Re: /proc filesystem allows bypassing directory permissions on Linux

[Casper.Dik () sun com]

Pavel Machek wrote:
> On Sat 2009-10-24 01:12:51, Dan Yefimov wrote:
> > On 24.10.2009 0:35, Matthew Bergin wrote:
> > > doesnt look like the original owner is trying to write to it. Shows it
> > > cant, it had guest write to it via the proc folders bad permissions.
> > > Looks legitimate
> > Please tell me, who issued 'chmod 0666 unwritable_file'? Was that an
> > attacker? No, that was the owner of 'unwritable_file', nobody else.
> > What the 0666 file mode means? It means, that everybody can write to
> > the file, can't he? So why do you believe that pretension
> > legitimate?
>
> Original owner did chmod 666... after making sure traditional unix
> permissions protect the file. Please look at original mail; it was
> subtle but I believe I got it right, and file would not be writable
> with /proc unmounted.


In Solaris, you don't have permission to access a file in /proc/<pid>/fd unless
you can control the process <pid>.

$ ls -l /proc/1/fd
/proc/1/fd: Permission denied

If you can control <pid>, then clearly you have access the file anyway 
simply by controlling it using a debugger.

I agree with Pavel's assessment here.

Casper