mmap_min_addr

Overview

mmap_min_addr is a kernel tunable that specifies the minimum virtual address that a process is allowed to mmap.Allowing processes to map low values increases the security implications of a class of defects known as "kernel NULL pointer dereference" defects. If a malicious local user finds a way to trigger one of these NULL pointer defects, they can exploit it to cause system hangs, crashes, or otherwise make parts of the system unusable. If this user is also able to map low portions of virtual memory, they can often further exploit this issue to gain increased privileges.

The downside to preventing applications from mmap'ing low virtual memory addresses is that certain applications depend on this functionality. dosemu, qemu and wine are three such applications that exist in Debian. See theapplication specific information below.

Current status

Starting with Debian 5.0.4, the 2.6.26 kernel is configured with a defaultmmap_min_addr of '4096'. If you find this prevents you from running certain applications, you can remove this restriction using the following commands:

# echo "vm.mmap_min_addr = 0" > /etc/sysctl.d/mmap_min_addr.conf# /etc/init.d/procps restart

If you only need to run the affected application infrequently, you can temporarily decrease the value using the following command:

# sysctl -w vm.mmap_min_addr="0"

You can then reactivate the restriction by running the following command:

# sysctl -w vm.mmap_min_addr="4096"

In Debian 5.0.0 through 5.0.3 inclusive, the 2.6.26 kernel is shipped with a defaultmmap_min_addr of '0'. This means that the Debian system, by default, is susceptible to these NULL-pointer privilege escalation techniques. It is recommended that you increase the value ofmmap_min_addr on your system by upgrading to 5.0.4, see the update announcement.

Application specific information

bitbake

From the OpenEmbedded User Manual: bitbake might complain that there is a problem with the setting in/proc/sys/vm/mmap_min_addr, which needs to be set to zero.

dosemu

dosemu, as shipped in Debian 5.0, must run withvm.mmap_min_addr set to 0 or be executed as root. This limitation has been removed upstream (505247), so dosemu should work with an increasedmmap_min_addr starting with Debian squeeze.

qemu

qemu, as shipped in Debian 5.0, requires low virtual memory mmaps.mmap_min_addr must be set to 0 to run qemu as a non-root user. This limitation has been removed upstream, so qemu should work with an increasedmmap_min_addr starting with Debian squeeze.

wine

Only Win16 binaries require the ability to mmap low addresses, Win32 binaries do not. It is recommended that you test your application with the increasemmap_min_addr setting. If the application starts up without issue, then you should not need to remove themmap_min_addr restriction.