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

[ira . weiny]

From: Ira Weiny <ira.we...@intel.com>

kmap is inefficient and we are trying to reduce the usage in the kernel.
There is no readily apparent reason why the initp_page page needs to be
allocated and kmap'ed() but sigstruct needs to be page aligned and token
512 byte aligned.

In this case page_address() can be used instead of kmap_local_page() as
a much more efficient way to use the address because the page is
allocated GFP_KERNEL.

Remove the kmap and replace with page_address() to get a kernel address
for the alloc'ed page.

In addition add a comment regarding the alignment requirements as well
as 2 BUILD_BUG_ON's to ensure future changes to sigstruct and token do
not go unnoticed and cause a bug.

Cc: Sean Christopherson <sea...@google.com>,
Cc: Jethro Beekman <jet...@fortanix.com>,
Signed-off-by: Ira Weiny <ira.we...@intel.com>

---
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.we...@intel.com/
---
 arch/x86/kernel/cpu/sgx/ioctl.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/sgx/ioctl.c b/arch/x86/kernel/cpu/sgx/ioctl.c
index 90a5caf76939..678b02d67c3c 100644
--- a/arch/x86/kernel/cpu/sgx/ioctl.c
+++ b/arch/x86/kernel/cpu/sgx/ioctl.c
@@ -615,11 +615,18 @@ 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;
 
+       /*
+        * sigstruct must be on a page boundry and token on a 512 byte boundry
+        * so use alloc_page/page_address instead of a kmalloc().
+        */
        initp_page = alloc_page(GFP_KERNEL);
        if (!initp_page)
                return -ENOMEM;
 
-       sigstruct = kmap(initp_page);
+       sigstruct = page_address(initp_page);
+
+       BUILD_BUG_ON(sizeof(*sigstruct) > (PAGE_SIZE/2));
+       BUILD_BUG_ON(SGX_LAUNCH_TOKEN_SIZE > (PAGE_SIZE/2));
        token = (void *)((unsigned long)sigstruct + PAGE_SIZE / 2);
        memset(token, 0, SGX_LAUNCH_TOKEN_SIZE);
 
@@ -645,7 +652,6 @@ 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);
        return ret;
 }
-- 
2.28.0.rc0.12.gb6a658bd00c9