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

[Sean Christopherson]

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@xxxxxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/kvm/emulate.c     | 9 +++++++--
 arch/x86/kvm/kvm_emulate.h | 6 +-----
 2 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 2837110e66ed..bf81fd017e7f 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -5377,8 +5377,13 @@ 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;
diff --git a/arch/x86/kvm/kvm_emulate.h b/arch/x86/kvm/kvm_emulate.h
index 68b420289d7e..bc1fecacccd4 100644
--- a/arch/x86/kvm/kvm_emulate.h
+++ b/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;
--
2.33.0.rc1.237.g0d66db33f3-goog

> +		bool rip_relative;
> +		u8 rex_prefix;
> +		u8 lock_prefix;
> +		u8 rep_prefix;
> +		/* bitmaps of registers in _regs[] that can be read */
> +		u32 regs_valid;
> +		/* bitmaps of registers in _regs[] that have been written */
> +		u32 regs_dirty;
> +	);
> +
>  	/* modrm */
>  	u8 modrm;
>  	u8 modrm_mod;
> -- 
> 2.30.2
>