Re: [2.6.25-rc5-mm1] regression: cannot run Postfix sendmail command as non-root

Benjamin Thery
2008-03-19T23:46:49Z

On Wed, Mar 19, 2008 at 10:16 PM, Andrew Morton
<akpm@linux-foundation.org> wrote:
>
> On Wed, 19 Mar 2008 18:52:41 +0100
> "Benjamin Thery" <ben.thery@gmail.com> wrote:
>
> > Tilman,
> >
> > I've finally managed to reproduce your problem with Postfix on one of
> > my victims.
> >
> > Earlier, in the afternoon, I wrote a piece of code that triggered a
> > similar behaviour,
> > but I wasn't sure it was exactly the problem you found. So, I've
> > rebuilt Postfix, added
> > some traces and, voila, same issue as yours.
> > (The version of Postfix originally installed on my machine seems to
> > have IPv6 disabled)
> >
> > I bisected the problem to the commit "[NET]: Make /proc/net a symlink
> > on /proc/self/net (v3)"
> >
> > Here is what happens:
> >
> > - Recently /proc/net has been moved to /proc/self/net, and
> > /proc/self/net is a symlink
> > on this directory.
> > - Before that everybody could access /proc/net and read /proc/net/if_inet6:
> > dr-xr-xr-x 6 root root 0 2008-03-05 15:23 /proc/net
> >
> > - Now, /proc/self/net has a more restrictive access mode and ony the
> > owner of the
> > process can enter the directory:
> > dr-xr--r-- 5 toto toto 0 Mar 19 17:30 net
> >
> > This is not a problem in most of the cases, but it becomes annoying
> > when a process
> > decides to change its UID or GID. It may loose access to its own
> > /proc/self/net entries.
> >
> > - What happens in the Postfix case is the 'sendmail' process executes the
> > '/usr/sbin/postdrop' binary to enqueue the message, but unfortunately
> > '/usr/bin/postdrop' has the setgid bit set:
> > -rwxr-sr-x 1 root postdrop 479475 Mar 19 17:14 /usr/sbin/postdrop
> >
> > The process egid changes and this seems to be problematic to access
> > /proc/self/net/if_inet6. :)
> >
> > I've attached a tiny test program that can be used to reproduce the problem
> > without Postfix.
> > - Either execute it as root and give it an unprivileged uid in argument
> > ./test-proc_net_if_inet6 1001
> >
> > - Or change its ownership and access mode to: -rwxr-sr-x root postdrop
> > and execute it as a lambda user.
> > chown root:postdrop test-proc_net_if_inet6; chmod 2755 test-proc_net_if_inet6
> > ./test-proc_net_if_inet6
> >
> > I've found the cause but not the fix. :)
> > (Adding Pavel in cc:)
> >
>
> Thanks for that - most useful.
>
> Although this is advertised as a 2.6.25-rc5-mm1 problem, I assume the
> regression is also in mainline? 2.6.25-rc6?

Yes, it is in mainline. I reproduced it on 2.6.25-rc5.

Benjamin