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Date:   Thu, 13 Sep 2018 16:12:38 +0200
From:   Alexander Potapenko <glider@...gle.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     syzbot+f5f6080811c849739212@...kaller.appspotmail.com,
        LKML <linux-kernel@...r.kernel.org>, mostrows@...thlink.net,
        Networking <netdev@...r.kernel.org>,
        syzkaller-bugs@...glegroups.com
Subject: Re: KMSAN: uninit-value in pppoe_rcv

On Thu, Sep 13, 2018 at 3:57 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
>
> On 09/12/2018 03:38 AM, Alexander Potapenko wrote:
> > On Wed, Sep 12, 2018 at 12:24 PM syzbot
> > <syzbot+f5f6080811c849739212@...kaller.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@...kaller.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.
> diff --git a/drivers/net/ppp/pppoe.c b/drivers/net/ppp/pppoe.c
> index ce61231e96ea5fe27f512fbd0d80d4609997e508..333e967ed968ea3ff2dda25289f7f657263db2b9 100644
> --- a/drivers/net/ppp/pppoe.c
> +++ b/drivers/net/ppp/pppoe.c
> @@ -423,6 +423,7 @@ static int pppoe_rcv(struct sk_buff *skb, struct net_device *dev,
>         struct pppoe_hdr *ph;
>         struct pppox_sock *po;
>         struct pppoe_net *pn;
> +       __be16 sid;
>         int len;
>
>         skb = skb_share_check(skb, GFP_ATOMIC);
> @@ -434,6 +435,7 @@ static int pppoe_rcv(struct sk_buff *skb, struct net_device *dev,
>
>         ph = pppoe_hdr(skb);
>         len = ntohs(ph->length);
> +       sid = ph->sid;
>
>         skb_pull_rcsum(skb, sizeof(*ph));
>         if (skb->len < len)
> @@ -447,7 +449,7 @@ static int pppoe_rcv(struct sk_buff *skb, struct net_device *dev,
>         /* Note that get_item does a sock_hold(), so sk_pppox(po)
>          * is known to be safe.
>          */
> -       po = get_item(pn, ph->sid, eth_hdr(skb)->h_source, dev->ifindex);
> +       po = get_item(pn, sid, eth_hdr(skb)->h_source, dev->ifindex);
>         if (!po)
>                 goto drop;
>
>
>
> --
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-- 
Alexander Potapenko
Software Engineer

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