Re: KMSAN: uninit-value in pppoe_rcv

Eric Dumazet
2018-09-13T13:58:02Z

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)

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;