[PATCH 01/17] KVM: x86: Replace memset() "optimization" with normal per-field writes

Kees Cook
2021-12-13T22:33:40Z

From: Sean Christopherson <seanjc@google.com>

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>
---
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 28b1a4e57827..7401a3133e17 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -5381,8 +5381,13 @@ 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;
--
2.30.2