dlsym(RTLD_DEFAULT, "getentropy") return non-NULL on Mac

[Mike StJohns]

Hi Max et al -

In general, I'm not a big fan of silent failover in security stuff.  
There ought to be a way - perhaps a system property - to signal that if 
you don't get the originally desired entropy source, you'll throw an error.

Also, have you considered: *

https://software.intel.com/en-us/articles/intel-digital-random-number-generator-drng-software-implementation-guide?language=fr

*If you're going into the raw libraries, it might actually be a better 
idea to go straight to the chipset if RDSEED is available.

(Hmm... also 
http://stackoverflow.com/questions/29372893/rdrand-and-rdseed-intrinsics-gcc-and-intel-c)

Mike


On 11/8/2015 6:18 AM, Dmitry Samersoff wrote:
> Wang Weijun,
>
>> The function is rather new in the latest Solaris beta [1] and it's
>> preferred to reading from /dev/random. There are already people
>> suggest adding it to Linux. If I use simply using dlsym then it will
>> automatically work on all current and future platforms. In fact, I
>> was planning to write
> 1. Please, check libc only not entire process image.
>
> 2. OS X random system is a different story: /dev/random never blocks and
> returns the output of Yarrow PRNG.  Also OS X uses SecurityServer
> process to feed entropy pool.
>
> So IMHO, it's better to don't attempt to use other functions on OS X
> until it appears officially.
>
> 3. Please notice that:
>
>     /dev/random will block if you request more bits than entropy pool can
> provide.
>
>     but (according to manual page) getentropy will return immediately
> with error if less that requested bytes of entropy is available and you
> can't request more than 256 bytes of entropy at once.
>
>      it might require changes in uplevel logic.
>
> -Dmitry
>
>
> On 2015-11-08 03:37, Wang Weijun wrote:
>>> On Nov 8, 2015, at 4:29 AM, Dmitry Samersoff
>>> <dmitry.samersoff at oracle.com> wrote:
>>>
>>> Wang Weijun,
>>>
>>> 1. RTLD_DEFAUL call is expensive and dangerous because it cause
>>> symbol search across all loaded images. So it can pick up something
>>> absolutely irrelevant to your expectations at any time.
>>>
>>> If you decide to play to this game, you have to use dladdr to check
>>> the origin of a symbol and try to guess whether it is what you are
>>> looking for or not.
>>>
>>> Please dlopen(libSystem.dylib, RTLD_NOW) and search through it
>>> only.
>>>
>>> 2. You shouldn't rely on return value of dlopen() or dlsym(). Call
>>> dlerror() and check whether it returns error or NULL.
>> I'll try.
>>
>>> PS: What is the goal of using a get entropy ?
>> The function is rather new in the latest Solaris beta [1] and it's
>> preferred to reading from /dev/random. There are already people
>> suggest adding it to Linux. If I use simply using dlsym then it will
>> automatically work on all current and future platforms. In fact, I
>> was planning to write
>>
>> getentropy = (GETENTROPY_FN)dlsym(RTLD_DEFAULT, "getentropy"); if
>> (getentropy) { return 1; } else { getrandom =
>> (GETRANDOM_FN)dlsym(RTLD_DEFAULT, "getrandom"); if (getrandom) {
>> return 2; } else { return -1;  // will read /dev/random } }
>>
>> Thanks Max
>>
>> [1]
>> https://blogs.oracle.com/darren/entry/solaris_new_system_calls_getentropy
>>
>>
>>> -Dmitry
>>>
>>> On 2015-11-07 05:51, Wang Weijun wrote:
>>>> I find something strange.
>>>>
>>>> Background: a new method getentropy() is available on OpenBSD [1]
>>>> and Solaris and people are also proposing it on other OSes.
>>>>
>>>> Therefore inside JDK I write a piece of native code to detect it,
>>>> something like
>>>>
>>>> typedef int (*GETENTROPY_FN)(char* buffer, int len);
>>>>
>>>> getentropy = (GETENTROPY_FN)dlsym(RTLD_DEFAULT, "getentropy"); if
>>>> (getentropy) { return 1; }
>>>>
>>>> On Mac, it returns non-NULL, but a later call to
>>>> (*getentropy)(cbuffer, length) shows
>>>>
>>>> #  SIGBUS (0xa) at pc=0x0000000103bfa030, pid=22434, tid=5891
>>>> ... # Problematic frame: # C  [libj2rand.dylib+0x1030]
>>>> getentropy+0x0
>>>>
>>>> However, "man getentropy" does not show anything, and the
>>>> following simple program also prints out 0x0
>>>>
>>>> #include <dlfcn.h> #include <stdlib.h>
>>>>
>>>> main() { void* g = dlsym(RTLD_DEFAULT, "getentropy");
>>>> printf("%p\n", g); }
>>>>
>>>> What does this mean? Is the JDK code loading another getentropy()
>>>> somewhere else? How do I detect it and what shall I do to avoid
>>>> it?
>>>>
>>>> Thanks Max
>>>>
>>>> [1]
>>>> http://www.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man2/getentropy.2
>>>>
>>>
>>>

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