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Date:   Wed, 22 May 2019 17:54:17 -0700
From:   Ralph Campbell <rcampbell@...dia.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        John Hubbard <jhubbard@...dia.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Arnd Bergmann <arnd@...db.de>,
        Balbir Singh <bsingharora@...il.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Souptick Joarder <jrdr.linux@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 5/5] mm/hmm: Fix mm stale reference use in hmm_free()


On 5/22/19 4:36 PM, Jason Gunthorpe wrote:
> On Mon, May 06, 2019 at 04:35:14PM -0700, rcampbell@...dia.com wrote:
>> From: Ralph Campbell <rcampbell@...dia.com>
>>
>> The last reference to struct hmm may be released long after the mm_struct
>> is destroyed because the struct hmm_mirror memory may be part of a
>> device driver open file private data pointer. The file descriptor close
>> is usually after the mm_struct is destroyed in do_exit(). This is a good
>> reason for making struct hmm a kref_t object [1] since its lifetime spans
>> the life time of mm_struct and struct hmm_mirror.
> 
>> The fix is to not use hmm->mm in hmm_free() and to clear mm->hmm and
>> hmm->mm pointers in hmm_destroy() when the mm_struct is
>> destroyed.
> 
> I think the right way to fix this is to have the struct hmm hold a
> mmgrab() on the mm so its memory cannot go away until all of the hmm
> users release the struct hmm, hmm_ranges/etc
> 
> Then we can properly use mmget_not_zero() instead of the racy/abnormal
> 'if (hmm->xmm == NULL || hmm->dead)' pattern (see the other
> thread). Actually looking at this, all these tests look very
> questionable. If we hold the mmget() for the duration of the range
> object, as Jerome suggested, then they all get deleted.
> 
> That just leaves mmu_notifier_unregister_no_relase() as the remaining
> user of hmm->mm (everyone else is trying to do range->mm) - and it
> looks like it currently tries to call
> mmu_notifier_unregister_no_release on a NULL hmm->mm and crashes :(
> 
> Holding the mmgrab fixes this as we can safely call
> mmu_notifier_unregister_no_relase() post exit_mmap on a grab'd mm.
> 
> Also we can delete the hmm_mm_destroy() intrustion into fork.c as it
> can't be called when the mmgrab is active.
> 
> This is the basic pattern we used in ODP when working with mmu
> notifiers, I don't know why hmm would need to be different.
> 
>> index 2aa75dbed04a..4e42c282d334 100644
>> +++ b/mm/hmm.c
>> @@ -43,8 +43,10 @@ static inline struct hmm *mm_get_hmm(struct mm_struct *mm)
>>   {
>>   	struct hmm *hmm = READ_ONCE(mm->hmm);
>>   
>> -	if (hmm && kref_get_unless_zero(&hmm->kref))
>> +	if (hmm && !hmm->dead) {
>> +		kref_get(&hmm->kref);
>>   		return hmm;
>> +	}
> 
> hmm->dead and mm->hmm are not being read under lock, so this went from
> something almost thread safe to something racy :(
> 
>> @@ -53,25 +55,28 @@ static inline struct hmm *mm_get_hmm(struct mm_struct *mm)
>>    * hmm_get_or_create - register HMM against an mm (HMM internal)
>>    *
>>    * @mm: mm struct to attach to
>> - * Returns: returns an HMM object, either by referencing the existing
>> - *          (per-process) object, or by creating a new one.
>> + * Return: an HMM object reference, either by referencing the existing
>> + *         (per-process) object, or by creating a new one.
>>    *
>> - * This is not intended to be used directly by device drivers. If mm already
>> - * has an HMM struct then it get a reference on it and returns it. Otherwise
>> - * it allocates an HMM struct, initializes it, associate it with the mm and
>> - * returns it.
>> + * If the mm already has an HMM struct then return a new reference to it.
>> + * Otherwise, allocate an HMM struct, initialize it, associate it with the mm,
>> + * and return a new reference to it. If the return value is not NULL,
>> + * the caller is responsible for calling hmm_put().
>>    */
>>   static struct hmm *hmm_get_or_create(struct mm_struct *mm)
>>   {
>> -	struct hmm *hmm = mm_get_hmm(mm);
>> -	bool cleanup = false;
>> +	struct hmm *hmm = mm->hmm;
>>   
>> -	if (hmm)
>> -		return hmm;
>> +	if (hmm) {
>> +		if (hmm->dead)
>> +			goto error;
> 
> Create shouldn't fail just because it is racing with something doing
> destroy
> 
> The flow should be something like:
> 
> spin_lock(&mm->page_table_lock); // or write side mmap_sem if you prefer
> if (mm->hmm)
>     if (kref_get_unless_zero(mm->hmm))
>          return mm->hmm;
>     mm->hmm = NULL
> 
> 
>> +		goto out;
>> +	}
>>   
>>   	hmm = kmalloc(sizeof(*hmm), GFP_KERNEL);
>>   	if (!hmm)
>> -		return NULL;
>> +		goto error;
>> +
>>   	init_waitqueue_head(&hmm->wq);
>>   	INIT_LIST_HEAD(&hmm->mirrors);
>>   	init_rwsem(&hmm->mirrors_sem);
>> @@ -83,47 +88,32 @@ static struct hmm *hmm_get_or_create(struct mm_struct *mm)
>>   	hmm->dead = false;
>>   	hmm->mm = mm;
>>   
