Re: [PATCH] x86/sgx: Fix deadlock and race conditions between fork() and EPC reclaim

[Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>]

On Wed, Mar 18, 2020 at 05:39:07PM +0200, Jarkko Sakkinen wrote:
> On Mon, Mar 16, 2020 at 10:15:39PM -0700, Sean Christopherson wrote:
> > 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.
> > 
> > Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> > ---
> >  arch/x86/kernel/cpu/sgx/encl.c    | 22 +++++++++++++++++++--
> >  arch/x86/kernel/cpu/sgx/encl.h    |  1 +
> >  arch/x86/kernel/cpu/sgx/reclaim.c | 33 +++++++++++++++++++++++++++++++
> >  3 files changed, 54 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
> > index d6a19bdd1921..b9a7c56f7c25 100644
> > --- a/arch/x86/kernel/cpu/sgx/encl.c
> > +++ b/arch/x86/kernel/cpu/sgx/encl.c
> > @@ -196,6 +196,12 @@ int sgx_encl_mm_add(struct sgx_encl *encl, struct mm_struct *mm)
> >  	struct sgx_encl_mm *encl_mm;
> >  	int ret;
> >  
> > +	/*
> > +	 * This flow relies on mmap_sem to provide mutual exclusivity (for a
> > +	 * given mm) to prevent duplicate instances of an encl_mm on the list.
> > +	 */
> > +	lockdep_assert_held_write(&mm->mmap_sem);
> > +
> >  	if (atomic_read(&encl->flags) & SGX_ENCL_DEAD)
> >  		return -EINVAL;
> >  
> > @@ -223,10 +229,22 @@ int sgx_encl_mm_add(struct sgx_encl *encl, struct mm_struct *mm)
> >  
> >  	spin_lock(&encl->mm_lock);
> >  	list_add_rcu(&encl_mm->list, &encl->mm_list);
> > +	/*
> > +	 * Ensure the mm is added to the list before updating the version.
> > +	 * Pairs with the smp_rmb() in sgx_reclaimer_block().
> > +	 */
> > +	smp_wmb();
> > +	encl->mm_list_version++;
> >  	spin_unlock(&encl->mm_lock);
> >  
> > -	synchronize_srcu(&encl->srcu);
> > -
> > +	/*
> > +	 * DO NOT call synchronize_srcu()!  When this is called via dup_mmap(),
> > +	 * mmap_sem is held for write in both the old mm and new mm, and the
> > +	 * reclaimer may be holding srcu for read while waiting on down_read()
> > +	 * for the old mm's mmap_sem, i.e. synchronizing will deadlock.
> > +	 * Incrementing the list version ensures readers that must not race
> > +	 * with a mm being added will see the updated list.
> > +	 */

For this comment, please completely remove it. We either call something
or do not call it. We do !call anything.

> >  	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..3b4b849c5b2e 100644
> > --- a/arch/x86/kernel/cpu/sgx/reclaim.c
> > +++ b/arch/x86/kernel/cpu/sgx/reclaim.c
> > @@ -155,6 +155,11 @@ static bool sgx_reclaimer_age(struct sgx_epc_page *epc_page)
> >  	bool ret = true;
> >  	int idx;
> >  
> > +	/*
> > +	 * Note, this can race with sgx_encl_mm_add(), but worst case scenario
> > +	 * a page will be reclaimed immediately after it's accessed in the new
> > +	 * process/mm.
> > +	 */
> >  	idx = srcu_read_lock(&encl->srcu);
> >  
> >  	list_for_each_entry_rcu(encl_mm, &encl->mm_list, list) {
> > @@ -184,10 +189,20 @@ 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;
> >  
> > +retry:
> > +	mm_list_version = encl->mm_list_version;
> > +	/*
> > +	 * Ensure the list version is read before walking the list to prevent
> > +	 * beginning the walk with the old list using the new version.  Pairs
> > +	 * with the smp_wmb() in sgx_encl_mm_add().
> > +	 */
> > +	smp_rmb();
> > +
> >  	idx = srcu_read_lock(&encl->srcu);
> >  
> >  	list_for_each_entry_rcu(encl_mm, &encl->mm_list, list) {
> > @@ -207,6 +222,19 @@ static void sgx_reclaimer_block(struct sgx_epc_page *epc_page)
> >  
> >  	srcu_read_unlock(&encl->srcu, idx);
> >  
> > +	/*
> > +	 * Redo the zapping if a mm was added to the list while zapping was in
> > +	 * progress.  dup_mmap() copies the PTEs for VM_PFNMAP VMAs, i.e. the
> > +	 * new mm won't take a page fault and so won't see that the page is
> > +	 * tagged RECLAIMED.  Note, vm_ops->open()/sgx_encl_mm_add() is called
> > +	 * _after_ PTEs are copied, and dup_mmap() holds the old mm's mmap_sem
> > +	 * for write, so the version check is only needed to protect against
> > +	 * dup_mmap() running after the list walk started but before the old
> > +	 * mm's PTEs were zapped.
> > +	 */
> > +	if (unlikely(encl->mm_list_version != mm_list_version))
> > +		goto retry;
> > +
> >  	mutex_lock(&encl->lock);
> >  
> >  	if (!(atomic_read(&encl->flags) & SGX_ENCL_DEAD)) {
> > @@ -250,6 +278,11 @@ static const cpumask_t *sgx_encl_ewb_cpumask(struct sgx_encl *encl)
> >  	struct sgx_encl_mm *encl_mm;
> >  	int idx;
> >  
> > +	/*
> > +	 * Note, this can race with sgx_encl_mm_add(), but ETRACK has already
> > +	 * been executed, so CPUs running in the new mm will enter the enclave
> > +	 * in a different epoch.
> > +	 */
> >  	cpumask_clear(cpumask);
> >  
> >  	idx = srcu_read_lock(&encl->srcu);
> > -- 
> > 2.24.1
> > 
> 
> Please recheck the remarks that I made about inline comments in the
> source code.

/Jarkko