Re: [PATCH] KVM: nVMX: Don't flush shadow VMCS12 to guest memory during vCPU teardown

[Sean Christopherson <seanjc@google.com>]

On Thu, Sep 10, 2026, Sean Christopherson wrote:
> On Thu, Sep 10, 2026, James Houghton wrote:
> > On Wed, Sep 9, 2026 at 12:00 PM Sean Christopherson <seanjc@google.com> wrote:
> > I wish we could just change the uaccess primitives, like
> > {,__}access_ok(), to check that `current->mm` is not NULL (and WARN
> > and return -EFAULT if it is NULL).
> 
> That wouldn't help at all in this case, because the access_ok() check is done
> when memslots are modified.  Which is the crux of KVM's problems: KVM decouples
> the initial checks from the accesses, relying on kvm->mm to
> 
> And even if we hardened all of the uaccess helpers, we'd _still_ have problems,
> because it's not just a NULL current->mm that's problematic.  The last reference
> to a VM file, i.e. to struct kvm, can be put by a different _process_.  I.e. KVM
> still needs to guard against reading/writing guest memory using a valid, non-NULL
> current->mm that isn't kvm->mm.  That can't be genericized in the uaccess APIs,
> because the rule that only a specific address space can be used is very much unique
> to KVM.

Oh, and I'm not saying we shouldn't try to harden the uaccess APIs to guard against
a NULL current->mm.  That absolutely would be worthwhile.  I'm just saying that for
KVM, it sadly isn't sufficient.