From: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Sent: Wednesday, May 31, 2023 8:49 AM > > On Wed, May 31, 2023 at 02:50:50PM +0000, Michael Kelley (LINUX) wrote: > > > I'm jumping in to answer some of the basic questions here. Yesterday, > > there was a discussion about nested #HV exceptions, so maybe some of > > this is already understood, but let me recap at a higher level, provide some > > references, and suggest the path forward. > > > 2) For the Restricted Interrupt Injection code, Tianyu will look at > > how to absolutely minimize the impact in the hot code paths, > > particularly when SEV-SNP is not active. Hopefully the impact can > > be a couple of instructions at most, or even less with the use of > > other existing kernel techniques. He'll look at the other things you've > > commented on and get the code into a better state. I'll work with > > him on writing commit messages and comments that explain what's > > going on. > > So from what I understand of all this SEV-SNP/#HV muck is that it is > near impossible to get right without ucode/hw changes. Hence my request > to Tom to look into that. > > The feature as specified in the AMD documentation seems fundamentally > buggered. > > Specifically #HV needs to be IST because hypervisor can inject at any > moment, irrespective of IF or anything else -- even #HV itself. This > means also in the syscall gap. > > Since it is IST, a nested #HV is instant stack corruption -- #HV can > attempt to play stack games as per the copied #VC crap (which I'm not at > all convinced about being correct itself), but this doesn't actually fix > anything, all you need is a single instruction window to wreck things. > > Because as stated, the whole premise is that the hypervisor is out to > get you, you must not leave it room to wiggle. As is, this is security > through prayer, and we don't do that. > > In short; I really want a solid proof that what you propose to implement > is correct and not wishful thinking. Fair enough. We will be sync'ing with the AMD folks to make sure that one way or another this really will work. Michael
