linux-hardening - [PATCH 50/64] KVM: x86: Use struct

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Date:   Tue, 27 Jul 2021 13:58:41 -0700
From:   Kees Cook <keescook@...omium.org>
To:     linux-hardening@...r.kernel.org
Cc:     Kees Cook <keescook@...omium.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Keith Packard <keithpac@...zon.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-wireless@...r.kernel.org,
        netdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-staging@...ts.linux.dev, linux-block@...r.kernel.org,
        linux-kbuild@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: [PATCH 50/64] KVM: x86: Use struct_group() to zero decode cache

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memset(), avoid intentionally writing across
neighboring fields.

Add struct_group() to mark region of struct x86_emulate_ctxt that should
be initialized to zero.

Signed-off-by: Kees Cook <keescook@...omium.org>
---
 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,
+		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