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

[Sean Christopherson <seanjc@google.com>]

On Fri, May 20, 2022, Kees Cook wrote:
> 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;

This is wrong, reg_write() confusingly returns a pointer the register to be written,
it doesn't actually do the write.  So if we want to guard against array overflow,
it would be better to cap @nr and continue on, i.e. assume some higher bit was
spuriously set.

The other oddity here is that VCPU_REGS_RIP should never be read, the RIP relative
code reads _eip directly.  I.e. _regs[] should really be VCPU_REGS_R15+1.  And
adding a #define for that would clean up this bit of code in writeback_registers()
that hardcodes 16 (rax - r15) GPRs:

	for_each_set_bit(reg, (ulong *)&ctxt->regs_dirty, 16)
		ctxt->ops->write_gpr(ctxt, reg, ctxt->_regs[reg]);

Lastly, casting regs_dirty to an unsigned long pointer is all kinds of gross, e.g.
if it were moved to the end of struct x86_emulate_ctxt then the above could trigger
an out-of-bounds read.

I'll whip up a small series to clean this code up and add WARNs similar to above.