Hello David, 

I tried to reproduce this today (with samba-4.3.8-0.fc23.x86_64) but I couldn't. Can you share your config and describe briefly your setup?

Thanks!
Guenther

client NTLMv2 auth = no

Here's 4.3.8-0.fc23 (the above was 4.3.9-0.fc23) with debuginfo:

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff511e38f in _IO_vfprintf_internal (s=s@entry=0x7fffffffca60, 
    format=<optimized out>, 
    format@entry=0x7ffff56b0988 "talloc: access after free error - first free may be at %s\n", ap=ap@entry=0x7fffffffcbf8) at vfprintf.c:1631
1631		  process_string_arg (((struct printf_spec *) NULL));
(gdb) bt
#0  0x00007ffff511e38f in _IO_vfprintf_internal (s=s@entry=0x7fffffffca60, 
    format=<optimized out>, 
    format@entry=0x7ffff56b0988 "talloc: access after free error - first free may be at %s\n", ap=ap@entry=0x7fffffffcbf8) at vfprintf.c:1631
#1  0x00007ffff51e3e76 in ___vsnprintf_chk (
    s=s@entry=0x7fffffffcc10 "talloc: access after free error - first free may be at ed symbol: gconv_end", maxlen=<optimized out>, maxlen@entry=1024, 
    flags=flags@entry=1, slen=slen@entry=1024, 
    format=format@entry=0x7ffff56b0988 "talloc: access after free error - first free may be at %s\n", args=args@entry=0x7fffffffcbf8) at vsnprintf_chk.c:63
#2  0x00007ffff56a8dbe in vsnprintf (__ap=0x7fffffffcbf8, 
    __fmt=<optimized out>, __n=1024, 
    __s=0x7fffffffcc10 "talloc: access after free error - first free may be at ed symbol: gconv_end") at /usr/include/bits/stdio2.h:77
#3  talloc_vasprintf (t=t@entry=0x0, 
    fmt=fmt@entry=0x7ffff56b0988 "talloc: access after free error - first free may be at %s\n", ap=ap@entry=0x7fffffffd068) at ../talloc.c:2440
#4  0x00007ffff56a477c in talloc_log (
    fmt=fmt@entry=0x7ffff56b0988 "talloc: access after free error - first free may be at %s\n") at ../talloc.c:370
#5  0x00007ffff56aab5a in talloc_chunk_from_ptr (ptr=0x7ffff673d9c3)
    at ../talloc.c:432
#6  _talloc_steal_loc (new_ctx=new_ctx@entry=0x5555557928b0, 
---Type <return> to continue, or q <return> to quit---
    ptr=0x7ffff673d9c3, 
    location=location@entry=0x7ffff56b0cef "../talloc.c:1927")
    at ../talloc.c:1219
#7  0x00007ffff56aab93 in _talloc_move (new_ctx=new_ctx@entry=0x5555557928b0, 
    _pptr=_pptr@entry=0x7fffffffd218) at ../talloc.c:1927
#8  0x00007ffff7374cfe in ntlmssp_client_challenge (
    gensec_security=0x555555792310, out_mem_ctx=0x555555791c40, in=..., 
    out=0x7fffffffd580) at ../auth/ntlmssp/ntlmssp_client.c:354
#9  0x00007ffff737360f in gensec_ntlmssp_update (
    gensec_security=0x555555792310, out_mem_ctx=0x555555791c40, 
    ev=<optimized out>, input=..., out=0x7fffffffd580)
    at ../auth/ntlmssp/ntlmssp.c:176
#10 0x00007ffff737d612 in gensec_update_ev (gensec_security=0x555555792310, 
    out_mem_ctx=0x555555791c40, ev=0x555555792dc0, ev@entry=0x0, in=..., 
    out=0x7fffffffd580) at ../auth/gensec/gensec.c:303
#11 0x00007ffff737d657 in gensec_update (gensec_security=<optimized out>, 
    out_mem_ctx=<optimized out>, in=..., out=<optimized out>)
    at ../auth/gensec/gensec.c:372
#12 0x000055555555bbfe in manage_gensec_request (
    stdio_helper_mode=NTLMSSP_CLIENT_1, lp_ctx=<optimized out>, 
    buf=<optimized out>, private1=<optimized out>, length=<optimized out>)
    at ../source3/utils/ntlm_auth.c:1467
#13 0x00005555555593b3 in manage_squid_request (
---Type <return> to continue, or q <return> to quit---
    stdio_helper_mode=NTLMSSP_CLIENT_1, lp_ctx=0x555555784050, 
    state=0x555555791800, fn=0x55555555c8d0 <manage_client_ntlmssp_request>, 
    private2=0x0) at ../source3/utils/ntlm_auth.c:2040
#14 0x0000555555558d5b in squid_stream (
    fn=0x55555555c8d0 <manage_client_ntlmssp_request>, lp_ctx=0x555555784050, 
    stdio_mode=NTLMSSP_CLIENT_1) at ../source3/utils/ntlm_auth.c:2074
#15 main (argc=<optimized out>, argv=<optimized out>)
    at ../source3/utils/ntlm_auth.c:2317

(Not the same run...)

#8  0x00007ffff7374cfe in ntlmssp_client_challenge (
    gensec_security=0x555555792360, out_mem_ctx=0x555555791470, in=..., 
    out=0x7fffffffd570) at ../auth/ntlmssp/ntlmssp_client.c:354
354		ntlmssp_state->server.netbios_domain = talloc_move(ntlmssp_state, &server_domain);
(gdb) p server_domain
$3 = 0x7ffff673d9c3 ""


In /proc/$pid/maps it seems that server_domain is pointing to a static "" string in libcliauth-samba4.so, instead of something that was allocated. We probably weren't expecting msrpc_parse() to do that?

The talloc library crapping itself when trying to report this, is obviously a separate issue.

Right, have it reproduced and a possible fix, hold on.

Created attachment 1155392 [details]
possible fix

The upstream bug is https://bugzilla.samba.org/show_bug.cgi?id=11912

Thanks Stefan, just saw I came up with the exact same patch 45 minutes later than yours :-)

Scratch build at http://koji.fedoraproject.org/koji/taskinfo?taskID=13991277

And for f24 at http://koji.fedoraproject.org/koji/taskinfo?taskID=14166983

samba-4.3.10-0.fc23 has been submitted as an update to Fedora 23. https://bodhi.fedoraproject.org/updates/FEDORA-2016-de8ba50085

samba-4.3.10-0.fc23 has been pushed to the Fedora 23 testing repository. If problems still persist, please make note of it in this bug report.
See https://fedoraproject.org/wiki/QA:Updates_Testing for
instructions on how to install test updates.
You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2016-de8ba50085

samba-4.3.10-0.fc23 has been pushed to the Fedora 23 stable repository. If problems still persist, please make note of it in this bug report.