>> -	spin_lock(&mm->page_table_lock);
>> -	if (!mm->hmm)
>> -		mm->hmm = hmm;
>> -	else
>> -		cleanup = true;
>> -	spin_unlock(&mm->page_table_lock);
> 
> BTW, Jerome this needs fixing too, it shouldn't fail the function just
> because it lost the race.
> 
> More like
> 
> spin_lock(&mm->page_table_lock);
> if (mm->hmm)
>     if (kref_get_unless_zero(mm->hmm)) {
>          kfree(hmm);
>          return mm->hmm;
>     }
> mm->hmm = hmm
> 
>> -	if (cleanup)
>> -		goto error;
>> -
>>   	/*
>> -	 * We should only get here if hold the mmap_sem in write mode ie on
>> -	 * registration of first mirror through hmm_mirror_register()
>> +	 * The mmap_sem should be held for write so no additional locking
> 
> Please let us have proper lockdep assertions for this kind of stuff.
> 
>> +	 * is needed. Note that struct_mm holds a reference to hmm.
>> +	 * It is cleared in hmm_release().
>>   	 */
>> +	mm->hmm = hmm;
> 
> Actually using the write side the mmap_sem seems sort of same if it is
> assured the write side is always held for this call..
> 
> 
> Hmm, there is a race with hmm_destroy touching mm->hmm that does
> hold the write lock.
> 
>> +
>>   	hmm->mmu_notifier.ops = &hmm_mmu_notifier_ops;
>>   	if (__mmu_notifier_register(&hmm->mmu_notifier, mm))
>>   		goto error_mm;
> 
> And the error unwind here is problematic as it should do
> kref_put. Actually after my patch to use container_of this
> mmu_notifier_register should go before the mm->hmm = hmm to avoid
> having to do the sketchy error unwind at all.
> 
>> +out:
>> +	/* Return a separate hmm reference for the caller. */
>> +	kref_get(&hmm->kref);
>>   	return hmm;
>>   
>>   error_mm:
>> -	spin_lock(&mm->page_table_lock);
>> -	if (mm->hmm == hmm)
>> -		mm->hmm = NULL;
>> -	spin_unlock(&mm->page_table_lock);
>> -error:
>> +	mm->hmm = NULL;
>>   	kfree(hmm);
>> +error:
>>   	return NULL;
>>   }
>>   
>>   static void hmm_free(struct kref *kref)
>>   {
>>   	struct hmm *hmm = container_of(kref, struct hmm, kref);
>> -	struct mm_struct *mm = hmm->mm;
>> -
>> -	mmu_notifier_unregister_no_release(&hmm->mmu_notifier, mm);
> 
> Where did the unregister go?
> 
>> -
>> -	spin_lock(&mm->page_table_lock);
>> -	if (mm->hmm == hmm)
>> -		mm->hmm = NULL;
>> -	spin_unlock(&mm->page_table_lock);
> 
> Well, we still need to NULL mm->hmm if the hmm was put before the mm
> is destroyed.
> 
>>   	kfree(hmm);
>>   }
>> @@ -135,25 +125,18 @@ static inline void hmm_put(struct hmm *hmm)
>>   
>>   void hmm_mm_destroy(struct mm_struct *mm)
>>   {
>> -	struct hmm *hmm;
>> +	struct hmm *hmm = mm->hmm;
>>   
>> -	spin_lock(&mm->page_table_lock);
>> -	hmm = mm_get_hmm(mm);
>> -	mm->hmm = NULL;
>>   	if (hmm) {
>> +		mm->hmm = NULL;
> 
> At this point The kref on mm is 0, so any other thread reading mm->hmm
> has a use-after-free bug. Not much point in doing this assignment , it
> is just confusing.
> 
>>   		hmm->mm = NULL;
>> -		hmm->dead = true;
>> -		spin_unlock(&mm->page_table_lock);
>>   		hmm_put(hmm);
>> -		return;
>>   	}
>> -
>> -	spin_unlock(&mm->page_table_lock);
>>   }
>>   
>>   static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm)
>>   {
>> -	struct hmm *hmm = mm_get_hmm(mm);
>> +	struct hmm *hmm = mm->hmm;
> 
> container_of is much safer/better
> 
>> @@ -931,20 +909,14 @@ int hmm_range_register(struct hmm_range *range,
>>   		return -EINVAL;
>>   	if (start >= end)
>>   		return -EINVAL;
>> +	hmm = mm_get_hmm(mm);
>> +	if (!hmm)
>> +		return -EFAULT;
>>   
>>   	range->page_shift = page_shift;
>>   	range->start = start;
>>   	range->end = end;
>> -
>> -	range->hmm = mm_get_hmm(mm);
>> -	if (!range->hmm)
>> -		return -EFAULT;
>> -
>> -	/* Check if hmm_mm_destroy() was call. */
>> -	if (range->hmm->mm == NULL || range->hmm->dead) {
> 
> This comment looks bogus too, we can't race with hmm_mm_destroy as the
> caller MUST have a mmgrab or mmget on the mm already to call this API
> - ie can't be destroyed.
> 
> As discussed in the other thread this should probably be
> mmget_not_zero.
> 
> Jason

I think you missed the main points which are:

1) mm->hmm holds a reference to struct hmm so hmm isn't going away until
    __mmdrop() is called. hmm->mm is not a reference to mm,
   just a "backward" pointer.
   Trying to make struct hmm hold a *reference* to mm seems wrong to me.

2) mm->hmm is only set with mm->mmap_sem held for write.
    mm->hmm is only cleared when __mmdrop() is called.
    hmm->mm is only cleared when __mmdrop() is called so it is long 
after the call to hmm_release().

3) The mmu notifier unregister happens only as part of exit_mmap().

The hmm->dead and hmm->mm == NULL checks are more for sanity checking
since hmm_mirror_register() shouldn't be called without holding mmap_sem.
A VM_WARN or other check makes sense like you said.

Anyway, I'll wait for Jerome to weigh in as to how to proceed.

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