RE: [PATCH 1/1] x86/hyperv: Disable IBT when hypercall page lacks ENDBR instruction

["Michael Kelley (LINUX)" <mikelley@microsoft.com>]

From: Peter Zijlstra <peterz@infradead.org> Sent: Friday, July 21, 2023 12:59 AM
> 
> On Fri, Jul 21, 2023 at 12:41:35AM +0000, Michael Kelley (LINUX) wrote:
> 
> > > Other than that, this seems fairly straight forward. One thing I
> > > wondered about; wouldn't it be possible to re-write the indirect
> > > hypercall thingies to a direct call? I mean, once we have the hypercall
> > > page mapped, the address is known right?
> >
> > Yes, the address is known.  It does not change across things like
> > hibernation.  But the indirect call instruction is part of an inline assembly
> > sequence, so the call instructions that need re-writing are scattered
> > throughout the code. There's also the SEV-SNP case from the
> > latest version of Tianyu Lan's patch set [1] where vmmcall may be used
> > instead, based on your recent enhancement for nested ALTERNATIVE.
> > Re-writing seems like that's more complexity than warranted for a
> > mostly interim situation until the Hyper-V patch is available and
> > users install it.
> 
> Well, we have a lot of infrastructure for this already. Specifically
> this is very like the paravirt patching.
> 
> Also, direct calls are both faster and have less speculation issues, so
> it might still be worth looking at.
> 
> The way to do something like this would be:
> 
> 
> 	asm volatile ("   ANNOTATE_RETPOLINE_SAFE	\n\t"
> 		      "1: call *hv_hypercall_page	\n\t"
> 		      ".pushsection .hv_call_sites	\n\t"
> 		      ".long 1b - .			\n\t"
> 		      ".popsection			\n\t");
> 
> 
> And then (see alternative.c for many other examples):
> 
> 
> patch_hypercalls()
> {
> 	s32 *s;
> 
> 	for (s = __hv_call_sites_begin; s < __hv_call_sites_end; s++) {
> 		void *addr = (void *)s + *s;
> 		struct insn insn;
> 
> 		ret = insn_decode_kernel(&insn, addr);
> 		if (WARN_ON_ONCE(ret < 0))
> 			continue;
> 
> 		/*
> 		 * indirect call: ff 15 disp32
> 		 * direct call:   2e e8 disp32
> 		 */
> 		if (insn.length == 6 &&
> 		    insn.opcode.bytes[0] == 0xFF &&
> 		    X86_MODRM_REG(insn.modrm.bytes[0]) == 2) {
> 
> 			/* verify it was calling hy_hypercall_page */
> 			if (WARN_ON_ONCE(addr + 6 + insn.displacement.value != &hv_hypercall_page))
> 				continue;
> 
> 			/*
> 			 * write a CS padded direct call -- assumes the
> 			 * hypercall page is in the 2G immediate range
> 			 * of the kernel text

Probably not true -- the hypercall page has a vmalloc address.

> 			 */
> 			addr[0] = 0x2e; /* CS prefix */
> 			addr[1] = CALL_INSN_OPCODE;
> 			(s32 *)&Addr[2] = *hv_hypercall_page - (addr + 6);
> 		}
> 	}
> }
> 
> 
> See, easy :-)

OK, worth looking into.  This is a corner of the Linux kernel code that
I've never looked at before.  I appreciate the pointers.

Hypercall sites also exist in loadable modules, so would need to hook
into module_finalize() as well.  Processing a new section type looks
straightforward.

But altogether, this feels like more change than should go as a bug
fix to be backported to stable kernels.  It's something to look at for a
future kernel release.

Michael