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

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Message-ID: <20210205050850.GC5033@iweiny-DESK2.sc.intel.com>
Date:   Thu, 4 Feb 2021 21:08:50 -0800
From:   Ira Weiny <ira.weiny@...el.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>
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 Wed, Feb 03, 2021 at 12:45:33AM +0200, Jarkko Sakkinen wrote:
> 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.

I don't see why these would be a bad thing as they don't have any run time
implications.  But I've removed them for v4 because getting rid of the kmap()
is more important for me right now.

Ira

> 
> >  	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