Re: bpf: use-after-free in array_map_alloc

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Mon, May 23, 2016 at 05:35:01PM -0400, Tejun Heo wrote:
> Hello,
> 
> Can you please test whether this patch resolves the issue?  While
> adding support for atomic allocations, I reduced alloc_mutex covered
> region too much.

after the patch the use-after-free is no longer seen.
Tested-by: Alexei Starovoitov <ast@kernel.org>

> 
> Thanks.
> 
> diff --git a/mm/percpu.c b/mm/percpu.c
> index 0c59684..bd2df70 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -162,7 +162,7 @@ static struct pcpu_chunk *pcpu_reserved_chunk;
>  static int pcpu_reserved_chunk_limit;
>  
>  static DEFINE_SPINLOCK(pcpu_lock);	/* all internal data structures */
> -static DEFINE_MUTEX(pcpu_alloc_mutex);	/* chunk create/destroy, [de]pop */
> +static DEFINE_MUTEX(pcpu_alloc_mutex);	/* chunk create/destroy, [de]pop, map extension */
>  
>  static struct list_head *pcpu_slot __read_mostly; /* chunk list slots */
>  
> @@ -435,6 +435,8 @@ static int pcpu_extend_area_map(struct pcpu_chunk *chunk, int new_alloc)
>  	size_t old_size = 0, new_size = new_alloc * sizeof(new[0]);
>  	unsigned long flags;
>  
> +	lockdep_assert_held(&pcpu_alloc_mutex);
> +
>  	new = pcpu_mem_zalloc(new_size);
>  	if (!new)
>  		return -ENOMEM;
> @@ -895,6 +897,9 @@ static void __percpu *pcpu_alloc(size_t size, size_t align, bool reserved,
>  		return NULL;
>  	}
>  
> +	if (!is_atomic)
> +		mutex_lock(&pcpu_alloc_mutex);
> +
>  	spin_lock_irqsave(&pcpu_lock, flags);
>  
>  	/* serve reserved allocations from the reserved chunk if available */
> @@ -967,12 +972,11 @@ static void __percpu *pcpu_alloc(size_t size, size_t align, bool reserved,
>  	if (is_atomic)
>  		goto fail;
>  
> -	mutex_lock(&pcpu_alloc_mutex);
> +	lockdep_assert_held(&pcpu_alloc_mutex);
>  
>  	if (list_empty(&pcpu_slot[pcpu_nr_slots - 1])) {
>  		chunk = pcpu_create_chunk();
>  		if (!chunk) {
> -			mutex_unlock(&pcpu_alloc_mutex);
>  			err = "failed to allocate new chunk";
>  			goto fail;
>  		}
> @@ -983,7 +987,6 @@ static void __percpu *pcpu_alloc(size_t size, size_t align, bool reserved,
>  		spin_lock_irqsave(&pcpu_lock, flags);
>  	}
>  
> -	mutex_unlock(&pcpu_alloc_mutex);
>  	goto restart;
>  
>  area_found:
> @@ -993,8 +996,6 @@ static void __percpu *pcpu_alloc(size_t size, size_t align, bool reserved,
>  	if (!is_atomic) {
>  		int page_start, page_end, rs, re;
>  
> -		mutex_lock(&pcpu_alloc_mutex);
> -
>  		page_start = PFN_DOWN(off);
>  		page_end = PFN_UP(off + size);
>  
> @@ -1005,7 +1006,6 @@ static void __percpu *pcpu_alloc(size_t size, size_t align, bool reserved,
>  
>  			spin_lock_irqsave(&pcpu_lock, flags);
>  			if (ret) {
> -				mutex_unlock(&pcpu_alloc_mutex);
>  				pcpu_free_area(chunk, off, &occ_pages);
>  				err = "failed to populate";
>  				goto fail_unlock;
> @@ -1045,6 +1045,8 @@ static void __percpu *pcpu_alloc(size_t size, size_t align, bool reserved,
>  		/* see the flag handling in pcpu_blance_workfn() */
>  		pcpu_atomic_alloc_failed = true;
>  		pcpu_schedule_balance_work();
> +	} else {
> +		mutex_unlock(&pcpu_alloc_mutex);
>  	}
>  	return NULL;
>  }
> @@ -1137,6 +1139,8 @@ static void pcpu_balance_workfn(struct work_struct *work)
>  	list_for_each_entry_safe(chunk, next, &to_free, list) {
>  		int rs, re;
>  
> +		cancel_work_sync(&chunk->map_extend_work);
> +
>  		pcpu_for_each_pop_region(chunk, rs, re, 0, pcpu_unit_pages) {
>  			pcpu_depopulate_chunk(chunk, rs, re);
>  			spin_lock_irq(&pcpu_lock);

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