Re: KMSAN: uninit-value in pppoe_rcv

Guillaume Nault
2018-09-13T17:35:46Z

On Thu, Sep 13, 2018 at 04:12:38PM +0200, Alexander Potapenko wrote:
> On Thu, Sep 13, 2018 at 3:57 PM Eric Dumazet <eric.dumazet@gmail.com> wrote:
> >
> >
> >
> > On 09/12/2018 03:38 AM, Alexander Potapenko wrote:
> > > 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.
> > >
> >
> > I guess the following patch would fix the issue
> >
> > (I will submit it more formally)
> No, as far as I can see it doesn't.
> Saving sid before __skb_pull() is still a good idea, but in this
> particular case |ph| doesn't change.

Yes, we probably need to save sid.

But I think the problem found by syzbot is related to
eth_hdr(skb)->h_source. PPPoE expects that Ethernet header has already
been parsed and is accessible at skb_mac_header(skb).
But here skb_mac_header(skb) == skb->data, and we may pull only 6 bytes
(sizeof(truct pppoe_hdr)). Therefore eth_hdr(skb)->h_source points past
skb's head length.

Not sure if something needs to be changed in tun.c for properly setting
skb_mac_header. But PPPoE has no reason to consider packets from
non-Ethernet devices anyway.