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

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