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

Lukas Slebodnik lslebodn at redhat.com
Wed May 27 08:34:27 UTC 2015


On (26/05/15 11:04), Pavel Březina wrote:
>On 05/26/2015 10:57 AM, Lukas Slebodnik wrote:
>>On (26/05/15 10:44), Pavel Březina wrote:
>>>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
>>>
>>Yes, it is pointer to ringbuffer.
>>But argument "__siginfo" was valid pointer to ringbuffer before calling
>>tevent_req_error and after it was NULL. That's the reason why it crashed.
>>Dereference of null pointer (in memset)
>
>Yes. But the reason why it is NULL should not be that we freed tevent_signal
>from the handler (unless it is a bug in tevent).
>
It was cleared by tevent_signal_destructor
so it is not bug in tevent. it is by design.

IMHO it make sense. If you release tevent_signal
You do not care anymore about sig_info

Maybe it could be documented in tevent that you should not release
tevent_signal within handler if you created tevent signal with SA_SIGINFO.



(gdb) bt
#0  0x00007f5d4d86cc74 in tevent_signal_destructor (se=0x7f5d5370f920) at ../tevent_signal.c:213
#1  0x00007f5d4d65f233 in _talloc_free_internal () from /lib64/libtalloc.so.2
#2  0x00007f5d4d6593a3 in _talloc_free () from /lib64/libtalloc.so.2
#3  0x00007f5d4342f3d4 in proxy_child_init_done (subreq=0x7f5d5370f600) at src/providers/proxy/proxy_auth.c:436
#4  0x00007f5d4d86b0c2 in _tevent_req_error (req=req at entry=0x7f5d5370f600, error=error at entry=5, location=location at entry=0x7f5d43433010 "src/providers/proxy/proxy_auth.c:356")
    at ../tevent_req.c:167
#5  0x00007f5d4342ef5e in pc_init_sig_handler (ev=<optimized out>, sige=<optimized out>, signum=<optimized out>, count=<optimized out>, __siginfo=<optimized out>, pvt=<optimized out>)
    at src/providers/proxy/proxy_auth.c:356
#6  0x00007f5d4d86d48c in tevent_common_check_signal (ev=0x7f5d536de670) at ../tevent_signal.c:428
#7  0x00007f5d4d86f28c in epoll_event_loop (tvalp=0x7fff7b568490, epoll_ev=0x7f5d536de8b0) at ../tevent_epoll.c:647
#8  epoll_event_loop_once (ev=<optimized out>, location=<optimized out>) at ../tevent_epoll.c:926
#9  0x00007f5d4d86d7d7 in std_event_loop_once (ev=0x7f5d536de670, location=0x7f5d50faedc3 "src/util/server.c:668") at ../tevent_standard.c:114
#10 0x00007f5d4d869fbd in _tevent_loop_once (ev=ev at entry=0x7f5d536de670, location=location at entry=0x7f5d50faedc3 "src/util/server.c:668") at ../tevent.c:530
#11 0x00007f5d4d86a15b in tevent_common_loop_wait (ev=0x7f5d536de670, location=0x7f5d50faedc3 "src/util/server.c:668") at ../tevent.c:634
#12 0x00007f5d4d86d777 in std_event_loop_wait (ev=0x7f5d536de670, location=0x7f5d50faedc3 "src/util/server.c:668") at ../tevent_standard.c:140
#13 0x00007f5d50f96863 in server_loop (main_ctx=0x7f5d536dfac0) at src/util/server.c:668
#14 0x00007f5d5180aa42 in main (argc=8, argv=<optimized out>) at src/providers/data_provider_be.c:2909

>>>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
>>>
>>That might be a solution too.
>>
I tried it and it works.

>>>6) freeing tevent_signal causes to run destructor of pc_init_ctx which sends
>>>kill(SIGKILL). Maybe it triggers some tevent bug?
>>Do you want to access to a machine to confirm wheter it's tevent bug or no?
>>I do not want to dig in tevent :-)
>
>Sure.
>
will do in private IRC message when you will be online :-)

It's better to double check.

LS



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