Commit 73ab4a35 authored by Sean Christopherson's avatar Sean Christopherson Committed by Kees Cook
Browse files

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: default avatarKees Cook <keescook@chromium.org>
Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/lkml/YR0jIEzEcUom/7rd@google.com


Signed-off-by: default avatarKees Cook <keescook@chromium.org>
parent dfd42fac

arch/x86/kvm/emulate.c

+7 −2
Original line number Diff line number Diff line
@@ -5380,8 +5380,13 @@ 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;

arch/x86/kvm/kvm_emulate.h

+1 −5
Original line number Diff line number Diff line
@@ -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;