Linux Today: Linux News On Internet Time.

Rafal Wojtczuk/Bugtraq: Flaws in recent Linux kernels

Oct 18, 2001, 22:15 (15 Talkback[s])

[ Users can solve this one with a quick visit to one of kernel.org's many mirrors and a download of 2.4.10 or later. Patches are also referenced below -ed. ]

From:   Rafal Wojtczuk 
To:     bugtraq@securityfocus.com
Subject:        Flaws in recent Linux kernels
Date:   18 Oct 2001 17:35:40 +0000      

        There are two bugs present in Linux kernels 2.2.x, x<=19 and 2.4.y, 
y<=9. The first vulnerability results in local DoS. The second one,
involving ptrace, can be used to gain root privileges locally (in case of 
default install of most popular distributions). Linux 2.0.x is not vulnerable 
to the ptrace bug mentioned.

I. Local DoS via deep symlinks
        An attacker can force the kernel to spend almost 
arbitrary amount of time on dereferencing a single symlink, which prevents
other processes from running. The attached 
script, mklink.sh, takes a single 
parameter N. The script creates 5 symlinks, each of 
them containing 2*N+1 path elements. When N=3, the symlinks look this way:
$ ls -lG
drwxr-xr-x    2 nergal       4096 wrz 21 14:46 l
lrwxrwxrwx    1 nergal         53 wrz 21 14:46 l0 ->
lrwxrwxrwx    1 nergal         19 wrz 21 14:46 l1 -> l2/../l2/../l2/../l
lrwxrwxrwx    1 nergal         19 wrz 21 14:46 l2 -> l3/../l3/../l3/../l
lrwxrwxrwx    1 nergal         19 wrz 21 14:46 l3 -> l4/../l4/../l4/../l
lrwxrwxrwx    1 nergal         19 wrz 21 14:46 l4 -> l5/../l5/../l5/../l
drwxr-xr-x    2 nergal       4096 wrz 21 14:46 l5
drwxr-xr-x    2 rybagowa     4096 lut 27  1999 still_here

The amount of time the command "head l0" consumes (measured with time(1)) 
N       system time
10:     sys     0m0.050s
20:     sys     0m1.400s
30:     sys     0m10.150s
40:     sys     0m41.840s

        When "head l0" is being executed, other processes are not scheduled to
run. Thus the possibility of local DoS (in case of SMP you may need to spawn 
one mklink.sh process per cpu). The time spent on dereferencing "l0" is 
proportional to the number of path elements in normalized "l0". So, when 
N=120, the scheduler should be locked out for about three hours. One can 
reach N=600, in case of 2.4.9; also in case of 2.4.9, one can create even more 
(up to eight) levels of symlinks.
        2.4.10 fixed this problem, but not completely. Under 2.4.10 "head
l0" command would not block the scheduler, but it cannot be killed. The
problem is fully solved in 2.4.12. 

II. Root compromise by ptrace(3)
        In order for this flaw to be exploitable, /usr/bin/newgrp must be 
setuid root and world-executable. Additionally, newgrp, when run with no
arguments, should not prompt for password. This 
conditions are satisfied in case of most popular Linux distributions (but
not Openwall GNU/*/Linux).
        Suppose the following flow of execution (initially, Process 1 and 
Process 2 are unprivileged):
Time    Process 1                                       Process 2
0       ptrace(PTRACE_ATTACH, pid of Process 2,...)
1       execve /usr/bin/newgrp 
2                                               execve /any/thing/suid
3       execve default user shell  
4       execve ./insert_shellcode

        The unexpected happens at moment 2. Process 2 is still traced, execve 
/any/thing/suid succeeds, and the setuid bit is honored ! This is so 
1) the property of "having an ptrace-attached child" survives the execve
2) at moment 2, the tracer (process 1) has CAP_SYS_PTRACE set (well, has all
root privs), therefore it is allowed to trace even execve of setuid binary.
        In moment 3, newgrp executes a shell, which is an usual behavior. 
This shell is still able to control the process 2 with ptrace. Therefore, the 
"./insert_shellcode" binary is able to insert arbitrary code into the address 
space of Process 2. Game over.
        In order to exploit this kernel vulnerability, one needs a setuid
root binary which execs an user-defined binary (or a shell). Newgrp is
appropriate on most distributions. On default install of slackware it does
not work (the password fields in /etc/group are empty, and newgrp demands a
password). However, one can use "su" on this distribution. "su" 
binary is compiled without PAM support on slackware, therefore it execs an
user shell.
        Do you remember the exploit against *BSD procfs, published in
January 2000 (http://www.securityfocus.com/cgi-bin/archive.pl?id=1&mid=43189) ? 
This one is very similar; a setuid binary is spawned so that the system treats
it as a tracing process. Observe that in case of newgrp, only CAP_SYS_SETGID
is required (plus probably some reserved egid E to read gshadow; provided that
gshadow would be readable by gid E). If the file system supported granting 
capabilities to programs (not only +s bit), this bug could have been benign. 
Similarly, "su" needs only CAP_SYS_SETUID+CAP_SYS_SETGID (and egid shadow). 
The "least privilege" rule, strictly applied, can save from a lot of 
unexpected trouble.
        This bug seems to be Linux-specific. I have tested FreeBSD, OpenBSD
and [older versions of] Irix and Solaris. None of the tested systems 
honored setuid bit when an executing process was traced, even when the 
tracer was root.

III. Vendor status
        The kernel developers were notified on 18th September. 
vendor-sec at lists dot de was notified on 9th October.

IV. Availability of patches.
        2.4.12 kernel fixes both presented problems. The attached patches,
2.2.19-deep-symlink.patch and 2.2.19-ptrace.patch, both blessed by Linus, 
can be used to close the vulnerability in 2.2.19. The (updated) 
Openwall GNU/*/Linux kernel patches can be retrieved from
Note that the default Owl installation is not vulnerable to the ptrace bug

V. The exploits
        The attached mklink.sh script creates malicious symlinks. 
        ptrace-exp.c and insert_shellcode.c exploit the ptrace bug on i386
architecture. You will probably need to adjust #define in the latter. Note 
that ptrace-exp uses LD_DEBUG variable to force a setuid program to generate 
output. This technique (stderr redirected to a pipe, LD_DEBUG set, especially 
LD_DEBUG=symbols) allows for forced suspending of a setuid binary in a 
precisely determined moments, which may be helpful to build exploits which 
rely on race-conditions. And finally, notice that under Owl LD_DEBUG is 
ignored in case of suid binaries. 

Save yourself,