cifs: fix incorrect validation for num_aces field of smb_acl

parse_dcal() validate num_aces to allocate ace array.

f (num_aces > ULONG_MAX / sizeof(struct smb_ace *))

It is an incorrect validation that we can create an array of size ULONG_MAX.
smb_acl has ->size field to calculate actual number of aces in response buffer
size. Use this to check invalid num_aces.

Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: namjaejeon

fs/smb/client/cifsacl.c

@@ -778,7 +778,8 @@ static void parse_dacl(struct smb_acl *pdacl, char *end_of_acl,
 	}
 
 	/* validate that we do not go past end of acl */
-	if (end_of_acl < (char *)pdacl + le16_to_cpu(pdacl->size)) {
+	if (end_of_acl < (char *)pdacl + sizeof(struct smb_acl) ||
+	    end_of_acl < (char *)pdacl + le16_to_cpu(pdacl->size)) {
 		cifs_dbg(VFS, "ACL too small to parse DACL\n");
 		return;
 	}
@@ -799,8 +800,11 @@ static void parse_dacl(struct smb_acl *pdacl, char *end_of_acl,
 	if (num_aces > 0) {
 		umode_t denied_mode = 0;
 
-		if (num_aces > ULONG_MAX / sizeof(struct smb_ace *))
+		if (num_aces > (le16_to_cpu(pdacl->size) - sizeof(struct smb_acl)) /
+				(offsetof(struct smb_ace, sid) +
+				 offsetof(struct smb_sid, sub_auth) + sizeof(__le16)))
 			return;
+
 		ppace = kmalloc_array(num_aces, sizeof(struct smb_ace *),
 				      GFP_KERNEL);
 		if (!ppace)