On 11/30/23 18:45, Edgecombe, Rick P wrote: > On Wed, 2023-11-29 at 15:07 -0600, Madhavan T. Venkataraman wrote: >> Threat Model >> ------------ >> >> In the threat model in Heki, the attacker is a user space attacker >> who exploits >> a kernel vulnerability to gain more privileges or bypass the kernel's >> access >> control and self-protection mechanisms. >> >> In the context of the guest page table, one of the things that the >> threat model translates >> to is a hacker gaining access to a guest page with RWX permissions. >> E.g., by adding execute >> permissions to a writable page or by adding write permissions to an >> executable page. >> >> Today, the permissions for a guest page in the extended page table >> are RWX by >> default. So, if a hacker manages to establish RWX for a page in the >> guest page >> table, then that is all he needs to do some damage. > > I had a few random comments from watching the plumbers talk online: > > Is there really a big difference between a page that is RWX, and a RW > page that is about to become RX? I realize that there is an addition of > timing, but when executable code is getting loaded it can be written to > then and later executed. I think that gap could be addressed in two > different ways, both pretty difficult: > 1. Verifying the loaded code before it gets marked > executable. This is difficult because the kernel does lots of > tweaks on the code it is loading (alternatives, etc). It can't > just check a signature. > 2. Loading the code in a protected environment. In this model the > (for example) module signature would be checked, then the code > would be loaded in some sort of protected environment. This way > integrity of the loaded code would be enforced. But extracting > module loading into a separate domain would be difficult. > Various scattered features all have their hands in the loading. > > Secondly, I wonder if another way to look at the memory parts of HEKI > could be that this is a way to protect certain page table bits from > stay writes. The RWX bits in the EPT are not directly writable, so more > steps are needed to change things than just a stray write (instead the > helpers involved in the operations need to be called). If that is a > fair way of looking at it, then I wonder how HEKI compares to a > solution like this security-wise: > https://lore.kernel.org/lkml/20210830235927.6443-1-rick.p.edgecombe@xxxxxxxxx/ > > Functional-wise it had the benefit of working on bare metal and > supporting the normal kernel features. Thanks for the comments. I will think about what you have said and will respond soon. Madhavan
