[PATCH net] inetpeer: fix uninit-value in inet_getpeer

[PATCH net] inetpeer: fix uninit-value in inet_getpeer

[Eric Dumazet]

syzbot/KMSAN reported that p->dtime was read while it was
not yet initialized in :

	delta = (__u32)jiffies - p->dtime;
	if (delta < ttl || !refcount_dec_if_one(&p->refcnt))
		gc_stack[i] = NULL;

This is a false positive, because the inetpeer wont be erased
from rb-tree if the refcount_dec_if_one(&p->refcnt) does not
succeed. And this wont happen before first inet_putpeer() call
for this inetpeer has been done, and ->dtime field is written
exactly before the refcount_dec_and_test(&p->refcnt).

The KMSAN report was :

BUG: KMSAN: uninit-value in inet_peer_gc net/ipv4/inetpeer.c:163 [inline]
BUG: KMSAN: uninit-value in inet_getpeer+0x1567/0x1e70 net/ipv4/inetpeer.c:228
CPU: 0 PID: 9494 Comm: syz-executor5 Not tainted 4.16.0+ #82
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:676
 inet_peer_gc net/ipv4/inetpeer.c:163 [inline]
 inet_getpeer+0x1567/0x1e70 net/ipv4/inetpeer.c:228
 inet_getpeer_v4 include/net/inetpeer.h:110 [inline]
 icmpv4_xrlim_allow net/ipv4/icmp.c:330 [inline]
 icmp_send+0x2b44/0x3050 net/ipv4/icmp.c:725
 ip_options_compile+0x237c/0x29f0 net/ipv4/ip_options.c:472
 ip_rcv_options net/ipv4/ip_input.c:284 [inline]
 ip_rcv_finish+0xda8/0x16d0 net/ipv4/ip_input.c:365
 NF_HOOK include/linux/netfilter.h:288 [inline]
 ip_rcv+0x119d/0x16f0 net/ipv4/ip_input.c:493
 __netif_receive_skb_core+0x47cf/0x4a80 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+0x6d88/0x7580 drivers/net/tun.c:1962
 tun_chr_write_iter+0x1d4/0x330 drivers/net/tun.c:1990
 do_iter_readv_writev+0x7bb/0x970 include/linux/fs.h:1776
 do_iter_write+0x30d/0xd40 fs/read_write.c:932
 vfs_writev fs/read_write.c:977 [inline]
 do_writev+0x3c9/0x830 fs/read_write.c:1012
 SYSC_writev+0x9b/0xb0 fs/read_write.c:1085
 SyS_writev+0x56/0x80 fs/read_write.c:1082
 do_syscall_64+0x309/0x430 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_hwframe+0x3d/0xa2
RIP: 0033:0x455111
RSP: 002b:00007fae0365cba0 EFLAGS: 00000293 ORIG_RAX: 0000000000000014
RAX: ffffffffffffffda RBX: 000000000000002e RCX: 0000000000455111
RDX: 0000000000000001 RSI: 00007fae0365cbf0 RDI: 00000000000000fc
RBP: 0000000020000040 R08: 00000000000000fc R09: 0000000000000000
R10: 000000000000002e R11: 0000000000000293 R12: 00000000ffffffff
R13: 0000000000000658 R14: 00000000006fc8e0 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
 kmem_cache_alloc+0xaab/0xb90 mm/slub.c:2756
 inet_getpeer+0xed8/0x1e70 net/ipv4/inetpeer.c:210
 inet_getpeer_v4 include/net/inetpeer.h:110 [inline]
 ip4_frag_init+0x4d1/0x740 net/ipv4/ip_fragment.c:153
 inet_frag_alloc net/ipv4/inet_fragment.c:369 [inline]
 inet_frag_create net/ipv4/inet_fragment.c:385 [inline]
 inet_frag_find+0x7da/0x1610 net/ipv4/inet_fragment.c:418
 ip_find net/ipv4/ip_fragment.c:275 [inline]
 ip_defrag+0x448/0x67a0 net/ipv4/ip_fragment.c:676
 ip_check_defrag+0x775/0xda0 net/ipv4/ip_fragment.c:724
 packet_rcv_fanout+0x2a8/0x8d0 net/packet/af_packet.c:1447
 deliver_skb net/core/dev.c:1897 [inline]
 deliver_ptype_list_skb net/core/dev.c:1912 [inline]
 __netif_receive_skb_core+0x314a/0x4a80 net/core/dev.c:4545
 __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+0x6d88/0x7580 drivers/net/tun.c:1962
 tun_chr_write_iter+0x1d4/0x330 drivers/net/tun.c:1990
 do_iter_readv_writev+0x7bb/0x970 include/linux/fs.h:1776
 do_iter_write+0x30d/0xd40 fs/read_write.c:932
 vfs_writev fs/read_write.c:977 [inline]
 do_writev+0x3c9/0x830 fs/read_write.c:1012
 SYSC_writev+0x9b/0xb0 fs/read_write.c:1085
 SyS_writev+0x56/0x80 fs/read_write.c:1082
 do_syscall_64+0x309/0x430 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_hwframe+0x3d/0xa2

