First of all, thanks for picking this back up. It looks to be going in a very positive direction! On 1/10/19 1:09 PM, Khalid Aziz wrote: > I implemented a solution to reduce performance penalty and > that has had large impact. When XPFO code flushes stale TLB entries, > it does so for all CPUs on the system which may include CPUs that > may not have any matching TLB entries or may never be scheduled to > run the userspace task causing TLB flush. ... > A rogue process can launch a ret2dir attack only from a CPU that has > dual mapping for its pages in physmap in its TLB. We can hence defer > TLB flush on a CPU until a process that would have caused a TLB > flush is scheduled on that CPU. This logic is a bit suspect to me. Imagine a situation where we have two attacker processes: one which is causing page to go from kernel->user (and be unmapped from the kernel) and a second process that *was* accessing that page. The second process could easily have the page's old TLB entry. It could abuse that entry as long as that CPU doesn't context switch (switch_mm_irqs_off()) or otherwise flush the TLB entry. As for where to flush the TLB... As you know, using synchronous IPIs is obviously the most bulletproof from a mitigation perspective. If you can batch the IPIs, you can get the overhead down, but you need to do the flushes for a bunch of pages at once, which I think is what you were exploring but haven't gotten working yet. Anything else you do will have *some* reduced mitigation value, which isn't a deal-breaker (to me at least). Some ideas: Take a look at the SWITCH_TO_KERNEL_CR3 in head_64.S. Every time that gets called, we've (potentially) just done a user->kernel transition and might benefit from flushing the TLB. We're always doing a CR3 write (on Meltdown-vulnerable hardware) and it can do a full TLB flush based on if X86_CR3_PCID_NOFLUSH_BIT is set. So, when you need a TLB flush, you would set a bit that ADJUST_KERNEL_CR3 would see on the next user->kernel transition on *each* CPU. Potentially, multiple TLB flushes could be coalesced this way. The downside of this is that you're exposed to the old TLB entries if a flush is needed while you are already *in* the kernel. You could also potentially do this from C code, like in the syscall entry code, or in sensitive places, like when you're returning from a guest after a VMEXIT in the kvm code.
