9p: p9dirent_read: check network-provided name length

BugLink: https://bugs.launchpad.net/bugs/1836668

[ Upstream commit ef5305f1 ]

strcpy to dirent->d_name could overflow the buffer, use strscpy to check
the provided string length and error out if the size was too big.

While we are here, make the function return an error when the pdu
parsing failed, instead of returning the pdu offset as if it had been a
success...

Link: http://lkml.kernel.org/r/1536339057-21974-4-git-send-email-asmadeus@codewreck.org
Addresses-Coverity-ID: 139133 ("Copy into fixed size buffer")
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

net/9p/protocol.c

@@ -622,13 +622,19 @@ int p9dirent_read(struct p9_client *clnt, char *buf, int len,
 	if (ret) {
 		p9_debug(P9_DEBUG_9P, "<<< p9dirent_read failed: %d\n", ret);
 		trace_9p_protocol_dump(clnt, &fake_pdu);
-		goto out;
+		return ret;
 	}
 
-	strcpy(dirent->d_name, nameptr);
+	ret = strscpy(dirent->d_name, nameptr, sizeof(dirent->d_name));
+	if (ret < 0) {
+		p9_debug(P9_DEBUG_ERROR,
+			 "On the wire dirent name too long: %s\n",
+			 nameptr);
+		kfree(nameptr);
+		return ret;
+	}
 	kfree(nameptr);
 
-out:
 	return fake_pdu.offset;
 }
 EXPORT_SYMBOL(p9dirent_read);