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

[Christian Borntraeger]

Adding Janosch (co-maintainer) and Claudio (author of that code)

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:
> 
>   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.
> 
> 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@xxxxxxxxxx>
> ---
> 
> 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).

I think its fine to send to the public lists. Its just a bug and with cc stable this
should cascade quickly to the right places. 
> 
> 
>  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");

I would prefer to not have a WARN_ON. Otherwise this would allow a malicious user
to spam the log. 


> +			return -EINVAL;
> +		}
>  		/*
>  		 * The second half of the bitmap is only used on x86,
>  		 * and would be wasted otherwise, so we put it to good
>