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SubjectRe: [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@intel.com wrote:
> > From: Ira Weiny <ira.weiny@intel.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@intel.com>
> > Cc: Sean Christopherson <seanjc@google.com>
> > Cc: Jethro Beekman <jethro@fortanix.com>
> > Signed-off-by: Ira Weiny <ira.weiny@intel.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

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