tssci security

Linux 2.6.20 kernel relocatable on x86

Linus released kernel v2.6.20 (tar.bz2) to the public today, adding virtualization support through KVM and relocatable kernel support for x86, among other changes. The latter feature is an interesting one from a security perspective and for kdump users. From the changelog:

Relocatable kernel support for x86 This feature (enabled with CONFIG_RELOCATABLE) isn't very noticeable for end-users but it's quite interesting from a kernel POV. Until now, it was a requeriment that a i386 kernel was loaded at a fixed memory address in order to work, loading it in a different place wouldn't work. This feature allows to compile a kernel that can be loaded at different 4K-aligned addresses, but always below 1 GB, with no runtime overhead. Kdump users (a feature introduced in 2.6.13 that it triggers kexec in a kernel crash in order to boot a kernel that has been previously loaded at a 'empty' address, then runs that kernel, saves the memory where the crashed kernel was placed, dumps it in a file and continues booting the system) will benefit from this because until now the "rescue kernel" need to be compiled with different configuration options in order to make it bootable at a different address. With a relocatable kernel, the same kernel can be boot at different addresses. (commit 1, 2, 3, 4)

Basically, kdump users can now use the same kernel to do standard boot and kexec boot without an additional kernel image. From the security side of things, you can move the kernel around in memory. Since most rootkits and exploits rely on static memory addresses, this makes it more difficult.

Posted by Marcin on Sunday, February 4, 2007 in News, Security and Tech.

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