Re: [PATCH v2] x86/sgx: Fix deadlock and race conditions between fork() and EPC reclaim

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

On Fri, Apr 03, 2020 at 12:35:50PM +0300, Jarkko Sakkinen wrote:
> From: Sean Christopherson <sean.j.christopherson@intel.com>
> 
> 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@linux.intel.com>
> Cc: Sean Christopherson <sean.j.christopherson@intel.com>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
> ---
> 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