Re: /proc filesystem allows bypassing directory permissions on Linux

[Pavel Machek <pavel () ucw cz>]

Hi!

> Your assumption that the same file descriptor is being re-opened is
> wrong!
> The file descriptor retrieved via /proc is a new one. It is not the
> same as the
> initial read-only.

True, we were just being sloppy with the words. But it does not matter
one iota.

> As Martin Rex already explained yesterday, /proc is all virtual.
> The item referred as fd in /proc is not a real file descriptor and as
> of that, that 'not-tfor-real file descriptor' is also not re-opend
> and so does
> not become read-write.

True.

> Imagen:
> - a house surrounded with a fence with all doors unlocked (file with
> perm 0666)
> - a drive-way leads to the gate in the fence and the gate is
> unlocked (dir with perms 777)
> - next we put a lock on the gate and don't give guest the key (dir
> with perms 700)
> - guest cannot access the house because he can't pass the gate
> - now we take an airplane and parachute guest straight into the
> perimeter of the fence (/proc access)
> - guest can access the house (write the file), because the house has
> all doors unlocked

Exactly, and I'm saying that airplanes should not exist (fix the 
/proc).  Martin Rex had another solution -- allow fcntl() to remove
read-only and append-only limitations, so that behaviour is at least
explicit.

                                                                        Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html