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
---
 net/ipv4/inetpeer.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/ipv4/inetpeer.c b/net/ipv4/inetpeer.c
index 1f04bd91fc2e999ddb82f4be92d39d229166b691..d757b9642d0d1c418bffad5bcd50e8e7bf336c66 100644
--- a/net/ipv4/inetpeer.c
+++ b/net/ipv4/inetpeer.c
@@ -211,6 +211,7 @@ struct inet_peer *inet_getpeer(struct inet_peer_base *base,
 		p = kmem_cache_alloc(peer_cachep, GFP_ATOMIC);
 		if (p) {
 			p->daddr = *daddr;
+			p->dtime = (__u32)jiffies;
 			refcount_set(&p->refcnt, 2);
 			atomic_set(&p->rid, 0);
 			p->metrics[RTAX_LOCK-1] = INETPEER_METRICS_NEW;
-- 
2.17.0.484.g0c8726318c-goog

Re: [PATCH net] inetpeer: fix uninit-value in inet_getpeer

[David Miller]

From: Eric Dumazet <edumazet@google.com>
Date: Mon,  9 Apr 2018 06:43:27 -0700

> syzbot/KMSAN reported that p->dtime was read while it was
> not yet initialized in :
> 
> 	delta = (__u32)jiffies - p->dtime;
> 	if (delta < ttl || !refcount_dec_if_one(&p->refcnt))
> 		gc_stack[i] = NULL;
> 
> This is a false positive, because the inetpeer wont be erased
> from rb-tree if the refcount_dec_if_one(&p->refcnt) does not
> succeed. And this wont happen before first inet_putpeer() call
> for this inetpeer has been done, and ->dtime field is written
> exactly before the refcount_dec_and_test(&p->refcnt).
> 
> The KMSAN report was :
 ...
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Reported-by: syzbot <syzkaller@googlegroups.com>

Applied, but it looks like we are just adding assignments simply
to placate these reports when the tools and facilities cannot
see through the logic properly.

Re: [PATCH net] inetpeer: fix uninit-value in inet_getpeer

[Eric Dumazet]

On 04/09/2018 07:58 AM, David Miller wrote:
> From: Eric Dumazet <edumazet@google.com>
> Date: Mon,  9 Apr 2018 06:43:27 -0700
> 
>> syzbot/KMSAN reported that p->dtime was read while it was
>> not yet initialized in :
>>
>> 	delta = (__u32)jiffies - p->dtime;
>> 	if (delta < ttl || !refcount_dec_if_one(&p->refcnt))
>> 		gc_stack[i] = NULL;
>>
>> This is a false positive, because the inetpeer wont be erased
>> from rb-tree if the refcount_dec_if_one(&p->refcnt) does not
>> succeed. And this wont happen before first inet_putpeer() call
>> for this inetpeer has been done, and ->dtime field is written
>> exactly before the refcount_dec_and_test(&p->refcnt).
>>
>> The KMSAN report was :
>  ...
>> Signed-off-by: Eric Dumazet <edumazet@google.com>
>> Reported-by: syzbot <syzkaller@googlegroups.com>
> 
> Applied, but it looks like we are just adding assignments simply
> to placate these reports when the tools and facilities cannot
> see through the logic properly.
> 

To be fair, this is because the check on ->dtime should be done a second time after
the refcount_dec_if_one(&p->refcnt)

It is a tiny race, and we do not really care given nature of inetpeer cache, best effort,
and DDOS candidate anyway.

If we purge one entry too soon, this is not a big deal.

I believe tool is fine.