Re: dynamic /dev security hole?

[Michael Buesch <mbuesch@freenet.de>]

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Quoting Albert Cahalan <albert@users.sf.net>:
> Pretty much, but you must change ownership first to
> keep the user from changing the mode back. There are
> ways for an evildoer to win this race if you don't
> change the ownership first.
>
> Now all we need is revoke() and we're all set.
> Ordering: chown, chmod, revoke, unlink
>
> BTW, I'm make revoke() just force re-verification
> of file access.

So what about this updated patch?
I'm running latest udev with klibc and my patch.
It seems to work. At least with my USB stick. :)

Greg? Any comments?


===== udev-remove.c 1.31 vs edited =====
- --- 1.31/udev-remove.c	2004-04-01 04:12:56 +02:00
+++ edited/udev-remove.c	2004-08-09 18:42:08 +02:00
@@ -65,6 +65,41 @@
 	return 0;
 }

+/** Remove all permissions on the device node, before
+  * unlinking it. This fixes a security issue.
+  * If the user created a hard-link to the device node,
+  * he can't use it any longer, because he lost permission
+  * to do so.
+  */
+static int secure_unlink(const char *filename)
+{
+	int retval;
+
+	retval = chown(filename, 0, 0);
+	if (retval) {
+		dbg("chown(%s, 0, 0) failed with error '%s'",
+		    filename, strerror(errno));
+		/* We continue nevertheless.
+		 * I think it's very unlikely for chown
+		 * to fail here, if the file exists.
+		 */
+	}
+	retval = chmod(filename, 0000);
+	if (retval) {
+		dbg("chmod(%s, 0000) failed with error '%s'",
+		    filename, strerror(errno));
+		/* We continue nevertheless. */
+	}
+	retval = unlink(filename);
+	if (errno == ENOENT)
+		retval = 0;
+	if (retval) {
+		dbg("unlink(%s) failed with error '%s'",
+			filename, strerror(errno));
+	}
+	return retval;
+}
+
 static int delete_node(struct udevice *dev)
 {
 	char filename[NAME_SIZE];
@@ -79,14 +114,9 @@
 	strfieldcat(filename, dev->name);

 	info("removing device node '%s'", filename);
- -	retval = unlink(filename);
- -	if (errno == ENOENT)
- -		retval = 0;
- -	if (retval) {
- -		dbg("unlink(%s) failed with error '%s'",
- -			filename, strerror(errno));
+	retval = secure_unlink(filename);
+	if (retval)
 		return retval;
- -	}

 	/* remove partition nodes */
 	if (dev->partitions > 0) {
@@ -94,7 +124,7 @@
 		for (i = 1; i <= dev->partitions; i++) {
 			strfieldcpy(partitionname, filename);
 			strintcat(partitionname, i);
- -			unlink(partitionname);
+			secure_unlink(partitionname);
 		}
 	}

@@ -108,14 +138,9 @@
 		strfieldcat(filename, linkname);

 		dbg("unlinking symlink '%s'", filename);
- -		retval = unlink(filename);
- -		if (errno == ENOENT)
- -			retval = 0;
- -		if (retval) {
- -			dbg("unlink(%s) failed with error '%s'",
- -				filename, strerror(errno));
+		retval = secure_unlink(filename);
+		if (retval)
 			return retval;
- -		}
 		if (strchr(dev->symlink, '/')) {
 			delete_path(filename);
 		}

- --
Regards Michael Buesch  [ http://www.tuxsoft.de.vu ]


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