Re: [PATCH] KVM: sev: Fix int overflow in send|recieve_update_data ioctls

[Tom Lendacky <thomas.lendacky@amd.com>]

On 1/9/23 10:08, Peter Gonda wrote:
> KVM_SEV_SEND_UPDATE_DATA and KVM_SEV_RECEIVE_UPDATE_DATA have an integer
> overflow issue. Params.guest_len and offset are both 32bite wide, with a
> large params.guest_len the check to confirm a page boundary is not
> crossed can falsely pass:
> 
>      /* Check if we are crossing the page boundary *
>      offset = params.guest_uaddr & (PAGE_SIZE - 1);
>      if ((params.guest_len + offset > PAGE_SIZE))
> 
> Add an additional check to this conditional to confirm that
> params.guest_len itself is not greater than PAGE_SIZE.
> 
> The current code is can only overflow with a params.guest_len of greater
> than 0xfffff000. And the FW spec says these commands fail with lengths
> greater than 16KB. So this issue should not be a security concern
> 
> Fixes: 15fb7de1a7f5 ("KVM: SVM: Add KVM_SEV_RECEIVE_UPDATE_DATA command")
> Fixes: d3d1af85e2c7 ("KVM: SVM: Add KVM_SEND_UPDATE_DATA command")
> Reported-by: Andy Nguyen <theflow@google.com>
> Suggested-by: Thomas Lendacky <thomas.lendacky@amd.com>
> Signed-off-by: Peter Gonda <pgonda@google.com>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Sean Christopherson <seanjc@google.com>
> Cc: kvm@vger.kernel.org
> Cc: stable@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> ---
>   arch/x86/kvm/svm/sev.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 273cba809328..9451de72f917 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -1294,7 +1294,7 @@ static int sev_send_update_data(struct kvm *kvm, struct kvm_sev_cmd *argp)
>   
>   	/* Check if we are crossing the page boundary */
>   	offset = params.guest_uaddr & (PAGE_SIZE - 1);
> -	if ((params.guest_len + offset > PAGE_SIZE))
> +	if (params.guest_len > PAGE_SIZE || (params.guest_len + offset > PAGE_SIZE))

I see the original if statement had double parentheses, which looks
strange. Should this if (and the one below) be:

	if (params.guest_len > PAGE_SIZE || (params.guest_len + offset) > PAGE_SIZE)

Thanks,
Tom

>   		return -EINVAL;
>   
>   	/* Pin guest memory */
> @@ -1474,7 +1474,7 @@ static int sev_receive_update_data(struct kvm *kvm, struct kvm_sev_cmd *argp)
>   
>   	/* Check if we are crossing the page boundary */
>   	offset = params.guest_uaddr & (PAGE_SIZE - 1);
> -	if ((params.guest_len + offset > PAGE_SIZE))
> +	if (params.guest_len > PAGE_SIZE || (params.guest_len + offset > PAGE_SIZE))
>   		return -EINVAL;
>   
>   	hdr = psp_copy_user_blob(params.hdr_uaddr, params.hdr_len);