On Wed, Nov 29, 2023 at 03:07:15PM -0600, Madhavan T. Venkataraman wrote: > Kernel Lockdown > --------------- > > But, we must provide at least some security in V2. Otherwise, it is useless. > > So, we have implemented what we call a kernel lockdown. At the end of kernel > boot, Heki establishes permissions in the extended page table as mentioned > before. Also, it adds an immutable attribute for kernel text and kernel RO data. > Beyond that point, guest requests that attempt to modify permissions on any of > the immutable pages will be denied. > > This means that features like FTrace and KProbes will not work on kernel text > in V2. This is a temporary limitation. Once authentication is in place, the > limitation will go away. So either you're saying your patch 17 / text_poke is broken (so why include it ?!?) or your statement above is incorrect. Pick one. > __text_poke() > This function is called by various features to patch text. > This calls heki_text_poke_start() and heki_text_poke_end(). > > heki_text_poke_start() is called to add write permissions to the > extended page table so that text can be patched. heki_text_poke_end() > is called to revert write permissions in the extended page table. This, if text_poke works, then static_call / jump_label / ftrace and everything else should work, they all rely on this. > Peter mentioned the following: > > "if you want to mirror the native PTEs why don't you hook into the > paravirt page-table muck and get all that for free?" > > We did consider using a shadow page table kind of approach so that guest page table > modifications can be intercepted and reflected in the page table entry. We did not > do this for two reasons: > > - there are bits in the page table entry that are not permission bits. We would like > the guest kernel to be able to modify them directly. This statement makes no sense. > - we cannot tell a genuine request from an attack. Why not? How is an explicit call different from an explicit call in a paravirt hook? >From a maintenance pov we already hate paravirt with a passion, but it is ever so much better than sprinkling yet another pile of crap (heki_*) around.
