Re: [PATCH v2 1/2] x86: sgx_vepc: extract sgx_vepc_remove_page

[Jarkko Sakkinen]

On Tue, 2021-10-12 at 06:57 -0400, Paolo Bonzini wrote:
> For bare-metal SGX on real hardware, the hardware provides guarantees
> SGX state at reboot.  For instance, all pages start out uninitialized.
> The vepc driver provides a similar guarantee today for freshly-opened
> vepc instances, but guests such as Windows expect all pages to be in
> uninitialized state on startup, including after every guest reboot.
> 
> One way to do this is to simply close and reopen the /dev/sgx_vepc file
> descriptor and re-mmap the virtual EPC.  However, this is problematic
> because it prevents sandboxing the userspace (for example forbidding
> open() after the guest starts; this is doable with heavy use of SCM_RIGHTS
> file descriptor passing).
> 
> In order to implement this, we will need a ioctl that performs
> EREMOVE on all pages mapped by a /dev/sgx_vepc file descriptor:
> other possibilities, such as closing and reopening the device,
> are racy.
> 
> Start the implementation by creating a separate function with just
> the __eremove wrapper.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
>         v1->v2: keep WARN in sgx_vepc_free_page
> 
>  arch/x86/kernel/cpu/sgx/virt.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/sgx/virt.c b/arch/x86/kernel/cpu/sgx/virt.c
> index 64511c4a5200..59cdf3f742ac 100644
> --- a/arch/x86/kernel/cpu/sgx/virt.c
> +++ b/arch/x86/kernel/cpu/sgx/virt.c
> @@ -111,10 +111,8 @@ static int sgx_vepc_mmap(struct file *file, struct vm_area_struct *vma)
>         return 0;
>  }
>  
> -static int sgx_vepc_free_page(struct sgx_epc_page *epc_page)
> +static int sgx_vepc_remove_page(struct sgx_epc_page *epc_page)
>  {
> -       int ret;
> -
>         /*
>          * Take a previously guest-owned EPC page and return it to the
>          * general EPC page pool.
> @@ -124,7 +122,12 @@ static int sgx_vepc_free_page(struct sgx_epc_page *epc_page)
>          * case that a guest properly EREMOVE'd this page, a superfluous
>          * EREMOVE is harmless.
>          */
> -       ret = __eremove(sgx_get_epc_virt_addr(epc_page));
> +       return __eremove(sgx_get_epc_virt_addr(epc_page));
> +}
> +
> +static int sgx_vepc_free_page(struct sgx_epc_page *epc_page)
> +{
> +       int ret = sgx_vepc_remove_page(epc_page);
>         if (ret) {
>                 /*
>                  * Only SGX_CHILD_PRESENT is expected, which is because of
> @@ -144,7 +147,6 @@ static int sgx_vepc_free_page(struct sgx_epc_page *epc_page)
>         }
>  
>         sgx_free_epc_page(epc_page);
> -
>         return 0;
>  }
>  

Reviewed-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>

/Jarkko