Re: /proc filesystem allows bypassing directory permissions on Linux

[Marco Verschuur <marco () osp nl>]

Pavel,

Proc does not need to be fixed, because /proc is referring to a file  
inode.
And due to the fact that it's being presented in /proc as an fd, you  
treat it
as an fd, therefor your expectations do not match.

Your assumptions is; because you accessed the world writable file via
a read-only fd, you also expect the file to appear as read-only in / 
proc.
There is the mis-match.

The idea of /proc is to refer to files being opened by the process,
while RETAINING the original INODE attributes, regardless of the fact  
that
this particular process accessed the file via read-only access.

You are expecting transactional behavior in /proc, where /proc only
registers object information.

I think this discussion had never existed if they used another name for
the 'fd' placeholder in /proc... Because then you wouldn't linked the
/proc fd to the fd being used within the actual process space.

And I think you would agree about your mis-interpretation earlier if  
not so
many people had the same mis-interpretation of the virtual /proc  
mechanism
(I hope you don't get me wrong; I'm not taking this personal or being  
rude to you,
because that's definitely NOT my intention).

Best regards,
        Marco

On 30 okt 2009, at 18:38, Pavel Machek wrote:

> 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