Full Disclosure: Re: Symlink vulnerabilities

[James Condron <james () zero-internet org uk>]

On 22 Oct 2011, at 07:06, Raj Mathur (राज माथुर) wrote:
> At first sight, the best option from that point of view seems to be a 
> per-user tmp under /tmp/$USER/ and mount /tmp noexec, nosuid.  If you 
> choose the ~$USER/tmp option, you'll probably have to do some userfs 
> jugglery to achieve the same objective.
Actually, no; per user /tmp could only be accomplished, without a major redesign and without breaking almost every 
application, by turning /tmp into a pseudofilesystem a la procfs. Consider /proc/self for instance, accessing it runs a 
subroutine which first must get the PPID of the stat() to work out which information the user wants.

As such /tmp must stay where it is to ensure backwards compat (Otherwise you introduce a new /tmp directory with no 
benefit) but where the UID of the caller determines to where the actual /tmp directory links to. Dynamic symlinking, if 
you've ever done any fuse programming. In which case from a security point of view we use ~/.tmp or similar.

This solves a couple fo problems further; it allows for greater control of what can and cannot be done, nosuid is 
effectively covered, noexec can be enforced, and only root can see other people's /tmp if we implement it correctly.

As an aside; we generally mount /tmp over the loopback for obvious reasons.
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