Memory leak in ntlm_auth ?

Stefan Metzmacher metze at
Fri Apr 7 09:27:07 UTC 2017

Hi Rebecca,

I've filed
for this...


> thanks for the report,
> can you please also file a bug at,
> this is required to get the fix into the release branches.
> While I think your patch fixes the problem, I'd propose the attached
> patch for upstream.
> Can you test if the attached patch also works for you?
> Thanks!
> metze
> Am 04.04.2017 um 11:20 schrieb Rebecca Gellman via samba-technical:
>> Hi, 
>> I wish to tell you a tale of investigation and suspicion of a memory
>> leak in the ntlm_auth program (as compiled from
>> source3/utils/ntlm_auth.c). 
>> So in our system, we use ntlm_auth to provide both Kerberos and NTLM
>> authentication for transparent (and not-so transparent) web proxy
>> connections. In this particular instance, Kerberos is the auth of
>> choice, but I believe this is relevant to NTLM too. 
>> We spawn up a pool of ntlm_auth processes on demand. If a process is not
>> available when a connection comes in, we create a new one up to a
>> maximum. 
>> The base software version is 4.2.10, as shipped with Debian 8 (Jessie). 
>> Now that the particulars of the environment are out of the way..... 
>> So we have a customer. And they phoned in a support call that their
>> system was falling over. We dive in remotely, and see each ntlm_auth
>> process is running at an impressive 1.7GB of memory in the "VIRT" column
>> of top. EEP! 
>> Suspecting a memory leak, we compiled up version 4.2.10 with a call to
>> talloc_enable_leak_reporting_full() in source3/utils/ntlm_auth.c (why
>> isn't this a command line option at the least?), along with a mod to our
>> own processes to send CTRL+D to ntlm_auth prior to terminating the
>> process, and gathering the report on STDERR. 
>> Left it running for a day and examined the report. Each process had on
>> average over 19,000 DATA_BLOBs of around 1.5K in size, reportedly
>> created at lib/util/base64.c line 35, the function
>> base64_decode_data_blob_talloc(). 
>> After digging through the code, this function is called on (pseudo-code)
>> &line_of_STDIN[3] to decode the base64 blob input to a binary blob.
>> There's then a load of logic surrounding the 2-letter command code and
>> processing gensec, etc, etc, etc. 
>> Now... the crux of the matter. If I compare with an earlier (3.x.x)
>> branch, we see that data_blob_free() is called on this returned blob
>> whenever the command handler returns. In 4.x.x this code has been
>> refactored, and the function manage_gensec_request() doesn't always call
>> data_blob_free(). In fact, it rarely calls it at all. 
>> There's also a couple of returns that omit talloc_free(mem_ctx) that
>> seems to precede every other return in this function. 
>> So, on this hunch, I added in the missing data_blob_free() calls and the
>> two talloc_free(mem_ctx) calls, and rebuilt. Now the same customer
>> system is sitting happily at a low memory usage, not showing so much of
>> a hint of a memory leak. 
>> I've attached the patch we used to "correct" ntlm_auth.c. I'd be
>> interested to here some technical commentary from knowledge Samba bods
>> :) 
>> Regards 
>> Rebecca Gellman (On behalf of SmoothWall) 

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