x86/cpu: Disable FRED when PTI is forced on FRED and PTI were never intended to work together. No FRED hardware is vulnerable to Meltdown and all of it should have LASS anyway. Nevertheless, if you boot a system with pti=on and fred=on, the kernel tries to do what is asked of it and dies a horrible death on the first attempt to run userspace (since it never switches to the user page tables). Disable FRED when PTI is forced on, and print a warning about it. A quick brain dump about what a FRED+PTI implementation would look like is below. I'm not sure it would make any sense to do it, but never say never. All I know is that it's way too complicated to be worth it today. The SWITCH_TO_USER/KERNEL_CR3 bits are simple to fix (or at least we have the assembly tools to do it already), as is sticking the FRED entry text in .entry.text (it's not in there today). The nasty part is the stacks. Today, the CPU pops into the kernel on MSR_IA32_FRED_RSP0 which is normal old kernel memory and not mapped to userspace. The hardware pushes gunk on to MSR_IA32_FRED_RSP0, which is currently the task stacks. MSR_IA32_FRED_RSP0 would need to point elsewhere, probably cpu_entry_stack(). Then, start playing games with stacks on entry/exit, including copying gunk to and from the task stack. While I'd *like* to have PTI everywhere, I'm not sure it's worth mucking up the FRED code with PTI kludges. If a user wants fast entry/exit, they use FRED. If you want PTI (and sekuritay), you certainly don't care about fast entry and FRED isn't going to help you *all* that much, so you can just stay with the IDT. Plus, FRED hardware should have LASS which gives you a similar security profile to PTI without the CR3 munging. Signed-off-by: Dave Hansen Reported-by: Gayatri Kammela Cc: stable@vger.kernel.org Cc: Andy Lutomirski Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: Ingo Molnar Cc: Borislav Petkov Cc: x86@kernel.org Cc: "H. Peter Anvin" Reviewed-by: Borislav Petkov (AMD)

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