Re: dynamic /dev security hole?

Michael Buesch
2004-08-09T16:58:31Z

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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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