linux-kernel - Re: Setreuid distinction about (uid

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Tue, 17 Jul 2012 14:52:01 +0100
From:	Athanasius <link@...gy.org>
To:	Adrián <adrianbn@...il.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Setreuid distinction about (uid_t)-1

On Tue, Jul 17, 2012 at 10:27:55AM +0100, Adrián wrote:
>         uid = atoi(argv[1]);
>         printf("%u\n",uid);
>         if (setreuid(uid,uid)==-1){
>                 printf("Setreuid to %u failed\n ",uid);
>                 perror("E");
>                 exit(1);
>         }
>         execve("/bin/sh",args,NULL);
> 
> I've been calling this binary with a bunch of different uid numbers,
> and I came across this weird behaviour with the (uid_t) -1 value:

  From the man page:

       Supplying a value of -1 for either the real or effective user ID forces
       the system to leave that ID unchanged.
	
> adrian@...e-pc:~$ /tmp/suid-tests
> Usage: /tmp/suid-tests target_uid
> adrian@...e-pc:~$ /tmp/suid-tests 0
> 0
> Setreuid to 0 failed
> E: Operation not permitted
> adrian@...e-pc:~$ /tmp/suid-tests -1
> 4294967295

  So this succeeded, by actually doing nothing.

> $ id
> uid=1000(adrian) gid=1000(adrian)
> groups=1000(adrian),4(adm),20(dialout),24(cdrom),46(plugdev),109(lpadmin),110(sambashare),111(admin)
> adrian@...e-pc:~$ /tmp/suid-tests -2
> 4294967294
> Setreuid to 4294967294 failed
> E: Operation not permitted
> adrian@...e-pc:~$ /tmp/suid-tests -3
> 4294967293
> Setreuid to 4294967293 failed
> E: Operation not permitted
> 
> If the binary is setuid, the -1 call effectively rises the euid to
> root (0), although other arbitrary values are properly being set:

  Because, again, -1 asks to leave things as is.  And as you've made
the binary setuid and owned by root when you run it euid is set to 0, and
the -1 leaves it alone.

> adrian@...e-pc:~$ ls -hl /tmp/suid-tests
> -rwsr-x--- 1 root adrian 8,5K 2012-07-17 10:53 /tmp/suid-tests
> adrian@...e-pc:~$ /tmp/suid-tests -1
> 4294967295
> # id
> uid=1000(adrian) gid=1000(adrian) euid=0(root)
> groups=0(root),4(adm),20(dialout),24(cdrom),46(plugdev),109(lpadmin),110(sambashare),111(admin),1000(adrian)

  Yup, totally as expected.

> adrian@...e-pc:~$ /tmp/suid-tests -2
> 4294967294
> $ id
> uid=4294967294 gid=1000(adrian)
> groups=4(adm),20(dialout),24(cdrom),46(plugdev),109(lpadmin),110(sambashare),111(admin),1000(adrian)

  -2 isn't a magic value, but as you're euid == 0 the kernel will do
what you asked and set uid to '-2', with some signed/unsigned conversion
going on you get 4294967294.

-- 
- Athanasius = Athanasius(at)miggy.org / http://www.miggy.org/
                  Finger athan(at)fysh.org for PGP key
	   "And it's me who is my enemy. Me who beats me up.
Me who makes the monsters. Me who strips my confidence." Paula Cole - ME

Download attachment "signature.asc" of type "application/pgp-signature" (199 bytes)