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Subject[PATCH] x86/sgx: rename and document SGX bit lock
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SGX ioctl() calls are serialized with a lock. It's a weird open-coded
lock that is not even called a "lock". That makes it a weird beast,
but Sean has convinced me it's a good idea without better alternatives.

Give the lock bit a better name, and document what it actually trying
to do.

Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Jarkko Sakkinen <jarkko@kernel.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: x86@kernel.org

---

b/arch/x86/kernel/cpu/sgx/encl.h | 2 +-
b/arch/x86/kernel/cpu/sgx/ioctl.c | 19 ++++++++++++++++---
2 files changed, 17 insertions(+), 4 deletions(-)

diff -puN arch/x86/kernel/cpu/sgx/ioctl.c~sgx-encl-flags arch/x86/kernel/cpu/sgx/ioctl.c
--- a/arch/x86/kernel/cpu/sgx/ioctl.c~sgx-encl-flags 2021-01-12 14:02:24.480689006 -0800
+++ b/arch/x86/kernel/cpu/sgx/ioctl.c 2021-01-12 14:02:24.486689006 -0800
@@ -690,8 +690,21 @@ long sgx_ioctl(struct file *filep, unsig
struct sgx_encl *encl = filep->private_data;
int ret;

- if (test_and_set_bit(SGX_ENCL_IOCTL, &encl->flags))
- return -EBUSY;
+ /*
+ * Behold, the Big SGX Lock
+ *
+ * The primary function of this "lock" is to actively discourage
+ * attempts at multi-threaded enclave management. Enclave management
+ * is fundamentally a single-threaded affair. Enclave measurement,
+ * for instance would be worthless if two ADD_PAGES instances raced
+ * and occurred in different orders.
+ *
+ * encl->lock is ill suited for this because it would need to be
+ * conditionally dropped and reqacuired for operations like enclave
+ * page allocation and reclaim.
+ */
+ if (test_and_set_bit(SGX_ENCL_IOCTL_LOCK, &encl->flags))
+ return -EINVAL;

switch (cmd) {
case SGX_IOC_ENCLAVE_CREATE:
@@ -711,6 +724,6 @@ long sgx_ioctl(struct file *filep, unsig
break;
}

- clear_bit(SGX_ENCL_IOCTL, &encl->flags);
+ clear_bit(SGX_ENCL_IOCTL_LOCK, &encl->flags);
return ret;
}
diff -puN arch/x86/kernel/cpu/sgx/encl.h~sgx-encl-flags arch/x86/kernel/cpu/sgx/encl.h
--- a/arch/x86/kernel/cpu/sgx/encl.h~sgx-encl-flags 2021-01-12 14:02:24.482689006 -0800
+++ b/arch/x86/kernel/cpu/sgx/encl.h 2021-01-12 14:16:37.511686879 -0800
@@ -34,7 +34,7 @@ struct sgx_encl_page {
};

enum sgx_encl_flags {
- SGX_ENCL_IOCTL = BIT(0),
+ SGX_ENCL_IOCTL_LOCK = BIT(0), /* See sgx_ioctl() */
SGX_ENCL_DEBUG = BIT(1),
SGX_ENCL_CREATED = BIT(2),
SGX_ENCL_INITIALIZED = BIT(3),
_
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