On Fri, Apr 03, 2020 at 12:35:50PM +0300, Jarkko Sakkinen wrote:
> From: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> @@ -221,12 +224,16 @@ int sgx_encl_mm_add(struct sgx_encl *encl, struct mm_struct *mm)
> return ret;
> }
>
> + /*
> + * The page reclaimer uses list version for synchronization instead of
> + * synchronize_scru() because otherwise we could conflict with
> + * dup_mmap().
> + */
> spin_lock(&encl->mm_lock);
> list_add_rcu(&encl_mm->list, &encl->mm_list);
You dropped the smp_wmb().
> + encl->mm_list_version++;
> spin_unlock(&encl->mm_lock);
>
> - synchronize_srcu(&encl->srcu);
> -
> return 0;
> }
>
> diff --git a/arch/x86/kernel/cpu/sgx/encl.h b/arch/x86/kernel/cpu/sgx/encl.h
> index 44b353aa8866..f0f72e591244 100644
> --- a/arch/x86/kernel/cpu/sgx/encl.h
> +++ b/arch/x86/kernel/cpu/sgx/encl.h
> @@ -74,6 +74,7 @@ struct sgx_encl {
> struct mutex lock;
> struct list_head mm_list;
> spinlock_t mm_lock;
> + unsigned long mm_list_version;
> struct file *backing;
> struct kref refcount;
> struct srcu_struct srcu;
> diff --git a/arch/x86/kernel/cpu/sgx/reclaim.c b/arch/x86/kernel/cpu/sgx/reclaim.c
> index 39f0ddefbb79..5fb8bdfa6a1f 100644
> --- a/arch/x86/kernel/cpu/sgx/reclaim.c
> +++ b/arch/x86/kernel/cpu/sgx/reclaim.c
> @@ -184,28 +184,38 @@ static void sgx_reclaimer_block(struct sgx_epc_page *epc_page)
> struct sgx_encl_page *page = epc_page->owner;
> unsigned long addr = SGX_ENCL_PAGE_ADDR(page);
> struct sgx_encl *encl = page->encl;
> + unsigned long mm_list_version;
> struct sgx_encl_mm *encl_mm;
> struct vm_area_struct *vma;
> int idx, ret;
>
> - idx = srcu_read_lock(&encl->srcu);
> + do {
> + mm_list_version = encl->mm_list_version;
>
> - list_for_each_entry_rcu(encl_mm, &encl->mm_list, list) {
> - if (!mmget_not_zero(encl_mm->mm))
> - continue;
> + /* Fence reads as the CPU can reorder them. This guarantees
> + * that we don't access old list with a new version.
This comment is flat out wrong. This has nothing to do the CPU reordering
things. The smp_{r,w}mb() are nothing more than compiler barriers, and
even those go away when the kernel is built with SMP=0.
I don't mind gutting the other comments, but there is a well established
pattern form smb_wmb()/smp_rmb() pairs, I would strongly prefer to keep
the exact comment I submitted.
> + */
> + smp_rmb();
>
> - down_read(&encl_mm->mm->mmap_sem);
> + idx = srcu_read_lock(&encl->srcu);
>
> - ret = sgx_encl_find(encl_mm->mm, addr, &vma);
> - if (!ret && encl == vma->vm_private_data)
> - zap_vma_ptes(vma, addr, PAGE_SIZE);
> + list_for_each_entry_rcu(encl_mm, &encl->mm_list, list) {
> + if (!mmget_not_zero(encl_mm->mm))
> + continue;
>
> - up_read(&encl_mm->mm->mmap_sem);
> + down_read(&encl_mm->mm->mmap_sem);
>
> - mmput_async(encl_mm->mm);
> - }
> + ret = sgx_encl_find(encl_mm->mm, addr, &vma);
> + if (!ret && encl == vma->vm_private_data)
> + zap_vma_ptes(vma, addr, PAGE_SIZE);
>
> - srcu_read_unlock(&encl->srcu, idx);
> + up_read(&encl_mm->mm->mmap_sem);
> +
> + mmput_async(encl_mm->mm);
> + }
> +
> + srcu_read_unlock(&encl->srcu, idx);
> + } while (unlikely(encl->mm_list_version != mm_list_version));
>
> mutex_lock(&encl->lock);
>
> @@ -250,6 +260,11 @@ static const cpumask_t *sgx_encl_ewb_cpumask(struct sgx_encl *encl)
> struct sgx_encl_mm *encl_mm;
> int idx;
>
> + /*
> + * Can race with sgx_encl_mm_add(), but ETRACK has already been
> + * executed, which means that the CPUs running in the new mm will enter
> + * into the enclave with a fresh epoch.
> + */
> cpumask_clear(cpumask);
>
> idx = srcu_read_lock(&encl->srcu);
> --
> 2.25.1
>
