On 11/30/23 05:33, Peter Zijlstra wrote: > 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. > It has been included so that people can be aware of the changes. I will remove the text_poke() changes from the patchset and send it later when I have some authentication in place. It will make sense then. > >> __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. I only said that the idea was considered. We can resume the discussion on this topic when I send the text_poke() changes in a later version of the Heki patchset. Madhavan
