Re: hard links create local DoS vulnerability and security problems

[Richard B. Johnson]

On Mon, 24 Nov 2003, Jakob Lell wrote:

> Hello,
> on Linux it is possible for any user to create a hard link to a file belonging
> to another user. This hard link continues to exist even if the original file
> is removed by the owner. However, as the link still belongs to the original
> owner, it is still counted to his quota. If a malicious user creates hard
> links for every temp file created by another user, this can make the victim
> run out of quota (or even fill up the hard disk). This makes a local DoS
> attack possible.
>

You can create hard-links to any file that a user has given you
permission to read or execute. This is correct. The new hard-link
still belongs to the original user, which is also correct.

To prevent this, a user can set his default permissions so that
neither group nor world can read the files. This is usually done
by setting the attributes in the user's top directory.

> Furthermore, users can even create links to a setuid binary. If there is a
> security whole like a buffer overflow in any setuid binary, a cracker can
> create a hard link to this file in his home directory. This link still exists
> when the administrator has fixed the security whole by removing or replacing
> the insecure program. This makes it possible for a cracker to keep a security
> whole open until an exploit is available. It is even possible to create links
> to every setuid program on the system. This doesn't create new security
> wholes but makes it more likely that they are exploited.
>

A setuid binary created with a hard-link will only work as a setuid
binary if the directory it's in is owned by root. If you have users
that can create files or hard-links within such directories, you
have users who either know the root password already or have used
some exploit to become root. In any case, it's not a hard-link
problem

> To solve the problem, the kernel shouldn't allow users to create hard
> links to
> files belonging to someone else.
>

No. Users must be able to create hard links to files that belong
to somebody else if they are readable. It's a requirement.

Cheers,
Dick Johnson
Penguin : Linux version 2.4.22 on an i686 machine (797.90 BogoMips).
            Note 96.31% of all statistics are fiction.


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