Re: /proc filesystem allows bypassing directory permissions on- Linux

From: Pavel Machek <>
To: Dan Yefimov <>
Subject: Re: /proc filesystem allows bypassing directory permissions on- Linux

> >pavel@toy:/tmp/my_priv$ chmod 700 .
> ># relax file permissions, directory is private, so this is safe
> ># check link count on unwritable_file. We would not want someone
> ># to have a hard link to work around our permissions, would we?
> >pavel@toy:/tmp/my_priv$ chmod 666 unwritable_file
> >pavel@toy:/tmp/my_priv$ cat unwritable_file
> >this file should never be writable
> >pavel@toy:/tmp/my_priv$ cat unwritable_file
> >got you
> ># Security problem here
> >
> >[Please pause here for a while before reading how guest did it.]
> ># Linux correctly prevents guest from writing to that file
> >guest@toy:/tmp/my_priv$ cat unwritable_file
> >cat: unwritable_file: Permission denied
> >guest@toy:/tmp/my_priv$ echo got you>&3
> >bash: echo: write error: Bad file descriptor
> >
> ># ...until we take a way around it with /proc filesystem. Oops.
> >guest@toy:/tmp/my_priv$ echo got you>  /proc/self/fd/3
> >
> That can hardly be called a real security hole, since the behaviour
> described above is expected, and is as it was conceived by design.
> If the file owner in fact allows writing to it, why should Linux
> prevent that from happening?

No, I do not think this is expected. You could not write to that file
under traditional unix, and you can not write into that file when
/proc is unmounted.

I do not think mounting /proc should change access control semantics.

Plus, you may run traditional unix/POSIX application, expecting
directory access controls to prevent the write. (Or can you see a way
to write to that file when /proc is unmounted?)
(cesky, pictures)

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