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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id 21sm228958pfh.103.2021.08.18.09.23.43 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 18 Aug 2021 09:23:44 -0700 (PDT) Date: Wed, 18 Aug 2021 09:23:43 -0700 From: Kees Cook To: Sean Christopherson Cc: linux-kernel@vger.kernel.org, Paolo Bonzini , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Thomas Gleixner , Ingo Molnar , Borislav Petkov , x86@kernel.org, "H. Peter Anvin" , kvm@vger.kernel.org, "Gustavo A. R. Silva" , Greg Kroah-Hartman , Andrew Morton , linux-wireless@vger.kernel.org, netdev@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-staging@lists.linux.dev, linux-block@vger.kernel.org, linux-kbuild@vger.kernel.org, clang-built-linux@googlegroups.com, Rasmus Villemoes , linux-hardening@vger.kernel.org Subject: Re: [PATCH v2 53/63] KVM: x86: Use struct_group() to zero decode cache Message-ID: <202108180922.6C9E385A1@keescook> References: <20210818060533.3569517-1-keescook@chromium.org> <20210818060533.3569517-54-keescook@chromium.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-hardening@vger.kernel.org On Wed, Aug 18, 2021 at 03:11:28PM +0000, Sean Christopherson wrote: > On Tue, Aug 17, 2021, Kees Cook wrote: > > arch/x86/kvm/emulate.c | 3 +-- > > arch/x86/kvm/kvm_emulate.h | 19 +++++++++++-------- > > 2 files changed, 12 insertions(+), 10 deletions(-) > > > > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c > > index 2837110e66ed..2608a047e769 100644 > > --- a/arch/x86/kvm/emulate.c > > +++ b/arch/x86/kvm/emulate.c > > @@ -5377,8 +5377,7 @@ 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); > > + memset(&ctxt->decode_cache, 0, sizeof(ctxt->decode_cache)); > > > > 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..9b8afcb8ad39 100644 > > --- a/arch/x86/kvm/kvm_emulate.h > > +++ b/arch/x86/kvm/kvm_emulate.h > > @@ -341,14 +341,17 @@ struct x86_emulate_ctxt { > > * the rest are initialized unconditionally in x86_decode_insn > > * or elsewhere > > */ > > - 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; > > + struct_group(decode_cache, > > This is somewhat misleading because half of this struct is the so called "decode > cache", not just these six fields. > > KVM's "optimization" is quite ridiculous as this has never been such a hot path > that saving a few mov instructions would be noticeable. And hilariously, the > "optimization" is completely unnecessary because both gcc and clang are clever > enough to batch the first five into a movq even when zeroing the fields individually. > > So, I would much prefer to go with the following: Sounds good to me! > > From dbdca1f4cd01fee418c252e54c360d518b2b1ad6 Mon Sep 17 00:00:00 2001 > From: Sean Christopherson > Date: Wed, 18 Aug 2021 08:03:08 -0700 > Subject: [PATCH] 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 > Signed-off-by: Sean Christopherson Reviewed-by: Kees Cook Do you want me to take this patch into my tree, or do you want to carry it for KVM directly? Thanks! -Kees > --- > 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 > > -- Kees Cook