From: Ira Weiny <[email protected]>
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 <[email protected]>,
Cc: Jethro Beekman <[email protected]>,
Signed-off-by: Ira Weiny <[email protected]>
---
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/[email protected]/
---
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
On 2/1/21 5:37 PM, [email protected] wrote:
> 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.
Hi Ira,
It's a *relatively* recent guaranteed, but:
https://www.kernel.org/doc/Documentation/core-api/memory-allocation.rst
says:
> The address of a chunk allocated with `kmalloc` is aligned to at least
> ARCH_KMALLOC_MINALIGN bytes. For sizes which are a power of two, the
> alignment is also guaranteed to be at least the respective size.
So, if you allocate a page with kmalloc(), you get an aligned page. Yay!
On Tue, Feb 02, 2021 at 10:55:36AM -0800, Dave Hansen wrote:
> On 2/1/21 5:37 PM, [email protected] wrote:
> > 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.
>
> Hi Ira,
>
> It's a *relatively* recent guaranteed, but:
>
> https://www.kernel.org/doc/Documentation/core-api/memory-allocation.rst
>
> says:
>
> > The address of a chunk allocated with `kmalloc` is aligned to at least
> > ARCH_KMALLOC_MINALIGN bytes. For sizes which are a power of two, the
> > alignment is also guaranteed to be at least the respective size.
>
> So, if you allocate a page with kmalloc(), you get an aligned page. Yay!
And this what we do sgx_ioc_enclave_create() anyway, as I stated in my
earlier response. Better to use the same pattern everywhere consitently
when it makes sense.
/Jarkko