Re: [Xen-devel] [PATCH for-4.10] x86/hvm: Don't corrupt the HVM context stream when writing the MSR record

[Julien Grall <julien.grall@xxxxxxxxxx>]

Hi,

On 11/16/2017 10:45 PM, Andrew Cooper wrote:
> Ever since it was introduced in c/s bd1f0b45ff, hvm_save_cpu_msrs() has had a
> bug whereby it corrupts the HVM context stream if some, but fewer than the
> maximum number of MSRs are written.
>
> _hvm_init_entry() creates an hvm_save_descriptor with length for
> msr_count_max, but in the case that we write fewer than max, h->cur only moves
> forward by the amount of space used, causing the subsequent
> hvm_save_descriptor to be written within the bounds of the previous one.
>
> To resolve this, reduce the length reported by the descriptor to match the
> actual number of bytes used.
>
> A typical failure on the destination side looks like:
>
>      (XEN) HVM4 restore: CPU_MSR 0
>      (XEN) HVM4.0 restore: not enough data left to read 56 MSR bytes
>      (XEN) HVM4 restore: failed to load entry 20/0
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> CC: Julien Grall <julien.grall@xxxxxxx>
>
> This wants backporting to all stable trees, so should also be considered for
> inclusion into 4.10 at this point.
Release-acked-by: Julien Grall <julien.grall@xxxxxxxxxx>

Cheers,

> ---
>   xen/arch/x86/hvm/hvm.c | 6 ++++++
>   1 file changed, 6 insertions(+)
>
> diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
> index 0af498a..c5e8467 100644
> --- a/xen/arch/x86/hvm/hvm.c
> +++ b/xen/arch/x86/hvm/hvm.c
> @@ -1330,6 +1330,7 @@ static int hvm_save_cpu_msrs(struct domain *d, 
> hvm_domain_context_t *h)
>   
>       for_each_vcpu ( d, v )
>       {
> +        struct hvm_save_descriptor *d = _p(&h->data[h->cur]);
>           struct hvm_msr *ctxt;
>           unsigned int i;
>   
> @@ -1348,8 +1349,13 @@ static int hvm_save_cpu_msrs(struct domain *d, hvm_domain_context_t *h)
>               ctxt->msr[i]._rsvd = 0;
>   
>           if ( ctxt->count )
> +        {
> +            /* Rewrite length to indicate how much space we actually used. */
> +            d->length = HVM_CPU_MSR_SIZE(ctxt->count);
>               h->cur += HVM_CPU_MSR_SIZE(ctxt->count);
> +        }
>           else
> +            /* or rewind and remove the descriptor from the stream. */
>               h->cur -= sizeof(struct hvm_save_descriptor);
>       }
>   
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