Re: [Devel] [PATCH -mm] utrace: fix double free re __rcu_process_callbacks()

[Kirill Korotaev <dev@openvz.org>]

Roland,

can you please help with it?
current utrace state is far from being stable,
RHEL5 and -mm kernels can be quite easily crashed with some of the exploits
we collected so far.
Alexey can help you with any information needed - call traces, test cases,
but without your help we can't fix it all ourselfes :/

Thanks,
Kirill

Alexey Dobriyan wrote:
> The following patch fixes double free manifesting itself as crash in
> __rcu_process_callbasks():
> http://marc.info/?l=linux-kernel&m=117518764517017&w=2
> https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=229112
> 
> The problem is with check_dead_utrace() conditionally scheduling
> "struct utrace" for freeing but not cleaning struct task_struct::utrace
> pointer leaving it reachable:
> 
> 	tsk->utrace_flags = flags;
> 	if (flags)
> 		spin_unlock(&utrace->lock);
> 	else
> 		rcu_utrace_free(utrace);
> 
> OTOH, utrace_release_task() first clears ->utrace pointer, then frees
> struct utrace itself:
> 
> Roland inserted some debugging into 2.6.21-rc6-mm1 so that aforementined
> double free couldn't be reproduced without seeing
> BUG at kernel/utrace.c:176 first. It triggers if one struct utrace were
> passed to rcu_utrace_free() second time.
> 
> With patch applied I no longer see¹ BUG message and double frees on
> 2-way P3, 8-way ia64, Core 2 Duo boxes. Testcase is at the first link.
> 
> I _think_ it adds leak if utrace_reap() takes branch without freeing
> but, well, I hope Roland will give me some clue on how to fix it too.
> 
> Signed-off-by: Alexey Dobriyan <adobriyan@sw.ru>
> ---
> 
>  kernel/utrace.c |    6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
> 
> ¹ But I see whole can of other bugs! I think they were already lurking
>   but weren't easily reproducable without hitting double-free first.
>   FWIW, it's
>   	BUG_ON(!list_empty(&tsk->ptracees));
> 	oops at the beginning of remove_engine()
> 	NULL ->report_quiesce call which is absent in ptrace utrace ops
> 	BUG_ON(tracehook_check_released(p));
> 
> --- a/kernel/utrace.c
> +++ b/kernel/utrace.c
> @@ -205,7 +205,6 @@ utrace_clear_tsk(struct task_struct *tsk
>  	if (utrace->u.live.signal == NULL) {
>  		task_lock(tsk);
>  		if (likely(tsk->utrace != NULL)) {
> -			rcu_assign_pointer(tsk->utrace, NULL);
>  			tsk->utrace_flags &= UTRACE_ACTION_NOREAP;
>  		}
>  		task_unlock(tsk);
> @@ -305,10 +304,7 @@ check_dead_utrace(struct task_struct *ts
>  	}
>  
>  	tsk->utrace_flags = flags;
> -	if (flags)
> -		spin_unlock(&utrace->lock);
> -	else
> -		rcu_utrace_free(utrace);
> +	spin_unlock(&utrace->lock);
>  
>  	/*
>  	 * Now we're finished updating the utrace state.
> 
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