linux-kernel - Re: [PATCH 1/4] KVM: x86: Grab regs

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Message-ID: <202205260832.0301216FB6@keescook>
Date:   Thu, 26 May 2022 08:33:03 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Robert Dinse <nanook@...imo.com>
Subject: Re: [PATCH 1/4] KVM: x86: Grab regs_dirty in local 'unsigned long'

On Wed, May 25, 2022 at 10:26:01PM +0000, Sean Christopherson wrote:
> Capture ctxt->regs_dirty in a local 'unsigned long' instead of casting
> ctxt->regs_dirty to an 'unsigned long *' for use in for_each_set_bit().
> The bitops helpers really do read the entire 'unsigned long', even though
> the walking of the read value is capped at the specified size.  I.e. KVM
> is reading memory beyond ctxt->regs_dirty.  Functionally it's not an
> issue because regs_dirty is in the middle of x86_emulate_ctxt, i.e. KVM
> is just reading its own memory, but relying on that coincidence is gross
> and unsafe.

Right; there have bean a handful of other bitops-using-stuff-cast-to-ulong
that has been recently fixed elsewhere. Better to get them all squashed.
:)

> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook