Re: /proc filesystem allows bypassing directory permissions on Linux

[Pavel Machek <pavel () ucw cz>]

> > 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?)
                                                                        Pavel
-- 
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