Re: 3.9-rc1 NULL pointer crash at find_pid_ns

[ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org (Eric W. Biederman)]

Sasha Levin <sasha.levin-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org> writes:

> On 03/07/2013 12:46 PM, Eric Dumazet wrote:
>> On Thu, 2013-03-07 at 12:36 -0500, Sasha Levin wrote:
>> 
>>> Looks like the hlist change is probably the issue, though it specifically
>>> uses:
>>>
>>> 	#define hlist_entry_safe(ptr, type, member) \
>>>         	(ptr) ? hlist_entry(ptr, type, member) : NULL
>>>
>>> I'm still looking at the code in question and it's assembly, but I can't
>>> figure out what's going wrong. I was also trying to see what's so special
>>> about this loop in find_pid_ns as opposed to the rest of the kernel code
>>> that uses hlist_for_each_entry_rcu() but couldn't find out why.
>>>
>>> Is it somehow possible that if we rcu_dereference_raw() the same thing twice
>>> inside the same rcu_read_lock() section we'll get different results? That's
>>> really the only reason for this crash that comes to mind at the moment, very
>>> unlikely - but that's all I have right now.
>>>
>> 
>> Yep
>> 
>> #define hlist_entry_safe(ptr, type, member) \
>> 	(ptr) ? hlist_entry(ptr, type, member) : NULL
>> 
>> Is not safe, as ptr can be evaluated twice, and thats not good at all...
>
> ptr is being evaluated twice, but in this case this is an
> rcu_dereference_raw() value within the same rcu_read_lock() section.
>
> Is it still problematic?

Definitely.

Head in this instance the expression: &pid_hash[pid_hashfn(nr, ns)]

And the crash clearly shows that when hilst_entry is being evaluated the
HEAD is NULL.

Perhaps this shows up in proc because the hash chains are short and
frequently NULL?

Eric