Re: [Patch net-next resend v2] net: use %px to print skb address in trace_netif_receive_skb

On Fri, Jul 23, 2021 at 12:09 AM Kees Cook [off-list ref] wrote:

On Wed, Jul 14, 2021 at 10:59:23PM -0700, Cong Wang wrote:

quoted

From: Qitao Xu <redacted>

The print format of skb adress in tracepoint class net_dev_template
is changed to %px from %p, because we want to use skb address
as a quick way to identify a packet.

No; %p was already hashed to uniquely identify unique addresses. This
is needlessly exposing kernel addresses with no change in utility. See
[1] for full details on when %px is justified (almost never).

This is clearly not true. Trust us, we _did_ use %p in the beginning,
it does not work at all. See the trace below captured with %p:

<idle>-0 [012] ..s. 385.248024: netif_receive_skb: dev=eth0
skbaddr=000000003fa575fb len=56 <idle>-0 [012] ..s. 385.261602:
netif_receive_skb: dev=eth0 skbaddr=000000003fa575fb len=38 <idle>-0
[012] ..s. 387.245617: netif_receive_skb: dev=eth0
skbaddr=000000003fa575fb len=38 <idle>-0 [012] ..s. 389.293621:
netif_receive_skb: dev=eth0 skbaddr=000000003fa575fb len=38 <idle>-0
[012] ..s. 391.277637: netif_receive_skb: dev=eth0
skbaddr=000000003fa575fb len=38 <idle>-0 [012] ..s. 392.939159:
netif_receive_skb: dev=eth0 skbaddr=000000003fa575fb len=60 <idle>-0
[012] .Ns. 392.939197: ip_rcv: skbaddr=000000003fa575fb <idle>-0 [012]
.Ns. 392.939286: ip_local_deliver_finish: skbaddr=000000003fa575fb
kworker/12:1-122 [012] ..s. 392.939369: netif_receive_skb: dev=eth0
skbaddr=000000003fa575fb len=52 kworker/12:1-122 [012] ..s.
392.939371: ip_rcv: skbaddr=000000003fa575fb kworker/12:1-122 [012]
..s. 392.939409: ip_local_deliver_finish: skbaddr=000000003fa575fb
<idle>-0 [012] ..s. 393.261605: netif_receive_skb: dev=eth0
skbaddr=000000003fa575fb len=38 <idle>-0 [012] ..s. 395.174268:
netif_receive_skb: dev=eth0 skbaddr=000000003fa575fb len=56 <idle>-0
[012] ..s. 395.174275: ip_rcv: skbaddr=000000003fa575fb <idle>-0 [012]
..s. 395.174284: sk_data_ready: skaddr=0000000049f7d0d9,
skbaddr=000000003fa575fb <idle>-0 [012] .Ns. 395.174337: tcp_v4_rcv:
sport=8801 dport=15884 saddr=192.168.122.139 daddr=192.168.122.1
saddrv6=::ffff:192.168.122.139 daddrv6=::ffff:192.168.122.1
state=TCP_ESTABLISHED skbaddr=000000003fa575fb <idle>-0 [012] .Ns.
395.174338: ip_local_deliver_finish: skbaddr=000000003fa575fb <idle>-0
[012] ..s. 395.245605: netif_receive_skb: dev=eth0
skbaddr=000000003fa575fb len=38 <idle>-0 [012] ..s. 397.293632:
netif_receive_skb: dev=eth0 skbaddr=000000003fa575fb len=38 <idle>-0
[012] ..s. 397.352766: netif_receive_skb: dev=eth0
skbaddr=000000003fa575fb len=56 <idle>-0 [012] ..s. 397.352771:
ip_rcv: skbaddr=000000003fa575fb <idle>-0 [012] ..s. 397.352813:
sk_data_ready: skaddr=0000000049f7d0d9, skbaddr=000000003fa575fb
<idle>-0 [012] .Ns. 397.352819: tcp_v4_rcv: sport=8801 dport=15884
saddr=192.168.122.139 daddr=192.168.122.1
saddrv6=::ffff:192.168.122.139 daddrv6=::ffff:192.168.122.1
state=TCP_ESTABLISHED skbaddr=000000003fa575fb <idle>-0 [012] .Ns.
397.352819: ip_local_deliver_finish: skbaddr=000000003fa575fb <idle>-0
[012] ..s. 399.277623: netif_receive_skb: dev=eth0
skbaddr=000000003fa575fb len=38 <idle>-0 [012] ..s. 401.261640:
netif_receive_skb: dev=eth0 skbaddr=000000003fa575fb len=38 <idle>-0
[012] ..s. 402.671285: netif_receive_skb: dev=eth0
skbaddr=000000003fa575fb len=56 <idle>-0 [012] ..s. 402.671294:
ip_rcv: skbaddr=000000003fa575fb <idle>-0 [012] ..s. 402.671344:
sk_data_ready: skaddr=0000000049f7d0d9, skbaddr=000000003fa575fb
<idle>-0 [012] .Ns. 402.671355: tcp_v4_rcv: sport=8801 dport=15884
saddr=192.168.122.139 daddr=192.168.122.1
saddrv6=::ffff:192.168.122.139 daddrv6=::ffff:192.168.122.1
state=TCP_ESTABLISHED skbaddr=000000003fa575fb <idle>-0 [012] .Ns.
402.671356: ip_local_deliver_finish: skbaddr=000000003fa575fb <idle>-0
[012] ..s. 403.245617: netif_receive_skb: dev=eth0
skbaddr=000000003fa575fb len=38 <idle>-0 [012] ..s. 403.688075:
netif_receive_skb: dev=eth0 skbaddr=000000003fa575fb len=52 <idle>-0
[012] ..s. 403.688080: ip_rcv: skbaddr=000000003fa575fb <idle>-0 [012]
.Ns. 403.688092: sk_data_ready: skaddr=0000000049f7d0d9,
skbaddr=000000003fa575fb <idle>-0 [012] .Ns. 403.688094: tcp_v4_rcv:
sport=8801 dport=15884 saddr=192.168.122.139 daddr=192.168.122.1
saddrv6=::ffff:192.168.122.139 daddrv6=::ffff:192.168.122.1
state=TCP_CLOSE_WAIT skbaddr=000000003fa575fb <idle>-0 [012] .Ns.
403.688095: ip_local_deliver_finish: skbaddr=000000003fa575fb nc-1044
[012] ..s. 403.688222: netif_receive_skb: dev=eth0
skbaddr=000000003fa575fb len=52 nc-1044 [012] ..s. 403.688222: ip_rcv:
skbaddr=000000003fa575fb nc-1044 [012] ..s. 403.688237: tcp_v4_rcv:
sport=8801 dport=15884 saddr=192.168.122.139 daddr=192.168.122.1
saddrv6=::ffff:192.168.122.139 daddrv6=::ffff:192.168.122.1
state=TCP_CLOSE skbaddr=000000003fa575fb nc-1044 [012] ..s.
403.688246: ip_local_deliver_finish: skbaddr=000000003fa575fb <idle>-0
[012] ..s. 405.293625: netif_receive_skb: dev=eth0
skbaddr=000000003fa575fb len=38 <idle>-0 [012] ..s. 407.137723:
netif_receive_skb: dev=eth0 skbaddr=000000003fa575fb len=222

quoted

Note, trace ring buffer is only accessible to privileged users,
it is safe to use a real kernel address here.

That's not accurate either; there is a difference between uid 0 and
kernel mode privilege levels.

Not sure how accurate we can be just to make you happy. Please
give us a pointer on the accuracy.

Thanks.