On Fri, Oct 26, 2012 at 02:37:29PM -0400, Mimi Zohar wrote: > On Fri, 2012-10-26 at 13:06 -0400, Vivek Goyal wrote: > > On Fri, Oct 26, 2012 at 03:39:16AM +0100, Matthew Garrett wrote: > > > On Thu, Oct 25, 2012 at 09:15:58PM -0400, Mimi Zohar wrote: > > > > > > > On a running system, the package installer, after verifying the package > > > > integrity, would install each file with the associated 'security.ima' > > > > extended attribute. The 'security.evm' digital signature would be > > > > installed with an HMAC, calculated using a system unique key. > > > > > > The idea isn't to prevent /sbin/kexec from being modified after > > > installation - it's to prevent it from being possible to install a > > > system that has a modified /sbin/kexec. Leaving any part of this up to > > > the package installer means that it doesn't solve the problem we're > > > trying to solve here. It must be impossible for the kernel to launch any > > > /sbin/kexec that hasn't been signed by a trusted key that's been built > > > into the kernel, and it must be impossible for anything other than > > > /sbin/kexec to make the kexec system call. > > > > I am kind of lost now so just trying to summarize whatever I have > > learned so far from this thread. > > Thanks for summarizing. > > > - So say we can sign /sbin/kexec at build time and distros can do that. > > - Verify the signature at exec time using kernel keyring and if > > verification happens successfully, say process gains extra capability. > > - Use this new capability to determine whether kexec_load() will be > > successful or not. > > > > Even if we can do all this, it still has the issue of being able to > > stop the process in user space and replace the code at run time > > and be able to launch unsigned kernel. Thinking more about it. Can we just keep track whether a process was ptraced or not and disallow kexec_load() syscall if it was ptraced. (I am assuming that ptrace is the only way to change process code/data). So binaries can be signed offline. Signature verification can take place using kernel keyring at exec() time. And we can keep track of ptraced processes and disallow calling kexec_load() for such processes. If this is implementable, this should take care of following requirement raised by matthew. ************************************************************************ It must be impossible for the kernel to launch any /sbin/kexec that hasn't been signed by a trusted key that's been built into the kernel, and it must be impossible for anything other than /sbin/kexec to make the kexec system call. ************************************************************************* Thoughts? Thanks Vivek
