Re: [PATCH] kvm_s390_vm_start_migration: check dirty_bitmap before using it as target for memset()

[Igor Mammedov <imammedo@redhat.com>]

On Mon, 9 Sep 2019 17:47:37 +0200
David Hildenbrand <david@redhat.com> wrote:

> On 09.09.19 16:55, Igor Mammedov wrote:
> > If userspace doesn't set KVM_MEM_LOG_DIRTY_PAGES on memslot before calling
> > kvm_s390_vm_start_migration(), kernel will oops with:
> >   
> 
> We usually have the subject in a "KVM: s390: ..." format. Like
> 
> "KVM: s390: check dirty_bitmap before using it as target for memset()"
> 
> >   Unable to handle kernel pointer dereference in virtual kernel address space
> >   Failing address: 0000000000000000 TEID: 0000000000000483
> >   Fault in home space mode while using kernel ASCE.
> >   AS:0000000002a2000b R2:00000001bff8c00b R3:00000001bff88007 S:00000001bff91000 P:000000000000003d
> >   Oops: 0004 ilc:2 [#1] SMP
> >   ...
> >   Call Trace:
> >   ([<001fffff804ec552>] kvm_s390_vm_set_attr+0x347a/0x3828 [kvm])
> >    [<001fffff804ecfc0>] kvm_arch_vm_ioctl+0x6c0/0x1998 [kvm]
> >    [<001fffff804b67e4>] kvm_vm_ioctl+0x51c/0x11a8 [kvm]
> >    [<00000000008ba572>] do_vfs_ioctl+0x1d2/0xe58
> >    [<00000000008bb284>] ksys_ioctl+0x8c/0xb8
> >    [<00000000008bb2e2>] sys_ioctl+0x32/0x40
> >    [<000000000175552c>] system_call+0x2b8/0x2d8
> >   INFO: lockdep is turned off.
> >   Last Breaking-Event-Address:
> >    [<0000000000dbaf60>] __memset+0xc/0xa0
> > 
> > due to ms->dirty_bitmap being NULL, which migh crash the host.  
> 
> s/migh/might/
> 
> > 
> > Make sure that ms->dirty_bitmap is set before using it or
> > print a warning and return -ENIVAL otherwise.
> > 
> > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> > ---
> > 
> > PS:
> >   keeping it private for now as issue might DoS host,
> >   I'll leave it upto maintainers to decide if it should be handled as security
> >   bug (I'm not sure what process for handling such bugs should be used).
> > 
> > 
> >  arch/s390/kvm/kvm-s390.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> > index f329dcb3f44c..dfba51c9d60c 100644
> > --- a/arch/s390/kvm/kvm-s390.c
> > +++ b/arch/s390/kvm/kvm-s390.c
> > @@ -1018,6 +1018,10 @@ static int kvm_s390_vm_start_migration(struct kvm *kvm)
> >  	/* mark all the pages in active slots as dirty */
> >  	for (slotnr = 0; slotnr < slots->used_slots; slotnr++) {
> >  		ms = slots->memslots + slotnr;
> > +		if (!ms->dirty_bitmap) {
> > +			WARN(1, "ms->dirty_bitmap == NULL\n");
> > +			return -EINVAL;
> > +		}  
> 
> if (WARN_ON_ONCE(!ms->dirty_bitmap))
> 	return -EINVAL;
> 
> But I wonder if the WARN is really needed. (or simply a wrong usage of the interface)
I added WARN because of there is no any visible sign that something
went wrong in userspace, this way at least we would have a trace of
invalid API usage.

But I can drop it if you prefer.

> 
> 
> >  		/*
> >  		 * The second half of the bitmap is only used on x86,
> >  		 * and would be wasted otherwise, so we put it to good
> >   
> 
> You should add
> 
> Fixes: afdad61615cc ("KVM: s390: Fix storage attributes migration with memory slots")
> Cc: stable@vger.kernel.org # v4.19+
> 
> Thanks!