linux-kernel - Re: [patch 7/8] fdmap v2

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Date:	Sat, 9 Jun 2007 15:27:43 -0400
From:	Kyle Moffett <mrmacman_g4@....com>
To:	Al Viro <viro@....linux.org.uk>
Cc:	Ulrich Drepper <drepper@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Davide Libenzi <davidel@...ilserver.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Theodore Tso <tytso@....edu>,
	Eric Dumazet <dada1@...mosbay.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [patch 7/8] fdmap v2 - implement sys_socket2

On Jun 09, 2007, at 13:24:29, Al Viro wrote:
> On Sat, Jun 09, 2007 at 10:08:59AM -0700, Ulrich Drepper wrote:
>> - - there are two interface to use: open + fcntl.  This is racy.   
>> And don't tell me this doesn't matter.
> Racy with respect to what?  Return-to-libc exploits from another  
> thread?

How about racy with respect to normal open or fork+exec from another  
thread?  Specifically there are cases where libc or other libraries  
want to create a backend thread dealing with file descriptors in  
response to the program's straightforward calls into that library  
(Examples include using syslets or event-based polling threads).

SCENARIO 1:

Program Thread:   Library Thread:
                   fd = socket(AF_*, SOCK_*, 0);
fork();
                   int x = FD_CLOEXEC;
                   fcntl(fd, F_SETFD, &x);

New Process:
setgroups(...);
seteuid(...);
exec(....);

Whoops!!! Suddenly the user process executed by the (theoretically)  
single-threaded program got a handle to a netlink socket affecting  
some system resource!!!

SCENARIO 2:

Program Thread:     Async libc getpwent()-cache syslet
close(0);
                     fd = open("/etc/shadow");
open("/dev/null");
code_which_insecurely_reads_from_stdin();

Here we were trying to safely call into code which reads from stdin  
and shouldn't be given privileged data, but the syslet makes the  
common paradigm 'close(0); open("/dev/null");' horribly insecure.

If you extend all the FD syscalls to all take a "flags" parameter and  
add the appropriate flags, then you can pass O_CLOEXEC|O_RANDFD to  
whatever syscall you are using and both problems vanish.

Cheers,
Kyle Moffett

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