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

Lukas Slebodnik lslebodn at redhat.com
Tue May 26 08:57:09 UTC 2015


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)

>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.

>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 :-)

LS



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