[PATCH] KVM: x86/emulator: Bounds check reg nr against reg array size

Kees Cook
2022-05-20T16:57:18Z

GCC 12 sees that it might be possible for "nr" to be outside the _regs
array. Add explicit bounds checking.

In function 'reg_read',
inlined from 'reg_rmw' at ../arch/x86/kvm/emulate.c:266:2:
../arch/x86/kvm/emulate.c:254:27: warning: array subscript 32 is above array bounds of 'long unsigned int[17]' [-Warray-bounds]
254 | return ctxt->_regs[nr];
| ~~~~~~~~~~~^~~~
In file included from ../arch/x86/kvm/emulate.c:23:
../arch/x86/kvm/kvm_emulate.h: In function 'reg_rmw':
../arch/x86/kvm/kvm_emulate.h:366:23: note: while referencing '_regs'
366 | unsigned long _regs[NR_VCPU_REGS];
| ^~~~~

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Wanpeng Li <wanpengli@tencent.com>
Cc: Jim Mattson <jmattson@google.com>
Cc: x86@kernel.org
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: kvm@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
arch/x86/kvm/emulate.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 89b11e7dca8a..fbcbc012a3ae 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -247,6 +247,8 @@ enum x86_transfer_type {

static ulong reg_read(struct x86_emulate_ctxt *ctxt, unsigned nr)
{
+ if (WARN_ON(nr >= ARRAY_SIZE(ctxt->_regs)))
+ return 0;
if (!(ctxt->regs_valid & (1 << nr))) {
ctxt->regs_valid |= 1 << nr;
ctxt->_regs[nr] = ctxt->ops->read_gpr(ctxt, nr);
@@ -256,6 +258,8 @@ static ulong reg_read(struct x86_emulate_ctxt *ctxt, unsigned nr)

static ulong *reg_write(struct x86_emulate_ctxt *ctxt, unsigned nr)
{
+ if (WARN_ON(nr >= ARRAY_SIZE(ctxt->_regs)))
+ return 0;
ctxt->regs_valid |= 1 << nr;
ctxt->regs_dirty |= 1 << nr;
return &ctxt->_regs[nr];
--
2.32.0