This has been shown in tests:

[ +0.000008] WARNING: CPU: 3 PID: 7620 at kernel/rcu/srcutree.c:374 cleanup_srcu_struct+0xed/0x100

There are two functions that drain encl->mm_list:

- sgx_release() (i.e. VFS release) removes the remaining mm_list entries. - sgx_mmu_notifier_release() removes mm_list entry for the registered process, if it still exists.

If encl->refcount is taken only for VFS, this can lead to sgx_encl_release() being executed before sgx_mmu_notifier_release() completes, which is exactly what happens in the above klog entry.

Each process also needs its own enclave reference.

In order to fix the race condition, increase encl->refcount when an entry to encl->mm_list added for a process. Release this reference when the mm_list entry is cleaned up, either in sgx_mmu_notifier_release() or sgx_release().

Cc: stable@vger.kernel.org Fixes: 1728ab54b4be ("x86/sgx: Add a page reclaimer") Reported-by: Haitao Huang haitao.huang@linux.intel.com Signed-off-by: Jarkko Sakkinen jarkko@kernel.org --- v6: - Maintain refcount for each encl->mm_list entry. v5: - To make sure that the instance does not get deleted use kref_get() kref_put(). This also removes the need for additional synchronize_srcu(). v4: - Rewrite the commit message. - Just change the call order. *_expedited() is out of scope for this bug fix. v3: Fine-tuned tags, and added missing change log for v2. v2: Switch to synchronize_srcu_expedited(). arch/x86/kernel/cpu/sgx/driver.c | 6 ++++++ arch/x86/kernel/cpu/sgx/encl.c | 8 ++++++++ 2 files changed, 14 insertions(+)

diff --git a/arch/x86/kernel/cpu/sgx/driver.c b/arch/x86/kernel/cpu/sgx/driver.c index f2eac41bb4ff..8d8fcc91c0d6 100644 --- a/arch/x86/kernel/cpu/sgx/driver.c +++ b/arch/x86/kernel/cpu/sgx/driver.c @@ -72,6 +72,12 @@ static int sgx_release(struct inode *inode, struct file *file) synchronize_srcu(&encl->srcu); mmu_notifier_unregister(&encl_mm->mmu_notifier, encl_mm->mm); kfree(encl_mm); + + /* + * Release the mm_list reference, as sgx_mmu_notifier_release() + * will only do this only, when it grabs encl_mm. + */ + kref_put(&encl->refcount, sgx_encl_release); }

kref_put(&encl->refcount, sgx_encl_release); diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c index ee50a5010277..c1d9c86c0265 100644 --- a/arch/x86/kernel/cpu/sgx/encl.c +++ b/arch/x86/kernel/cpu/sgx/encl.c @@ -474,6 +474,7 @@ static void sgx_mmu_notifier_release(struct mmu_notifier *mn, if (tmp == encl_mm) { synchronize_srcu(&encl_mm->encl->srcu); mmu_notifier_put(mn); + kref_put(&encl_mm->encl->refcount, sgx_encl_release); } }

@@ -545,6 +546,13 @@ int sgx_encl_mm_add(struct sgx_encl *encl, struct mm_struct *mm) }

spin_lock(&encl->mm_lock); + + /* + * Take a reference to guarantee that the enclave is not destroyed, + * while sgx_mmu_notifier_release() is active. + */ + kref_get(&encl->refcount); + list_add_rcu(&encl_mm->list, &encl->mm_list); /* Pairs with smp_rmb() in sgx_reclaimer_block(). */ smp_wmb();