[SSSD] [PATCH] proxy: Do not crash if proxy_child returns error

Pavel Březina pbrezina at redhat.com
Tue May 26 08:44:44 UTC 2015


On 05/25/2015 02:58 PM, Lukas Slebodnik wrote:
> On (22/05/15 11:02), Pavel Březina wrote:
>> On 05/20/2015 02:20 PM, Lukas Slebodnik wrote:
>>> ehlo,
>>>
>>> attached patch fixes a crash if return code of proxy_child is not 0.
>>> You can reproduce it with sssd in non-root mode or
>>> you can replace proxy_child with bash script.
>>>
>>> I'm not sure about the name of tevent_immediate callback
>>> so feel free to propose better name.
>>>
>>> LS
>>
>> Hi,
>> the whole proxy auth code does not really follow tevent style but I think
>> your name is good enough.
>> However, I don't see why it should crash, can you
>> explain it in more detail please?
>
> a) proxy_child_init_send is called
>     -- request with state "struct pc_init_ctx" is created
>     -- process is forked
>     -- parent register signal SIGCHLD with clallback pc_init_sig_handler
> b) child return non-zero
>     -- parent recieve pc_init_sig_handler
> c) pc_init_sig_handler is executed
>     -- unused argument "__siginfo" contains NON null data
>     -- tevent_req_error(req, EIO) is called
>     -- proxy_child_init_done callback is called
>     -- request is released (including "struct tevent_signal" and all related data)
>     -- unused argument "__siginfo" is NULL now
> d) function pc_init_sig_handler finished
>     -- we are back in tevent code from singnal handler
>     -- tevent wants to do some clean-up and dereference NULL in memset
>
> It is not clear from back trace but the 1st argument of memset is NULL.
>
> (gdb) bt
> #0  0x00007fd7ba400505 in memset (__len=<optimized out>, __ch=<optimized out>, __dest=<optimized out>) at /usr/include/bits/string3.h:84
> #1  tevent_common_check_signal (ev=0x7fd7bfddf670) at ../tevent_signal.c:459
> #2  0x00007fd7ba40228c in epoll_event_loop (tvalp=0x7fff85536430, epoll_ev=0x7fd7bfddf8b0) at ../tevent_epoll.c:647
> #3  epoll_event_loop_once (ev=<optimized out>, location=<optimized out>) at ../tevent_epoll.c:926
> #4  0x00007fd7ba4007d7 in std_event_loop_once (ev=0x7fd7bfddf670, location=0x7fd7bdb417c3 "src/util/server.c:668") at ../tevent_standard.c:114
> #5  0x00007fd7ba3fcfbd in _tevent_loop_once (ev=ev at entry=0x7fd7bfddf670, location=location at entry=0x7fd7bdb417c3 "src/util/server.c:668") at ../tevent.c:530
> #6  0x00007fd7ba3fd15b in tevent_common_loop_wait (ev=0x7fd7bfddf670, location=0x7fd7bdb417c3 "src/util/server.c:668") at ../tevent.c:634
> #7  0x00007fd7ba400777 in std_event_loop_wait (ev=0x7fd7bfddf670, location=0x7fd7bdb417c3 "src/util/server.c:668") at ../tevent_standard.c:140
> #8  0x00007fd7bdb29343 in server_loop (main_ctx=0x7fd7bfde0ac0) at src/util/server.c:668
> #9  0x00007fd7be39ca42 in main (argc=8, argv=<optimized out>) at src/providers/data_provider_be.c:2909
>
>
> BTW it's easily reproducible with proxy provider. I can give you an access
> to such machine.
>
> I hope it is clear now.
>
> LS

Hi,
I'm sorry, but I don't think this is the problem. I'm not saying the 
solution may not be right, but the cause is different.

1) siginfo is not attached to tevent_signal context, so freeing 
tevent_signal does not do anything with siginfo

> static struct tevent_sig_state {
> 	struct tevent_common_signal_list *sig_handlers[TEVENT_NUM_SIGNALS+1];
> 	struct sigaction *oldact[TEVENT_NUM_SIGNALS+1];
> 	struct tevent_sigcounter signal_count[TEVENT_NUM_SIGNALS+1];
> 	struct tevent_sigcounter got_signal;
> #ifdef SA_SIGINFO
> 	/* with SA_SIGINFO we get quite a lot of info per signal */
> 	siginfo_t *sig_info[TEVENT_NUM_SIGNALS+1];
> 	struct tevent_sigcounter sig_blocked[TEVENT_NUM_SIGNALS+1];
> #endif
> } *sig_state;

2) it is a ring buffer, that is the reason why it is cleared in tevent 
after sighandler is finished

3) freeing tevent_signal inside the handler is a valid operation, you 
can see a pretty clever check whether the structure was freed or not in 
the tevent code

> 			/*
> 			 * We have to be careful to not touch "se"
> 			 * after it was deleted in its handler. Thus
> 			 * we allocate a child whose destructor will
> 			 * tell by nulling out itself that its parent
> 			 * is gone.
> 			 */
> 			exists = talloc(se, struct tevent_se_exists);
> 			if (exists == NULL) {
> 				continue;
> 			}
> 			exists->myself = &exists;
> 			talloc_set_destructor(
> 				exists, tevent_se_exists_destructor);

4) so it is definitely not supposed to crash because you free 
tevent_signal inside the handler

5) I'm not sure why it crashes, however we can stop using flag 
SA_SIGINFO since it is unused anyway, it should do the trick

6) freeing tevent_signal causes to run destructor of pc_init_ctx which 
sends kill(SIGKILL). Maybe it triggers some tevent bug?





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