linux-kernel - Re: [PATCH V3] x86: Remove unnecessary kmap() from sgx_ioc_enclave

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Date:   Wed, 3 Feb 2021 00:45:33 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     ira.weiny@...el.com
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Jethro Beekman <jethro@...tanix.com>,
        linux-kernel@...r.kernel.org, linux-sgx@...r.kernel.org
Subject: Re: [PATCH V3] x86: Remove unnecessary kmap() from
 sgx_ioc_enclave_init()

On Tue, Feb 02, 2021 at 11:47:19AM -0800, ira.weiny@...el.com wrote:
> From: Ira Weiny <ira.weiny@...el.com>
> 
> kmap is inefficient and we are trying to reduce the usage in the kernel.
> There is no readily apparent reason why initp_page needs to be allocated
> and kmap'ed() but sigstruct needs to be page aligned and token
> 512 byte aligned.
> 
> kmalloc() can give us this alignment but we need to allocate PAGE_SIZE
> bytes to do so.  Rather than change this kmap() to kmap_local_page() use
> kmalloc() instead.
> 
> Remove the alloc_page()/kmap() and replace with kmalloc() to get a
> kernel address to use.
> 
> In addition add a comment to document the alignment requirements so that
> others like myself don't attempt to 'fix' this again.  Finally, add 2
> BUILD_BUG_ON's to ensure future changes to sigstruct and token do not go
> unnoticed and cause a bug.
> 
> Cc: Dave Hansen <dave.hansen@...el.com>
> Cc: Sean Christopherson <seanjc@...gle.com>
> Cc: Jethro Beekman <jethro@...tanix.com>
> Signed-off-by: Ira Weiny <ira.weiny@...el.com>
> 
> ---
> Changes from v2[1]:
> 	When allocating a power of 2 size kmalloc() now guarantees the
> 	alignment of the respective size.  So go back to using kmalloc() but
> 	with a PAGE_SIZE allocation to get the alignment.  This also follows
> 	the pattern in sgx_ioc_enclave_create()
> 
> Changes from v1[1]:
> 	Use page_address() instead of kcmalloc() to ensure sigstruct is
> 	page aligned
> 	Use BUILD_BUG_ON to ensure token and sigstruct don't collide.
> 
> [1] https://lore.kernel.org/lkml/20210129001459.1538805-1-ira.weiny@intel.com/
> [2] https://lore.kernel.org/lkml/20210202013725.3514671-1-ira.weiny@intel.com/
> ---
>  arch/x86/kernel/cpu/sgx/ioctl.c | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/sgx/ioctl.c b/arch/x86/kernel/cpu/sgx/ioctl.c
> index 90a5caf76939..e0c3301ccd67 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;
>  
> @@ -615,11 +614,16 @@ 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 must be on a page boundry and token on a 512 byte boundry
> +	 * kmalloc() gives us this alignment when allocating PAGE_SIZE bytes
> +	 */
> +	sigstruct = kmalloc(PAGE_SIZE, GFP_KERNEL);
> +	if (!sigstruct)
>  		return -ENOMEM;
>  
> -	sigstruct = kmap(initp_page);
> +	BUILD_BUG_ON(sizeof(*sigstruct) > (PAGE_SIZE/2));
> +	BUILD_BUG_ON(SGX_LAUNCH_TOKEN_SIZE > (PAGE_SIZE/2));

Please remove these.

>  	token = (void *)((unsigned long)sigstruct + PAGE_SIZE / 2);
>  	memset(token, 0, SGX_LAUNCH_TOKEN_SIZE);
>  
> @@ -645,8 +649,7 @@ 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(sigstruct);
>  	return ret;
>  }
>  
> -- 
> 2.28.0.rc0.12.gb6a658bd00c9
> 
> 

/Jarkko