linux-kernel - Re: [PATCH 1/2] KVM: x86: Fix partially uninitialized integer in emulate

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Date:   Thu, 5 Oct 2023 13:48:30 +0000
From:   Julian Stecklina <julian.stecklina@...erus-technology.de>
To:     "seanjc@...gle.com" <seanjc@...gle.com>
CC:     "x86@...nel.org" <x86@...nel.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "bp@...en8.de" <bp@...en8.de>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] KVM: x86: Fix partially uninitialized integer in
 emulate_pop

On Wed, 2023-10-04 at 08:07 -0700, Sean Christopherson wrote:
> On Wed, Oct 04, 2023, Julian Stecklina wrote:
> > Most code gives a pointer to an uninitialized unsigned long as dest in
> > emulate_pop. len is usually the word width of the guest.
> > 
> > If the guest runs in 16-bit or 32-bit modes, len will not cover the
> > whole unsigned long and we end up with uninitialized data in dest.
> > 
> > Looking through the callers of this function, the issue seems
> > harmless, but given that none of this is performance critical, there
> > should be no issue with just always initializing the whole value.
> > 
> > Fix this by explicitly requiring a unsigned long pointer and
> > initializing it with zero in all cases.
> 
> NAK, this will break em_leave() as it will zero RBP regardless of how many
> bytes
> are actually supposed to be written.  Specifically, KVM would incorrectly
> clobber
> RBP[31:16] if LEAVE is executed with a 16-bit stack.

Thanks, Sean! Great catch. I didn't see this. Is there already a test suite for
this?

> I generally like defense-in-depth approaches, but zeroing data that the caller
> did not ask to be written is not a net positive.

I'll rewrite the patch to just initialize variables where they are currently
not. This should be a bit more conservative and have less risk of breaking
anything.

Julian