Smack: prevent underflow in smk_set_cipso()

BugLink: https://bugs.launchpad.net/bugs/1892822

[ Upstream commit 42a2df3e ]

We have an upper bound on "maplevel" but forgot to check for negative
values.

Fixes: e114e473 ("Smack: Simplified Mandatory Access Control Kernel")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Ian May <ian.may@canonical.com>
Signed-off-by: Kelsey Skunberg <kelsey.skunberg@canonical.com>

security/smack/smackfs.c

@@ -918,7 +918,7 @@ static ssize_t smk_set_cipso(struct file *file, const char __user *buf,
 	}
 
 	ret = sscanf(rule, "%d", &maplevel);
-	if (ret != 1 || maplevel > SMACK_CIPSO_MAXLEVEL)
+	if (ret != 1 || maplevel < 0 || maplevel > SMACK_CIPSO_MAXLEVEL)
 		goto out;
 
 	rule += SMK_DIGITLEN;