KVM: x86: Replace memset() "optimization" with normal per-field writes

Explicitly zero select fields in the emulator's decode cache instead of
zeroing the fields via a gross memset() that spans six fields. gcc and
clang are both clever enough to batch the first five fields into a single
quadword MOV, i.e. memset() and individually zeroing generate identical
code.

Removing the wart also prepares KVM for FORTIFY_SOURCE performing
compile-time and run-time field bounds checking for memset().

No functional change intended.

Reported-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/lkml/YR0jIEzEcUom/7rd@google.com
Signed-off-by: Kees Cook <keescook@chromium.org>

Author: sean-jc

arch/x86/kvm/emulate.c

@@ -5380,8 +5380,13 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop)
 
 void init_decode_cache(struct x86_emulate_ctxt *ctxt)
 {
-	memset(&ctxt->rip_relative, 0,
-	       (void *)&ctxt->modrm - (void *)&ctxt->rip_relative);
+	/* Clear fields that are set conditionally but read without a guard. */
+	ctxt->rip_relative = false;
+	ctxt->rex_prefix = 0;
+	ctxt->lock_prefix = 0;
+	ctxt->rep_prefix = 0;
+	ctxt->regs_valid = 0;
+	ctxt->regs_dirty = 0;
 
 	ctxt->io_read.pos = 0;
 	ctxt->io_read.end = 0;

arch/x86/kvm/kvm_emulate.h

@@ -336,11 +336,7 @@ struct x86_emulate_ctxt {
 		fastop_t fop;
 	};
 	int (*check_perm)(struct x86_emulate_ctxt *ctxt);
-	/*
-	 * The following six fields are cleared together,
-	 * the rest are initialized unconditionally in x86_decode_insn
-	 * or elsewhere
-	 */
+
 	bool rip_relative;
 	u8 rex_prefix;
 	u8 lock_prefix;