linux-kernel - Re: [PATCH 4.19 STABLE 2/2] KVM: VMX: Mark RCX, RDX and RSI as clobbered in vmx_vcpu

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Date:   Tue, 5 May 2020 09:02:59 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     stable@...r.kernel.org,
        Ben Hutchings <ben.hutchings@...ethink.co.uk>,
        Sasha Levin <sashal@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        linux-kernel@...r.kernel.org,
        Tobias Urdin <tobias.urdin@...ero.com>
Subject: Re: [PATCH 4.19 STABLE 2/2] KVM: VMX: Mark RCX, RDX and RSI as
 clobbered in vmx_vcpu_run()'s asm blob

On Mon, May 04, 2020 at 11:27:31PM -0700, Sean Christopherson wrote:
> On Tue, May 05, 2020 at 08:15:02AM +0200, Greg Kroah-Hartman wrote:
> > On Mon, May 04, 2020 at 06:23:48PM -0700, Sean Christopherson wrote:
> > > Save RCX, RDX and RSI to fake outputs to coerce the compiler into
> > > treating them as clobbered.  RCX in particular is likely to be reused by
> > > the compiler to dereference the 'struct vcpu_vmx' pointer, which will
> > > result in a null pointer dereference now that RCX is zeroed by the asm
> > > blob.
> > > 
> > > Add ASM_CALL_CONSTRAINT to fudge around an issue where <something>
> > > during modpost can't find vmx_return when specifying output constraints.
> > > 
> > > Reported-by: Tobias Urdin <tobias.urdin@...ero.com>
> > > Fixes: b4be98039a92 ("KVM: VMX: Zero out *all* general purpose registers after VM-Exit")
> > > Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> > > ---
> > >  arch/x86/kvm/vmx.c | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> > > index 5b06a98ffd4c..54c8b4dc750d 100644
> > > --- a/arch/x86/kvm/vmx.c
> > > +++ b/arch/x86/kvm/vmx.c
> > > @@ -10882,7 +10882,8 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
> > >  		".global vmx_return \n\t"
> > >  		"vmx_return: " _ASM_PTR " 2b \n\t"
> > >  		".popsection"
> > > -	      : : "c"(vmx), "d"((unsigned long)HOST_RSP), "S"(evmcs_rsp),
> > > +	      : ASM_CALL_CONSTRAINT, "=c"((int){0}), "=d"((int){0}), "=S"((int){0})
> > > +	      : "c"(vmx), "d"((unsigned long)HOST_RSP), "S"(evmcs_rsp),
> > >  		[launched]"i"(offsetof(struct vcpu_vmx, __launched)),
> > >  		[fail]"i"(offsetof(struct vcpu_vmx, fail)),
> > >  		[host_rsp]"i"(offsetof(struct vcpu_vmx, host_rsp)),
> > > -- 
> > > 2.26.0
> > > 
> > 
> > What is the git commit id of this patch in Linus's tree?
> 
> There is none.  In upstream at the time of the offending commit (b4be98039a92
> in 4.19, 0e0ab73c9a024 upstream), the inline asm blob had previously been
> moved to a dedicated helper, __vmx_vcpu_run(), that was intentionally put
> into a separate compilation unit, i.e. consuming the clobbered register
> was effectively impossible because %rcx is volatile and __vmx_vcpu_run()
> couldn't itself be inlined.
> 
> To make things more confusing, the inline asm blob got moved into a proper
> asm subroutine shortly thereafter.  Things really start to diverge from
> current upstream right around the time of this commit.

Then you need to document the heck out of the fact that this is not
upstream, why it is different from upstream, and why we can't just take
what upstream did instead in the changelog.  That way, when this patch
turns out to be buggy (hint, 90% of the times they are), we know why
this was done the way it was so we can revert it and know who to
complain to :)

thanks,

greg k-h