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KVM: x86: Replace memset() "optimization" with normal per-field writes

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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>
This commit is contained in:
Sean Christopherson 2021-08-18 08:03:08 -07:00 committed by Kees Cook
parent dfd42facf1
commit 73ab4a3509
2 changed files with 8 additions and 7 deletions
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9
arch/x86/kvm/emulate.c
View File

@ -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;

6
arch/x86/kvm/kvm_emulate.h
View File

@ -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;