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

[Kees Cook <keescook@xxxxxxxxxxxx>]

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@xxxxxxxxxx>
Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
Cc: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
Cc: Wanpeng Li <wanpengli@xxxxxxxxxxx>
Cc: Jim Mattson <jmattson@xxxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: kvm@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 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