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

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Message-ID: <939d53060803191514i4f225043g961d4de932a209ce@mail.gmail.com>
Date:	Wed, 19 Mar 2008 23:14:21 +0100
From:	"Benjamin Thery" <ben.thery@...il.com>
To:	"Andrew Morton" <akpm@...ux-foundation.org>
Cc:	tilman@...p.cc, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, davem@...emloft.net, pekkas@...core.fi,
	yoshfuji@...ux-ipv6.org, dlezcano@...ibm.com, xemul@...nvz.org,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [2.6.25-rc5-mm1] regression: cannot run Postfix sendmail command as non-root

On Wed, Mar 19, 2008 at 10:16 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
>
> On Wed, 19 Mar 2008 18:52:41 +0100
>  "Benjamin Thery" <ben.thery@...il.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
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