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

From: ira . weiny
Date: Wed Mar 24 2021 - 14:23:41 EST


From: Ira Weiny <ira.weiny@xxxxxxxxx>

kmap is inefficient and is being replaced by kmap_local_page(), if
possible.

That said, there is no readily apparent reason why initp_page needs to
be allocated and kmap'ed() except that 'sigstruct' needs to be page
aligned and 'token' 512 byte aligned.

Rather than change this kmap() to kmap_local_page() use kmalloc()
instead because kmalloc() can gives this alignment when allocating
PAGE_SIZE bytes.

Remove the alloc_page()/kmap() and replace with kmalloc(PAGE_SIZE, ...)
to get a page aligned kernel address.

In addition add a comment to document the alignment requirements so that
others don't attempt to 'fix' this again.

Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
Cc: Jethro Beekman <jethro@xxxxxxxxxxxx>
Cc: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>

---
Changes from v5[5]:
From Boris
Clean up commit msg and comment

Changes from v4[4]:
Add Ack and Reviews
Send to the correct maintainers

Changes from v3[3]:
Remove BUILD_BUG_ONs

Changes from v2[2]:
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@xxxxxxxxx/
[2] https://lore.kernel.org/lkml/20210202013725.3514671-1-ira.weiny@xxxxxxxxx/
[3] https://lore.kernel.org/lkml/20210205050850.GC5033@xxxxxxxxxxxxxxxxxxxxxxxxx/#t
[4] https://lore.kernel.org/lkml/YCBY02iEKLVyj7Ix@xxxxxxxxxx/
[5] https://lore.kernel.org/lkml/20210306002058.303796-1-ira.weiny@xxxxxxxxx/
---
arch/x86/kernel/cpu/sgx/ioctl.c | 14 ++++++++------
1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/cpu/sgx/ioctl.c b/arch/x86/kernel/cpu/sgx/ioctl.c
index 90a5caf76939..2e10367ea66c 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 +614,15 @@ 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 boundary and 'token' on a 512 byte
+ * boundary. kmalloc() will give this alignment when allocating
+ * PAGE_SIZE bytes.
+ */
+ sigstruct = kmalloc(PAGE_SIZE, GFP_KERNEL);
+ if (!sigstruct)
return -ENOMEM;

- sigstruct = kmap(initp_page);
token = (void *)((unsigned long)sigstruct + PAGE_SIZE / 2);
memset(token, 0, SGX_LAUNCH_TOKEN_SIZE);

@@ -645,8 +648,7 @@ 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