smackfs: restrict bytes count in smackfs write functions

syzbot found WARNINGs in several smackfs write operations where
bytes count is passed to memdup_user_nul which exceeds
GFP MAX_ORDER. Check count size if bigger than PAGE_SIZE.

Per smackfs doc, smk_write_net4addr accepts any label or -CIPSO,
smk_write_net6addr accepts any label or -DELETE. I couldn't find
any general rule for other label lengths except SMK_LABELLEN,
SMK_LONGLABEL, SMK_CIPSOMAX which are documented.

Let's constrain, in general, smackfs label lengths for PAGE_SIZE.
Although fuzzer crashes write to smackfs/netlabel on 0x400000 length.

Here is a quick way to reproduce the WARNING:
python -c "print('A' * 0x400000)" > /sys/fs/smackfs/netlabel

Reported-by: syzbot+a71a442385a0b2815497@syzkaller.appspotmail.com
Signed-off-by: Sabyrzhan Tasbolatov <snovitoll@gmail.com>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>

security/smack/smackfs.c

@@ -1167,7 +1167,7 @@ static ssize_t smk_write_net4addr(struct file *file, const char __user *buf,
 		return -EPERM;
 	if (*ppos != 0)
 		return -EINVAL;
-	if (count < SMK_NETLBLADDRMIN)
+	if (count < SMK_NETLBLADDRMIN || count > PAGE_SIZE - 1)
 		return -EINVAL;
 
 	data = memdup_user_nul(buf, count);
@@ -1427,7 +1427,7 @@ static ssize_t smk_write_net6addr(struct file *file, const char __user *buf,
 		return -EPERM;
 	if (*ppos != 0)
 		return -EINVAL;
-	if (count < SMK_NETLBLADDRMIN)
+	if (count < SMK_NETLBLADDRMIN || count > PAGE_SIZE - 1)
 		return -EINVAL;
 
 	data = memdup_user_nul(buf, count);
@@ -1834,6 +1834,10 @@ static ssize_t smk_write_ambient(struct file *file, const char __user *buf,
 	if (!smack_privileged(CAP_MAC_ADMIN))
 		return -EPERM;
 
+	/* Enough data must be present */
+	if (count == 0 || count > PAGE_SIZE)
+		return -EINVAL;
+
 	data = memdup_user_nul(buf, count);
 	if (IS_ERR(data))
 		return PTR_ERR(data);
@@ -2005,6 +2009,9 @@ static ssize_t smk_write_onlycap(struct file *file, const char __user *buf,
 	if (!smack_privileged(CAP_MAC_ADMIN))
 		return -EPERM;
 
+	if (count > PAGE_SIZE)
+		return -EINVAL;
+
 	data = memdup_user_nul(buf, count);
 	if (IS_ERR(data))
 		return PTR_ERR(data);
@@ -2092,6 +2099,9 @@ static ssize_t smk_write_unconfined(struct file *file, const char __user *buf,
 	if (!smack_privileged(CAP_MAC_ADMIN))
 		return -EPERM;
 
+	if (count > PAGE_SIZE)
+		return -EINVAL;
+
 	data = memdup_user_nul(buf, count);
 	if (IS_ERR(data))
 		return PTR_ERR(data);
@@ -2648,6 +2658,10 @@ static ssize_t smk_write_syslog(struct file *file, const char __user *buf,
 	if (!smack_privileged(CAP_MAC_ADMIN))
 		return -EPERM;
 
+	/* Enough data must be present */
+	if (count == 0 || count > PAGE_SIZE)
+		return -EINVAL;
+
 	data = memdup_user_nul(buf, count);
 	if (IS_ERR(data))
 		return PTR_ERR(data);
@@ -2740,10 +2754,13 @@ static ssize_t smk_write_relabel_self(struct file *file, const char __user *buf,
 		return -EPERM;
 
 	/*
+	 * No partial write.
 	 * Enough data must be present.
 	 */
 	if (*ppos != 0)
 		return -EINVAL;
+	if (count == 0 || count > PAGE_SIZE)
+		return -EINVAL;
 
 	data = memdup_user_nul(buf, count);
 	if (IS_ERR(data))