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Subject[PATCH 4.9 55/66] flow_dissector: properly cap thoff field
4.9-stable review patch.  If anyone has any objections, please let me know.


From: Eric Dumazet <>

[ Upstream commit d0c081b49137cd3200f2023c0875723be66e7ce5 ]

syzbot reported yet another crash [1] that is caused by
insufficient validation of DODGY packets.

Two bugs are happening here to trigger the crash.

1) Flow dissection leaves with incorrect thoff field.

2) skb_probe_transport_header() sets transport header to this invalid
thoff, even if pointing after skb valid data.

3) qdisc_pkt_len_init() reads out-of-bound data because it
trusts tcp_hdrlen(skb)

Possible fixes :

- Full flow dissector validation before injecting bad DODGY packets in
the stack.
This approach was attempted here :

- Have more robust functions in the core.
This might be needed anyway for stable versions.

This patch fixes the flow dissection issue.

CPU: 1 PID: 3144 Comm: syzkaller271204 Not tainted 4.15.0-rc4-mm1+ #49
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+0x194/0x257 lib/dump_stack.c:53
print_address_description+0x73/0x250 mm/kasan/report.c:256
kasan_report_error mm/kasan/report.c:355 [inline]
kasan_report+0x23b/0x360 mm/kasan/report.c:413
__asan_report_load2_noabort+0x14/0x20 mm/kasan/report.c:432
__tcp_hdrlen include/linux/tcp.h:35 [inline]
tcp_hdrlen include/linux/tcp.h:40 [inline]
qdisc_pkt_len_init net/core/dev.c:3160 [inline]
__dev_queue_xmit+0x20d3/0x2200 net/core/dev.c:3465
dev_queue_xmit+0x17/0x20 net/core/dev.c:3554
packet_snd net/packet/af_packet.c:2943 [inline]
packet_sendmsg+0x3ad5/0x60a0 net/packet/af_packet.c:2968
sock_sendmsg_nosec net/socket.c:628 [inline]
sock_sendmsg+0xca/0x110 net/socket.c:638
sock_write_iter+0x31a/0x5d0 net/socket.c:907
call_write_iter include/linux/fs.h:1776 [inline]
new_sync_write fs/read_write.c:469 [inline]
__vfs_write+0x684/0x970 fs/read_write.c:482
vfs_write+0x189/0x510 fs/read_write.c:544
SYSC_write fs/read_write.c:589 [inline]
SyS_write+0xef/0x220 fs/read_write.c:581

Fixes: 34fad54c2537 ("net: __skb_flow_dissect() must cap its return value")
Fixes: a6e544b0a88b ("flow_dissector: Jump to exit code in __skb_flow_dissect")
Signed-off-by: Eric Dumazet <>
Cc: Willem de Bruijn <>
Reported-by: syzbot <>
Acked-by: Jason Wang <>
Signed-off-by: David S. Miller <>
Signed-off-by: Greg Kroah-Hartman <>
net/core/flow_dissector.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -550,8 +550,8 @@ ip_proto_again:
ret = true;

- key_control->thoff = (u16)nhoff;
+ key_control->thoff = min_t(u16, nhoff, skb ? skb->len : hlen);
key_basic->n_proto = proto;
key_basic->ip_proto = ip_proto;

@@ -559,7 +559,6 @@ out:

ret = false;
- key_control->thoff = min_t(u16, nhoff, skb ? skb->len : hlen);
goto out;

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