nfs: account for selinux security context when deciding to share superblock

Keep track of whether or not there were LSM security context
options passed during mount (ie creation of the superblock).
Then, while deciding if the superblock can be shared for the new
mount, check if the newly passed in LSM security context options
are compatible with the existing superblock's ones by calling
security_sb_mnt_opts_compat().

Previously, with selinux enabled, NFS wasn't able to do the
following 2mounts:
mount -o vers=4.2,sec=sys,context=system_u:object_r:root_t:s0
<serverip>:/ /mnt
mount -o vers=4.2,sec=sys,context=system_u:object_r:swapfile_t:s0
<serverip>:/scratch /scratch

2nd mount would fail with "mount.nfs: an incorrect mount option was
specified" and var log messages would have:
"SElinux: mount invalid. Same superblock, different security
settings for.."

Signed-off-by: Olga Kornievskaia <[e-mail]>
[PM: tweak subject line]
Signed-off-by: Paul Moore <[e-mail]>

fs/nfs/fs_context.c

···
 	if (opt < 0)
 		return ctx->sloppy ? 1 : opt;
 
+	if (fc->security)
+		ctx->has_sec_mnt_opts = 1;
+
 	switch (opt) {
 	case Opt_source:
 		if (fc->source)

fs/nfs/internal.h

···
 	char			*fscache_uniq;
 	unsigned short		protofamily;
 	unsigned short		mountfamily;
+	bool			has_sec_mnt_opts;
 
 	struct {
 		union {

fs/nfs/super.c

···
 						 &sb->s_blocksize_bits);
 
 	nfs_super_set_maxbytes(sb, server->maxfilesize);
+	server->has_sec_mnt_opts = ctx->has_sec_mnt_opts;
 }
 
 static int nfs_compare_mount_options(const struct super_block *s, const struct nfs_server *b,
···
 	if (memcmp(&old->fsid, &server->fsid, sizeof(old->fsid)) != 0)
 		return 0;
 	if (!nfs_compare_userns(old, server))
+		return 0;
+	if ((old->has_sec_mnt_opts || fc->security) &&
+			security_sb_mnt_opts_compat(sb, fc->security))
 		return 0;
 	return nfs_compare_mount_options(sb, server, fc);
 }

include/linux/nfs_fs_sb.h

···
 
 	/* User namespace info */
 	const struct cred	*cred;
+	bool			has_sec_mnt_opts;
 };
 
 /* Server capabilities */