KVM: nSVM: Pass next RIP, not current RIP, for nested VM-Exit on emulation

Set "next_rip" in the emulation interception info passed to vendor code
using the emulator context's "_eip", not "eip".  "eip" holds RIP from the
start of emulation, i.e. the RIP of the instruction that's being emulated,
whereas _eip tracks the context's current position in decoding the code
stream, which at the time of the intercept checks is effectively the RIP
of the next instruction.

Passing the current RIP as next_rip causes SVM to stuff the wrong value
value into vmcb12->control.next_rip if a nested VM-Exit is generated, i.e.
if L1 wants to intercept the instruction, and could result in L1 putting
L2 into an infinite loop due to restarting L2 with the same RIP over and
over.

Fixes: 8a76d7f ("KVM: x86: Add x86 callback for intercept check")
Link: https://lore.kernel.org/r/20250201015518.689704-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/emulate.c

@@ -478,7 +478,7 @@ static int emulator_check_intercept(struct x86_emulate_ctxt *ctxt,
 		.src_bytes  = ctxt->src.bytes,
 		.dst_bytes  = ctxt->dst.bytes,
 		.ad_bytes   = ctxt->ad_bytes,
-		.next_rip   = ctxt->eip,
+		.next_rip   = ctxt->_eip,
 	};
 
 	return ctxt->ops->intercept(ctxt, &info, stage);