On Wed, 2012-10-24 at 13:36 -0400, Vivek Goyal wrote: > On Tue, Oct 23, 2012 at 09:19:27AM -0700, Eric W. Biederman wrote: > > Vivek Goyal <vgoyal at redhat.com> writes: > > > > > On Tue, Oct 23, 2012 at 09:18:54AM -0400, Vivek Goyal wrote: > > > > > > [..] > > >> > >> There are 3 options for trusting /sbin/kexec. There are IMA and EMA, > > >> > >> and it is conceivable to have ELF note sections with signatures for > > >> > >> executables. > > >> > > > > >> > > Can you please tell more about what is EMA and IMA. I did quick google > > >> > > and could not find much. > > >> > > > >> > That should have been EVM and IMA. Look under security/integrity/. I > > >> > don't know much about them but they appear to be security modules with a > > >> > focus on verifying checksum or perhaps encrypted hashes of executables > > >> > are consistent. > > >> > > >> I will do some quick search there and I see if I can understand something. > > >> > > > > > > Ok, I quickly went through following paper. > > > > > > http://mirror.transact.net.au/sourceforge/l/project/li/linux-ima/linux-ima/Integrity_overview.pdf > > > > > > So it looks like that IMA can store the hashes of files and at execute > > > time ensure those hashes are unchanged to protect against the possibility > > > of modification of files. IMA-appraisal originally was hashed based, but Dmitry Kasatkin added digital signature support. Both have been upstreamed. > > > But what about creation of a new program which can call kexec_load() > > > and execute an unsigned kernel. Doesn't look like that will be > > > prevented using IMA. Assuming the IMA policy syntax is updated to require 'security.ima' to contain a digital signature, then it is only a question of protecting the _ima and _evm keyrings. (Dmitry has such a patch waiting to be reviewed.) So the new program would have to be vetted by someone trusted. > > > Whole idea behind UEFI secure boot seems to be that all signing happens > > > outside the running system and now only signed code can run with higher > > > priviliges. > > > > No. UEFI secure boot has absolutely nothing todo with this. > > > > UEFI secure boot is about not being able to hijack the code EFI runs > > directly. Full stop. > > > > Some people would like to implment a security policy that says > > you can't boot an untrusted version of windows from linux if you have > > booted with UEFI secure boot, so they don't get their bootloader > > signatures revoked by microsoft. > > > > A security model relying on Microsoft's key is totally uniteresting to > > me. Either signing at the UEFI level is of no use or Microsofts key > > will fall again to the combined assult of every cracker and every > > governmental dirty cyber ops division attacking it. Not to mention that > > Microsoft has little incentive to keep linux booting. > > > > I think it is reasonable to be able to support a policy where we can't > > boot unsigned versions of Microsoft windows. However beyond being able > > to exclude booting windows being one criteria for our policy mechanism > > please don't even start to justify things with that ridiculous security > > policy even indirectly. > > > > > IMA seems to be only protecting against only making sure > > > existing binaries are not modifed but it does not seem to prevent against > > > installation of new binaries and these binaries take advantage of kexec > > > system call to load an unsigned kernel. The IMA/IMA-appraisal policy dictates what needs to be appraised. The default ima-appraisal policy appraises all files owned by root. > > I believe you can combine IMA with EVM signed security attributes where > > the EVM signing key is offline, and the verification key is in the > > kernel. > > > > The combination of IMA and EVM gets very close to being able to sign > > executables offline and be able to update them. > > [ Again CCing lkml and IMA/EVM folks ] > > After little reading, my understanding is EVM also does not support > offline signing. > > http://sourceforge.net/apps/mediawiki/linux-ima/index.php?title=Main_Page > > Given the fact EVM protects IMA data (security.ima), which is generated > inline, I am not sure how EVM can sign images offline. > > I might have misunderstood things, please correct me if that's not the > case. > > Thanks > Vivek > IMA-appraisal verifies the integrity of file data, while EVM verifies the integrity of the file metadata, such as LSM and IMA-appraisal labels. Both 'security.ima' and 'security.evm' can contain digital signatures. thanks, Mimi
