Re: [PATCH] locking/lockdep: Fix potential buffer overrun problem in stack_trace[]

[Bart Van Assche <bvanassche@acm.org>]

On 2019-12-19 10:28, Waiman Long wrote:
> If the lockdep code is really running out of the stack_trace entries,
> there is a possiblity that buffer overrun can happen and corrupt the
             ^^^^^^^^^^
             possibility?
> data immediately after stack_trace[].
> 
> If there is less than LOCK_TRACE_SIZE_IN_LONGS entries left before
> the call to save_trace(), the max_entries computation will leave it
> with a very large positive number because of its unsigned nature. The
> subsequent call to stack_trace_save() will then corrupt the data after
> stack_trace[]. Fix that by changing max_entries to a signed integer
> and check for negative value before calling stack_trace_save().
> 
> Fixes: 12593b7467f9 ("locking/lockdep: Reduce space occupied by stack traces")
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  kernel/locking/lockdep.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 32282e7112d3..56e260a7582f 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -482,7 +482,7 @@ static struct lock_trace *save_trace(void)
>  	struct lock_trace *trace, *t2;
>  	struct hlist_head *hash_head;
>  	u32 hash;
> -	unsigned int max_entries;
> +	int max_entries;
>  
>  	BUILD_BUG_ON_NOT_POWER_OF_2(STACK_TRACE_HASH_SIZE);
>  	BUILD_BUG_ON(LOCK_TRACE_SIZE_IN_LONGS >= MAX_STACK_TRACE_ENTRIES);
> @@ -490,10 +490,8 @@ static struct lock_trace *save_trace(void)
>  	trace = (struct lock_trace *)(stack_trace + nr_stack_trace_entries);
>  	max_entries = MAX_STACK_TRACE_ENTRIES - nr_stack_trace_entries -
>  		LOCK_TRACE_SIZE_IN_LONGS;
> -	trace->nr_entries = stack_trace_save(trace->entries, max_entries, 3);
>  
> -	if (nr_stack_trace_entries >= MAX_STACK_TRACE_ENTRIES -
> -	    LOCK_TRACE_SIZE_IN_LONGS - 1) {
> +	if (max_entries < 0) {
>  		if (!debug_locks_off_graph_unlock())
>  			return NULL;
>  
> @@ -502,6 +500,7 @@ static struct lock_trace *save_trace(void)
>  
>  		return NULL;
>  	}
> +	trace->nr_entries = stack_trace_save(trace->entries, max_entries, 3);
>  
>  	hash = jhash(trace->entries, trace->nr_entries *
>  		     sizeof(trace->entries[0]), 0);

I'm not sure whether it is useful to call stack_trace_save() if
max_entries == 0. How about changing the "max_entries < 0" test into
"max_entries <= 0"?

Thanks,

Bart.