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Message-ID: <1923bc2f330f576cd246856f976af448c035d02e.camel@huaweicloud.com>
Subject: NFS mount fail
From:   Roberto Sassu <roberto.sassu@huaweicloud.com>
To:     Casey Schaufler <casey@schaufler-ca.com>
Cc:     linux-security-module@vger.kernel.org, linux-nfs@vger.kernel.org,
        chuck.lever@oracle.com, jlayton@kernel.org
Date:   Thu, 04 May 2023 18:11:32 +0200
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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.

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

We are in this part of the code:

Thread 1 hit Breakpoint 2, netlbl_sock_setattr (sk=sk@entry=0xffff888025178000, family=family@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).

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.

Thanks

Roberto