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 5.0 ('lenny'). See the application specific information below.
Starting with Debian 5.0.4, the 2.6.26 kernel is configured with a default mmap_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 default mmap_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 of mmap_min_addr on your system by upgrading to 5.0.4, see the update announcement.
The 2.6.18 kernel in Debian 4.0 ('etch') does not support this kernel tunable. To obtain this safeguard, it is recommended that users install the 2.6.24 "EtchAndAHalf" kernel, then increase the value of mmap_min_addr on your system with the following commands:
# echo "vm.mmap_min_addr = 4096" > /etc/sysctl.d/mmap_min_addr.conf # /etc/init.d/procps restart
Alternatively, upgrade your systems to Debian 5.0 ('lenny').
Application specific information
Only Win16 binaries require the ability to mmap low addresses, Win32 binaries do not. It is recommended that you test your application with the increase mmap_min_addr setting. If the application starts up without issue, then you should not need to remove the mmap_min_addr restriction.
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 increase mmap_min_addr starting with Debian squeeze.