Nasty local linux kernel exploit based on some bad bounds checking in the kernel.
*UPDATE*: just emerge -uvp world, new gentoo-sources has been released.
To fix a gentoo box already running the latest kernel, you need to unmerge and re-emerge gentoo-sources:
mv /usr/src/linux/.config /usr/src
emerge --unmerge gentoo-sources
rm -r /usr/src/linux-2.4.20-gentoo-r8
rm /usr/src/linux
emerge gentoo-sources
mv /usr/src/.config /usr/src/linux/
cd /usr/src/linux
make dep && make clean && make bzImage
mount /boot
mv /usr/src/linux/arc/i368/boot/bzImage /boot
umount /boot
(Stolen from a forum post)