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

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Message-ID: <87tu9cqtje.fsf@redhat.com>
Date:   Thu, 26 May 2022 16:04:37 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     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>,
        Kees Cook <keescook@...omium.org>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 1/4] KVM: x86: Grab regs_dirty in local 'unsigned long'

Sean Christopherson <seanjc@...gle.com> writes:

> 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.

Your nearly perfect description misses one important part: in 'struct
x86_emulate_ctxt', 'regs_dirty' is actually 'u32' thus all this buzz :-)

>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/emulate.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 89b11e7dca8a..7226a127ccb4 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -269,9 +269,10 @@ static ulong *reg_rmw(struct x86_emulate_ctxt *ctxt, unsigned nr)
>  
>  static void writeback_registers(struct x86_emulate_ctxt *ctxt)
>  {
> +	unsigned long dirty = ctxt->regs_dirty;
>  	unsigned reg;
>  
> -	for_each_set_bit(reg, (ulong *)&ctxt->regs_dirty, 16)
> +	for_each_set_bit(reg, &dirty, 16)
>  		ctxt->ops->write_gpr(ctxt, reg, ctxt->_regs[reg]);
>  }

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly