Update: Security Hole in Kernel Allows Privilege Extensions

11/04/2009

A null-pointer dereference in the Linux kernel allows local users to assume root privileges. Brad Spengler, who claims first discovery, announced that he will present a corresponding exploit during the course of the day.

Concurrent with Spengler's discovery in mid-October, Earl Chew likewise brought attention to the bug in an lkml.org blog. The bug affects all kernel versions in the 2.6 series and was fixed in the upcoming 2.6.32 RC. So far Red Hat is the only distro providing patches for what is now the CVE-2009-3547 advisory. Users of other distros can apply a workaround that sets the pointer to a value higher than zero, which prevents the exploit. Admins can enter

# cat /proc/sys/vm/mmap_min_addr

to read the current pointer value. If larger than zero, no action need be taken, otherwise use the command

# sysctl -w vm.mmap_min_addr="4096"

to set the value to 4096, for example. The value, however, is only good until the next reboot. To give it a more lasting value, users of Debian or its derivatives should use the following commands:

# echo "vm.mmap_min_addr = 4096" > /etc/sysctl.d/mmap_min_addr.conf
# /etc/init.d/procps restart

Users of openSUSE should use the following command instead:

# echo -e "\nvm.mmap_min_addr=4096\n" >> /etc/sysctl.conf

followed by a system reboot.

First indications are that Ubuntu/Kubuntu 9.10 are not affected by the bug, because the initial counter is set to 65535. However, subsequently installed programs such as Wine or Qemu will reset the counter to zero. First tests with Wine installations show this to be evident. The openSUSE distro is potentially the most vulnerable in that the initial counter is also set to zero.

( Thomas Leichtenstern)

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