linux-kernel - [PATCH] x86/sgx: Remove unnecessary kmap() from sgx_ioc_enclave

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Date:   Wed, 24 Mar 2021 11:22:46 -0700
From:   ira.weiny@...el.com
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org
Cc:     Ira Weiny <ira.weiny@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Jethro Beekman <jethro@...tanix.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-sgx@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] x86/sgx: Remove unnecessary kmap() from sgx_ioc_enclave_init()

From: Ira Weiny <ira.weiny@...el.com>

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@...gle.com>
Cc: Jethro Beekman <jethro@...tanix.com>
Cc: Jarkko Sakkinen <jarkko@...nel.org>
Cc: Dave Hansen <dave.hansen@...el.com>
Signed-off-by: Ira Weiny <ira.weiny@...el.com>

---
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@intel.com/
[2] https://lore.kernel.org/lkml/20210202013725.3514671-1-ira.weiny@intel.com/
[3] https://lore.kernel.org/lkml/20210205050850.GC5033@iweiny-DESK2.sc.intel.com/#t
[4] https://lore.kernel.org/lkml/YCBY02iEKLVyj7Ix@kernel.org/
[5] https://lore.kernel.org/lkml/20210306002058.303796-1-ira.weiny@intel.com/
---
 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