Re: [patch 3/4] [PATCH] kvm: Fix tprot locking

[Avi Kivity <avi@redhat.com>]

On 11/17/2011 01:15 PM, Martin Schwidefsky wrote:
> On Thu, 17 Nov 2011 12:27:41 +0200
> Avi Kivity <avi@redhat.com> wrote:
>
> > On 11/17/2011 12:00 PM, Carsten Otte wrote:
> > > From: Christian Borntraeger <borntraeger@de.ibm.com> 
> > >
> > > There is a potential host deadlock in the tprot intercept handling.
> > > We must not hold the mmap semaphore while resolving the guest
> > > address. If userspace is remapping, then the memory detection in
> > > the guest is broken anyway so we can safely separate the 
> > > address translation from walking the vmas.
> > >
> > > Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com> 
> > > Signed-off-by: Carsten Otte <cotte@de.ibm.com>
> > > ---
> > >
> > >  arch/s390/kvm/priv.c |   10 ++++++++--
> > >  1 file changed, 8 insertions(+), 2 deletions(-)
> > >
> > > diff -urpN linux-2.6/arch/s390/kvm/priv.c linux-2.6-patched/arch/s390/kvm/priv.c
> > > --- linux-2.6/arch/s390/kvm/priv.c	2011-10-24 09:10:05.000000000 +0200
> > > +++ linux-2.6-patched/arch/s390/kvm/priv.c	2011-11-17 10:03:53.000000000 +0100
> > > @@ -336,6 +336,7 @@ static int handle_tprot(struct kvm_vcpu
> > >  	u64 address1 = disp1 + base1 ? vcpu->arch.guest_gprs[base1] : 0;
> > >  	u64 address2 = disp2 + base2 ? vcpu->arch.guest_gprs[base2] : 0;
> > >  	struct vm_area_struct *vma;
> > > +	unsigned long user_address;
> > >  
> > >  	vcpu->stat.instruction_tprot++;
> > >  
> > > @@ -349,9 +350,14 @@ static int handle_tprot(struct kvm_vcpu
> > >  		return -EOPNOTSUPP;
> > >  
> > >  
> > > +	/* we must resolve the address without holding the mmap semaphore.
> > > +	 * This is ok since the userspace hypervisor is not supposed to change
> > > +	 * the mapping while the guest queries the memory. Otherwise the guest
> > > +	 * might crash or get wrong info anyway. */
> > > +	user_address = (unsigned long) __guestaddr_to_user(vcpu, address1);
> > > +
> > >  	down_read(&current->mm->mmap_sem);
> > > -	vma = find_vma(current->mm,
> > > -			(unsigned long) __guestaddr_to_user(vcpu, address1));
> > > +	vma = find_vma(current->mm, user_address);
> > >  	if (!vma) {
> > >  		up_read(&current->mm->mmap_sem);
> > >  		return kvm_s390_inject_program_int(vcpu, PGM_ADDRESSING);
> > >
> > 
> > Unrelated to the patch, but I'm curious: it looks like __gmap_fault()
> > dereferences the guest page table?  How can it assume that it is mapped?
>
> The gmap code does not assume that the code is mapped. If the individual
> MB has not been mapped in the guest address space or the target memory
> is not mapped in the process address space __gmap_fault() returns -EFAULT. 
>
> > I'm probably misreading the code.

I did misread it - I assumed it was guest page table, whereas those are
host page tables mapping guest memory (here, "guest page table" mean
guest-managed virt to phys translation).


-- 
error compiling committee.c: too many arguments to function