skbuff.c:5234
> sock_alloc_send_pskb+0xb56/0x1190 net/core/sock.c:2085
> tun_alloc_skb drivers/net/tun.c:1532 [inline]
> tun_get_user+0x2242/0x7c60 drivers/net/tun.c:1829
> tun_chr_write_iter+0x1d4/0x330 drivers/net/tun.c:1990
> call_write_iter include/linux/fs.h:1782 [inline]
> new_sync_write fs/read_write.c:469 [inline]
> __vfs_write+0x7fb/0x9f0 fs/read_write.c:482
> vfs_write+0x463/0x8d0 fs/read_write.c:544
> SYSC_write+0x172/0x360 fs/read_write.c:589
> SyS_write+0x55/0x80 fs/read_write.c:581
> do_syscall_64+0x309/0x430 arch/x86/entry/common.c:287
> entry_SYSCALL_64_after_hwframe+0x3d/0xa2
> ==================================================================
I did a little digging before sending the bug upstream.
If I add memset(obj, 0xfe, size) to __kmalloc_reserve(), these 0xfe
bytes are visible in __get_item() at the place where KMSAN reports an
error.

The problem is somehow related to tun_get_user() creating a fragmented
sk_buff - when I change the call to tun_alloc_skb() so that it
allocates a single buffer the bug goes away.

>
> ---
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--
Alexander Potapenko
Software Engineer

Google Germany GmbH
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