On Fri, Apr 03, 2020 at 12:35:50PM +0300, Jarkko Sakkinen wrote:
> From: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
>
> Drop the synchronize_srcu() from sgx_encl_mm_add() and replace it with a
> mm_list versioning concept to avoid deadlock when adding a mm during
> dup_mmap()/fork(), and to ensure copied PTEs are zapped.
>
> When dup_mmap() runs, it holds mmap_sem for write in both the old mm and
> new mm. Invoking synchronize_srcu() while holding mmap_sem of a mm that
> is already attached to the enclave will deadlock if the reclaimer is in
> the process of walking mm_list, as the reclaimer will try to acquire
> mmap_sem (of the old mm) while holding encl->srcu for read.
>
> INFO: task ksgxswapd:181 blocked for more than 120 seconds.
> ksgxswapd D 0 181 2 0x80004000
> Call Trace:
> __schedule+0x2db/0x700
> schedule+0x44/0xb0
> rwsem_down_read_slowpath+0x370/0x470
> down_read+0x95/0xa0
> sgx_reclaim_pages+0x1d2/0x7d0
> ksgxswapd+0x151/0x2e0
> kthread+0x120/0x140
> ret_from_fork+0x35/0x40
>
> INFO: task fork_consistenc:18824 blocked for more than 120 seconds.
> fork_consistenc D 0 18824 18786 0x00004320
> Call Trace:
> __schedule+0x2db/0x700
> schedule+0x44/0xb0
> schedule_timeout+0x205/0x300
> wait_for_completion+0xb7/0x140
> __synchronize_srcu.part.22+0x81/0xb0
> synchronize_srcu_expedited+0x27/0x30
> synchronize_srcu+0x57/0xe0
> sgx_encl_mm_add+0x12b/0x160
> sgx_vma_open+0x22/0x40
> dup_mm+0x521/0x580
> copy_process+0x1a56/0x1b50
> _do_fork+0x85/0x3a0
> __x64_sys_clone+0x8e/0xb0
> do_syscall_64+0x57/0x1b0
> entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> Furthermore, doing synchronize_srcu() in sgx_encl_mm_add() does not
> prevent the new mm from having stale PTEs pointing at the EPC page to be
> reclaimed. dup_mmap() calls vm_ops->open()/sgx_encl_mm_add() _after_
> PTEs are copied to the new mm, i.e. blocking fork() until reclaim zaps
> the old mm is pointless as the stale PTEs have already been created in
> the new mm.
>
> All other flows that walk mm_list can safely race with dup_mmap() or are
> protected by a different mechanism. Add comments to all srcu readers
> that don't check the list version to document why its ok for the flow to
> ignore the version.
>
> Note, synchronize_srcu() is still needed when removing a mm from an
> enclave, as the srcu readers must complete their walk before the mm can
> be freed. Removing a mm is never done while holding mmap_sem.
>
> Cc: Haitao Huang <haitao.huang@xxxxxxxxxxxxxxx>
> Cc: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx>
> ---
> v2:
> * Remove smp_wmb() as x86 does not reorder writes in the pipeline.
> * Refine comments to be more to the point and more maintainable when
> things might change.
> * Replace the ad hoc (goto-based) loop construct with a proper loop
> construct.
> arch/x86/kernel/cpu/sgx/encl.c | 11 +++++++--
> arch/x86/kernel/cpu/sgx/encl.h | 1 +
> arch/x86/kernel/cpu/sgx/reclaim.c | 39 +++++++++++++++++++++----------
> 3 files changed, 37 insertions(+), 14 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
> index e0124a2f22d5..5b15352b3d4f 100644
> --- a/arch/x86/kernel/cpu/sgx/encl.c
> +++ b/arch/x86/kernel/cpu/sgx/encl.c
> @@ -196,6 +196,9 @@ int sgx_encl_mm_add(struct sgx_encl *encl, struct mm_struct *mm)
> struct sgx_encl_mm *encl_mm;
> int ret;
>
> + /* mm_list can be accessed only by a single thread at a time. */
> + lockdep_assert_held_write(&mm->mmap_sem);
> +
> if (atomic_read(&encl->flags) & SGX_ENCL_DEAD)
> return -EINVAL;
>
> @@ -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);
> + 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.
> + */
> + 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));
This is bad, or at least makes the code unnecessarily hard to understand
given the use of the fences: encl->mm_list_version is read twice per
iteration.
I'll change the code to use "for ( ; ; )" and have exactly one read per
iteration.
/Jarko
