Re: /proc filesystem allows bypassing directory permissions on Linux

[Pavel Machek <pavel () ucw cz>]

On Tue 2009-10-27 11:49:32, CaT wrote:
> On Tue, Oct 27, 2009 at 12:29:09AM +0300, Dan Yefimov wrote:
> > and testing them. Remember the scenario from the original mail and try 
> > finding a window, during which creating a hardlink would still work thus 
> > evading directory permissions check.
>
> The main thing this does is allow a hardlink-like attack to work across
> mountpoints afaics.

Yes, plus it allows "hardlinks" on deleted files, and this "strange
hard links" can not be seen on link count.

> You can't actually use /proc/*/fd to gain access to files opened by
> processes you do not own. Only ones you do (at least in a mainline kernel)
> which is fair enough. This means that you can't have user a open a file
> owned by user b and then let user c have access to it via
> /proc/$pid/fd.

No, but you can upgrade file from read-only to read-write using /proc.
                                                                        Pavel
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