NFS mount fail

[Casey Schaufler]

On 5/4/2023 9:11 AM, Roberto Sassu wrote:
> Hi Casey
>
> while developing the fix for overlayfs, I tried first to address the
> issue of a NFS filesystem failing to mount.
>
> The NFS server does not like the packets sent by the client:
>
> 14:52:20.827208 IP (tos 0x0, ttl 64, id 60628, offset 0, flags [DF], proto TCP (6), length 72, options (unknown 134,EOL))
>     localhost.localdomain.omginitialrefs > _gateway.nfs: Flags [S], cksum 0x7618 (incorrect -> 0xa18c), seq 455337903, win 64240, options [mss 1460,sackOK,TS val 2178524519 ecr 0,nop,wscale 7], length 0
> 14:52:20.827376 IP (tos 0xc0, ttl 64, id 5906, offset 0, flags [none], proto ICMP (1), length 112, options (unknown 134,EOL))
>     _gateway > localhost.localdomain: ICMP parameter problem - octet 22, length 80
>
> I looked at the possible causes. SELinux works properly.

SELinux was the reference LSM implementation for labeled networking.

> What it seems to happen is that there is a default netlabel mapping,
> that is used to send the packets out.

Correct. SELinux only uses CIPSO options for MLS. Smack uses CIPSO for
almost all packets.

> We are in this part of the code:
>
> Thread 1 hit Breakpoint 2, netlbl_sock_setattr (sk=sk at entry=0xffff888025178000, family=family at entry=2, secattr=0xffff88802504b200) at net/netlabel/netlabel_kapi.c:980
> 980	{
> (gdb) n
> 771		__rcu_read_lock();
> (gdb) 
> 985		dom_entry = netlbl_domhsh_getentry(secattr->domain, family);
> (gdb) 
> 986		if (dom_entry == NULL) {
> (gdb) 
> 990		switch (family) {
> (gdb) 
> 992			switch (dom_entry->def.type) {
>
> Here is the difference between Smack and SELinux.
>
> Smack:
>
> (gdb) p *dom_entry
> $2 = {domain = 0x0 <fixed_percpu_data>, family = 2, def = {type = 3, {addrsel = 0xffff888006bbef40, cipso = 0xffff888006bbef40, calipso = 0xffff888006bbef40}}, valid = 1, list = {next = 0xffff88800767f6e8, prev = 0xffff88800767f6e8}, rcu = {next = 0x0 <fixed_percpu_data>, 
>     func = 0x0 <fixed_percpu_data>}}
>
> SELinux:
>
> (gdb) p *dom_entry
> $5 = {domain = 0x0 <fixed_percpu_data>, family = 2, def = {type = 5, {addrsel = 0x0 <fixed_percpu_data>, cipso = 0x0 <fixed_percpu_data>, calipso = 0x0 <fixed_percpu_data>}}, valid = 1, list = {next = 0xffff888006012c88, prev = 0xffff888006012c88}, rcu = {
>     next = 0x0 <fixed_percpu_data>, func = 0x0 <fixed_percpu_data>}}
>
>
> type = 3 (for Smack) is NETLBL_NLTYPE_CIPSOV4.
> type = 5 (for SELinux) is NETLBL_NLTYPE_UNLABELED.
>
> This is why SELinux works (no incompatible options are sent).

SELinux "works" because that's the use case that was verified.

>
> The netlabel mapping is added here:
>
> static void smk_cipso_doi(void)
> {
>
> [...]
>
> 	rc = netlbl_cfg_cipsov4_map_add(doip->doi, NULL, NULL, NULL, &nai);
>
>
> Not sure exactly how we can solve this issue. Just checked that
> commenting the call to smk_cipso_doi() in init_smk_fs() allows the NFS
> filesystem to be mounted.

Are both the server and client using Smack? Are they on a network that can
propagate labeled packets? What are you using for a Smack rule configuration?

>
> Thanks
>
> Roberto
>