Re: [PATCH] New PT_GNU_COMPAT segment header extension

[Ramon de C Valle <rcvalle@redhat.com>]

> Hi,
> 
> This is a brief example of the behavior of the system I use for some
> time
> already. For an ELF binary with the PT_GNU_STACK segment header and
> the PF_X
> flag not set (i.e. the default), the following are its currently
> virtual
> memory mappings and their access permissions:
> 
> [rcvalle@Fedora-15-i386 ~]$ cat /proc/2253/maps
> 004e2000-004e3000 r-xp 00000000 00:00 0          [vdso]
> 08048000-08049000 r-xp 00000000 fd:02 926785     /home/rcvalle/a.out
> 08049000-0804a000 rw-p 00000000 fd:02 926785     /home/rcvalle/a.out
> 495c9000-495e8000 r-xp 00000000 fd:01 1971790    /lib/ld-2.13.90.so
> 495e8000-495e9000 r--p 0001f000 fd:01 1971790    /lib/ld-2.13.90.so
> 495e9000-495ea000 rw-p 00020000 fd:01 1971790    /lib/ld-2.13.90.so
> 495ec000-49774000 r-xp 00000000 fd:01 1971791    /lib/libc-2.13.90.so
> 49774000-49776000 r--p 00188000 fd:01 1971791    /lib/libc-2.13.90.so
> 49776000-49777000 rw-p 0018a000 fd:01 1971791    /lib/libc-2.13.90.so
> 49777000-4977a000 rw-p 00000000 00:00 0
> b780c000-b780d000 rw-p 00000000 00:00 0
> b7824000-b7825000 rw-p 00000000 00:00 0
> bfdc3000-bfde4000 rw-p 00000000 00:00 0          [stack]
> [rcvalle@Fedora-15-i386 ~]$
> 
> The following are its currently virtual memory mappings and their
> access
> permissions with the PT_GNU_STACK segment header and the PF_X flag
> unset:

s/unset/set

> 
> [rcvalle@Fedora-15-i386 ~]$ cat /proc/2257/maps
> 00dcc000-00dcd000 r-xp 00000000 00:00 0          [vdso]
> 08048000-08049000 r-xp 00000000 fd:02 926825     /home/rcvalle/a.out
> 08049000-0804a000 rw-p 00000000 fd:02 926825     /home/rcvalle/a.out
> 495c9000-495e8000 r-xp 00000000 fd:01 1971790    /lib/ld-2.13.90.so
> 495e8000-495e9000 r--p 0001f000 fd:01 1971790    /lib/ld-2.13.90.so
> 495e9000-495ea000 rw-p 00020000 fd:01 1971790    /lib/ld-2.13.90.so
> 495ec000-49774000 r-xp 00000000 fd:01 1971791    /lib/libc-2.13.90.so
> 49774000-49776000 r--p 00188000 fd:01 1971791    /lib/libc-2.13.90.so
> 49776000-49777000 rw-p 0018a000 fd:01 1971791    /lib/libc-2.13.90.so
> 49777000-4977a000 rw-p 00000000 00:00 0
> b7711000-b7712000 rw-p 00000000 00:00 0
> b7729000-b772a000 rw-p 00000000 00:00 0
> bfca7000-bfcc8000 rwxp 00000000 00:00 0          [stack]
> [rcvalle@Fedora-15-i386 ~]$
> 
> The following are its currently virtual memory mappings and their
> access
> permissions with the PT_GNU_COMPAT segment header and the PF_X flag
> set:
> 
> [rcvalle@Fedora-15-i386 ~]$ cat /proc/2349/maps
> 00850000-00851000 r-xp 00000000 00:00 0          [vdso]
> 00d29000-00d2a000 rwxp 00000000 00:00 0
> 00fd2000-00fd3000 rwxp 00000000 00:00 0
> 08048000-08049000 r-xp 00000000 fd:02 926785     /home/rcvalle/a.out
> 08049000-0804a000 rwxp 00000000 fd:02 926785     /home/rcvalle/a.out
> 495c9000-495e8000 r-xp 00000000 fd:01 1971790    /lib/ld-2.13.90.so
> 495e8000-495e9000 r-xp 0001f000 fd:01 1971790    /lib/ld-2.13.90.so
> 495e9000-495ea000 rwxp 00020000 fd:01 1971790    /lib/ld-2.13.90.so
> 495ec000-49774000 r-xp 00000000 fd:01 1971791    /lib/libc-2.13.90.so
> 49774000-49776000 r-xp 00188000 fd:01 1971791    /lib/libc-2.13.90.so
> 49776000-49777000 rwxp 0018a000 fd:01 1971791    /lib/libc-2.13.90.so
> 49777000-4977a000 rwxp 00000000 00:00 0
> bfd2b000-bfd4c000 rwxp 00000000 00:00 0          [stack]
> [rcvalle@Fedora-15-i386 ~]$
> 
> Notice the difference between its currently virtual memory mappings
> and
> their access permissions with the PT_GNU_STACK segment header and the
> PF_X
> flag unset and their access permissions with the PT_GNU_COMPAT
> segment
> header and the PF_X flag set. The latter is the current behavior of
> the
> Linux kernel for any ELF binary with the PT_GNU_STACK segment header
> and the
> PF_X flag unset (i.e. all its virtual memory mappings are

s/unset/set

> executable).

One of the various scenarios an attacker can take advantage of this behavior
to exploit a vulnerability is if, for example, a buffer overflow occurs
within the data segment of a such loaded ELF binary (e.g the recent telnetd
vulnerability) .

In addition, if you use a non-GNU toolchain that does not support the
PT_GNU_STACK segment header extension, or does not create this segment with
the PF_X flag unset by default. Upon loading the resulting compiled
binaries, you will end up with the third aforementioned layout (i.e. all
virtual memory mappings executable)--this completely disregards the ABI.

It is also important to note some embedded Linux distributions may use
non-GNU or custom toolchains that either does not support the PT_GNU_STACK
segment header extension, does not create this segment with the PF_X flag
unset by default, or use binaries with some segment headers stripped out.

In other words, in the absence of the PT_GNU_STACK segment header or with
the PT_GNU_STACK segment header with the PF_X flag set, you will end up with
all virtual memory mappings executable.


> 
> Any comments about these patches would be greatly appreciated.
> 
> Thanks,
> 
> 
> --
> Ramon de C Valle / Red Hat Security Response Team
> 

-- 
Ramon de C Valle / Red Hat Security Response Team