cifsd: fix static checker warning from smb_check_perm_dacl()

Dan reported static checker warning:

  fs/cifsd/smbacl.c:1140 smb_check_perm_dacl()
  error: we previously assumed 'pntsd' could be null (see line 1137)

This patch validate bounds of pntsd buffer.
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NNamjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: NSteve French <stfrench@microsoft.com>

fs/cifsd/smbacl.c

@@ -800,9 +800,13 @@ int parse_sec_desc(struct smb_ntsd *pntsd, int acl_len,
 		 le32_to_cpu(pntsd->gsidoffset),
 		 le32_to_cpu(pntsd->sacloffset), dacloffset);
 
-	if (dacloffset && dacl_ptr)
+	if (dacloffset) {
+		if (end_of_acl <= (char *)dacl_ptr ||
+		    end_of_acl < (char *)dacl_ptr + le16_to_cpu(dacl_ptr->size))
+			return -EIO;
 		total_ace_size =
 			le16_to_cpu(dacl_ptr->size) - sizeof(struct smb_acl);
+	}
 
 	pntsd_type = le16_to_cpu(pntsd->type);
 
@@ -1131,13 +1135,28 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, struct dentry *dentry,
 	struct smb_ace *others_ace = NULL;
 	struct posix_acl_entry *pa_entry;
 	unsigned int sid_type = SIDOWNER;
+	char *end_of_acl;
 
 	ksmbd_debug(SMB, "check permission using windows acl\n");
 	acl_size = ksmbd_vfs_get_sd_xattr(conn, dentry, &pntsd);
-	if (acl_size <= 0 || (pntsd && !pntsd->dacloffset))
+	if (acl_size <= 0 || !pntsd || !pntsd->dacloffset) {
+		kfree(pntsd);
 		return 0;
+	}
 
 	pdacl = (struct smb_acl *)((char *)pntsd + le32_to_cpu(pntsd->dacloffset));
+	end_of_acl = ((char *)pntsd) + acl_size;
+	if (end_of_acl <= (char *)pdacl) {
+		kfree(pntsd);
+		return 0;
+	}
+
+	if (end_of_acl < (char *)pdacl + le16_to_cpu(pdacl->size) ||
+	    le16_to_cpu(pdacl->size) < sizeof(struct smb_acl)) {
+		kfree(pntsd);
+		return 0;
+	}
+
 	if (!pdacl->num_aces) {
 		if (!(le16_to_cpu(pdacl->size) - sizeof(struct smb_acl)) &&
 		    *pdaccess & ~(FILE_READ_CONTROL_LE | FILE_WRITE_DAC_LE)) {
@@ -1156,6 +1175,8 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, struct dentry *dentry,
 		for (i = 0; i < le32_to_cpu(pdacl->num_aces); i++) {
 			granted |= le32_to_cpu(ace->access_req);
 			ace = (struct smb_ace *) ((char *)ace + le16_to_cpu(ace->size));
+			if (end_of_acl < (char *)ace)
+				goto err_out;
 		}
 
 		if (!pdacl->num_aces)
@@ -1177,6 +1198,8 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, struct dentry *dentry,
 			others_ace = ace;
 
 		ace = (struct smb_ace *) ((char *)ace + le16_to_cpu(ace->size));
+		if (end_of_acl < (char *)ace)
+			goto err_out;
 	}
 
 	if (*pdaccess & FILE_MAXIMAL_ACCESS_LE && found) {