lists.openwall.net   lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  linux-cve-announce  PHC 
Open Source and information security mailing list archives
 
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date: Fri, 16 Feb 2024 09:10:47 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: David Matlack <dmatlack@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Pasha Tatashin <tatashin@...gle.com>, Michael Krebs <mkrebs@...gle.com>
Subject: Re: [PATCH 1/2] KVM: x86: Mark target gfn of emulated atomic
 instruction as dirty

On Thu, Feb 15, 2024, Sean Christopherson wrote:
> On Thu, Feb 15, 2024, David Matlack wrote:
> > On Wed, Feb 14, 2024 at 5:00 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > >
> > > When emulating an atomic access on behalf of the guest, mark the target
> > > gfn dirty if the CMPXCHG by KVM is attempted and doesn't fault.  This
> > > fixes a bug where KVM effectively corrupts guest memory during live
> > > migration by writing to guest memory without informing userspace that the
> > > page is dirty.
> > >
> > > Marking the page dirty got unintentionally dropped when KVM's emulated
> > > CMPXCHG was converted to do a user access.  Before that, KVM explicitly
> > > mapped the guest page into kernel memory, and marked the page dirty during
> > > the unmap phase.
> > >
> > > Mark the page dirty even if the CMPXCHG fails, as the old data is written
> > > back on failure, i.e. the page is still written.  The value written is
> > > guaranteed to be the same because the operation is atomic, but KVM's ABI
> > > is that all writes are dirty logged regardless of the value written.  And
> > > more importantly, that's what KVM did before the buggy commit.
> > >
> > > Huge kudos to the folks on the Cc list (and many others), who did all the
> > > actual work of triaging and debugging.
> > >
> > > Fixes: 1c2361f667f3 ("KVM: x86: Use __try_cmpxchg_user() to emulate atomic accesses")
> > 
> > I'm only half serious but... Should we just revert this commit?
> 
> No.

David, any objection to this patch?  I'd like to get this on its way to Paolo
asap, but also want to make sure we all agree this is the right solution before
doing so.

Powered by blists - more mailing lists

Powered by Openwall GNU/*/Linux Powered by OpenVZ