[PATCH] KVM: s390: kvm_s390_vm_start_migration: check dirty_bitmap before using it as target for memset()

From: Igor Mammedov
Date: Tue Sep 10 2019 - 09:02:31 EST

If userspace doesn't set KVM_MEM_LOG_DIRTY_PAGES on memslot before calling
kvm_s390_vm_start_migration(), kernel will oops with:

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 might crash the host.

Make sure that ms->dirty_bitmap is set before using it or
print a warning and return -ENIVAL otherwise.

Fixes: afdad61615cc ("KVM: s390: Fix storage attributes migration with memory slots")
Signed-off-by: Igor Mammedov <imammedo@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # v4.19+

- drop WARN()

arch/s390/kvm/kvm-s390.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index f329dcb3f44c..2a40cd3e40b4 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -1018,6 +1018,8 @@ 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)
+ return -EINVAL;
* The second half of the bitmap is only used on x86,
* and would be wasted otherwise, so we put it to good