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SubjectRe: [PATCH V3] x86: Remove unnecessary kmap() from sgx_ioc_enclave_init()
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.

> 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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