Messages in this thread Patch in this message |  | | From | Kees Cook <> | Subject | [PATCH 01/17] KVM: x86: Replace memset() "optimization" with normal per-field writes | Date | Mon, 13 Dec 2021 14:33:15 -0800 |
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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
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