Re: [PATCH] x86: Remove unnecessary kmap() from sgx_ioc_enclave_init()

[Sean Christopherson <seanjc@google.com>]

On Thu, Jan 28, 2021, ira.weiny@intel.com wrote:
> From: Ira Weiny <ira.weiny@intel.com>
> 
> There is no reason to alloc a page and kmap it to store this temporary
> data from the user. 

Actually, there is, it's just poorly documented.  The sigstruct needs to be
page aligned, and the token needs to be 512-byte aligned.  kmcalloc doesn't
guarantee alignment.  IIRC things will work until slub_debug is enabled, at
which point the natural alignment behavior goes out the window.

> This is especially true when we are trying to
> remove kmap usages.  Also placing the token pointer 1/2 way into the
> page is fragile.
> 
> Replace this allocation with two kzalloc()'s which also removes the need
> for the memset().
> 
> Signed-off-by: Ira Weiny <ira.weiny@intel.com>
> ---
>  arch/x86/kernel/cpu/sgx/ioctl.c | 18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/sgx/ioctl.c b/arch/x86/kernel/cpu/sgx/ioctl.c
> index 90a5caf76939..9c9019760585 100644
> --- a/arch/x86/kernel/cpu/sgx/ioctl.c
> +++ b/arch/x86/kernel/cpu/sgx/ioctl.c
> @@ -604,7 +604,6 @@ static long sgx_ioc_enclave_init(struct sgx_encl *encl, void __user *arg)
>  {
>  	struct sgx_sigstruct *sigstruct;
>  	struct sgx_enclave_init init_arg;
> -	struct page *initp_page;
>  	void *token;
>  	int ret;
>  
> @@ -615,13 +614,15 @@ static long sgx_ioc_enclave_init(struct sgx_encl *encl, void __user *arg)
>  	if (copy_from_user(&init_arg, arg, sizeof(init_arg)))
>  		return -EFAULT;
>  
> -	initp_page = alloc_page(GFP_KERNEL);
> -	if (!initp_page)
> +	sigstruct = kzalloc(sizeof(*sigstruct), GFP_KERNEL);
> +	if (!sigstruct)
>  		return -ENOMEM;
>  
> -	sigstruct = kmap(initp_page);
> -	token = (void *)((unsigned long)sigstruct + PAGE_SIZE / 2);
> -	memset(token, 0, SGX_LAUNCH_TOKEN_SIZE);
> +	token = kzalloc(SGX_LAUNCH_TOKEN_SIZE, GFP_KERNEL);
> +	if (!token) {
> +		ret = -ENOMEM;
> +		goto free_sigstruct;
> +	}
>  
>  	if (copy_from_user(sigstruct, (void __user *)init_arg.sigstruct,
>  			   sizeof(*sigstruct))) {
> @@ -645,8 +646,9 @@ static long sgx_ioc_enclave_init(struct sgx_encl *encl, void __user *arg)
>  	ret = sgx_encl_init(encl, sigstruct, token);
>  
>  out:
> -	kunmap(initp_page);
> -	__free_page(initp_page);
> +	kfree(token);
> +free_sigstruct:
> +	kfree(sigstruct);
>  	return ret;
>  }
>  
> -- 
> 2.28.0.rc0.12.gb6a658bd00c9
>