Re: Backporting CVE-2016-10739

[Carlos O'Donell <carlos@redhat.com>]

On 2/4/19 2:25 PM, Florian Weimer wrote:
> * Carlos O'Donell:
> 
>> On 2/4/19 11:18 AM, Florian Weimer wrote:
>>> * Carlos O'Donell:
>>>
>>>> On 2/4/19 10:44 AM, Florian Weimer wrote:
>>>>> * Carlos O'Donell:
>>>>>
>>>>>>> +#include <arpa/inet.h>
>>>>>>> +
>>>>>>
>>>>>> Please add a comment explaining why this is here.
>>>>>
>>>>> You mean like this?
>>>>>
>>>>> /* Obtain the prototype for __inet_aton_exact.  */
>>>>
>>>> It should reference the bug or CVE to document the intent
>>>> of the changes.
>>>>
>>>> Post v3 and I'll sign off?
>>>
>>> This approach does not actually work because copying a prototype this
>>> way and adding a hidden visibility attribute does not actually make the
>>> symbol hidden.  The patch below however has the desired effect, mainly
>>> because interposition no longer happens and the __inet_aton_exact_hidden
>>> function is not added to the dynamic symbol table of the nscd
>>> executable.
>>>
>>> I suspect I would have had to use __attribute__ ((visibility
>>> ("hidden"))) directly because we define attribute_hidden thusly:
>>>
>>> #if defined SHARED || defined LIBC_NONSHARED \
>>>   || (BUILD_PIE_DEFAULT && IS_IN (libc))
>>> # define attribute_hidden __attribute__ ((visibility ("hidden")))
>>> #else
>>> # define attribute_hidden
>>> #endif
>>>
>>> I can post yet another patch which uses real hidden visibility and
>>> avoids the symbol redirect.
>>
>> This version is a little hack-ish, but I don't object to it. It does
>> the job.
>>
>> It is certainly a minimal set of changes, and that makes it quite
>> useful for the release branch.
>>
>> I don't think you need to go any further unless you think the version
>> using real hidden visibility is all that much better?
> 
> Well, at least it does not have the completely ineffective
> attribute_hidden. 8-)

I'm just providing high-level review here, I haven't tested these patches,
I rely on you for that :-}

> Patch below.  What do you think?

This looks good to me, you make direct use of "__attribute__ ((visibility ("hidden")))"
in an exceptional case, and that's fine.

If this becomes less rare for some reason we might want a libc-symbols.h macro to define
something that expresses the intent of the hidden visibility e.g. attr_dup_sym_hidden.

It's rare we have a fix that needs a new private interface, and that we can also
just dup the code to fix the problem. Most likely I expect we *can't* dup the code
and so need the new private interface to fix the bug, or we rework the fix entirely.

> This one gets the symbol visibility correct.

OK.

> $ eu-readelf -s ../build/nscd/nscd-inet_addr.o | grep inet_aton_exact
>    22: 0000000000000150     56 FUNC    GLOBAL HIDDEN         1 __inet_aton_exact
> $ eu-readelf -s ../build/nscd/gai.o | grep inet_aton_exact
>    99: 0000000000000000      0 NOTYPE  GLOBAL HIDDEN     UNDEF __inet_aton_exact

That's good.

> 
> And there's no __inet_aton_exact symbol in nscd.  I think this is fairly
> standard usage of a hidden symbol, so renaming the symbol should not be
> necessary.

Perfect, and interposition is also not possible.

> Thanks,
> Florian
> 
> nscd: Do not use __inet_aton_exact@GLIBC_PRIVATE [BZ #20018]
> 
> This commit avoids referencing the __inet_aton_exact@GLIBC_PRIVATE
> symbol from nscd.  In master, the separately-compiled getaddrinfo
> implementation in nscd needs it, however such an internal ABI change
> is not desirable on a release branch if it can be avoided.
> 
> 2019-02-04  Florian Weimer  <fweimer@redhat.com>
> 
> 	[BZ #20018]
> 	nscd: Do not rely on new GLIBC_PRIVATE ABI after CVE-2016-10739 fix.
> 	* nscd/nscd-inet_addr.c: New file.  Build resolv/inet_addr.c for
> 	nscd, without public symbols.
> 	* nscd/Makefile (nscd-modules): Add it.
> 	* nscd/gai.c: Include <arpa/inet.h> and change visibility of
> 	__inet_aton_exact.

OK for release/2.28/master.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

> diff --git a/nscd/Makefile b/nscd/Makefile
> index b713a84c49..eb23c01a39 100644
> --- a/nscd/Makefile
> +++ b/nscd/Makefile
> @@ -36,7 +36,7 @@ nscd-modules := nscd connections pwdcache getpwnam_r getpwuid_r grpcache \
>  		getsrvbynm_r getsrvbypt_r servicescache \
>  		dbg_log nscd_conf nscd_stat cache mem nscd_setup_thread \
>  		xmalloc xstrdup aicache initgrcache gai res_hconf \
> -		netgroupcache
> +		netgroupcache nscd-inet_addr

OK.

>  
>  ifeq ($(build-nscd)$(have-thread-library),yesyes)
>  
> diff --git a/nscd/gai.c b/nscd/gai.c
> index f57f396f57..68a4abd30e 100644
> --- a/nscd/gai.c
> +++ b/nscd/gai.c
> @@ -33,6 +33,12 @@
>  #define __getifaddrs getifaddrs
>  #define __freeifaddrs freeifaddrs
>  
> +/* We do not want to export __inet_aton_exact.  Get the prototype and
> +   change its visibility to hidden.  */
> +#include <arpa/inet.h>
> +__typeof__ (__inet_aton_exact) __inet_aton_exact
> +  __attribute__ ((visibility ("hidden")));

OK.

> +
>  /* We are nscd, so we don't want to be talking to ourselves.  */
>  #undef  USE_NSCD
>  
> diff --git a/nscd/nscd-inet_addr.c b/nscd/nscd-inet_addr.c
> new file mode 100644
> index 0000000000..f366b9567d
> --- /dev/null
> +++ b/nscd/nscd-inet_addr.c
> @@ -0,0 +1,32 @@
> +/* Legacy IPv4 text-to-address functions.  Version for nscd.
> +   Copyright (C) 2019 Free Software Foundation, Inc.

OK.

> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>.  */
> +
> +#include <arpa/inet.h>
> +
> +/* We do not want to export __inet_aton_exact.  Get the prototype and
> +   change the visibility to hidden.  */
> +#include <arpa/inet.h>
> +__typeof__ (__inet_aton_exact) __inet_aton_exact
> +  __attribute__ ((visibility ("hidden")));

OK.

> +
> +/* Do not provide definitions of the public symbols exported from
> +   libc.  */
> +#undef weak_alias
> +#define weak_alias(from, to)

OK.

> +
> +#include <resolv/inet_addr.c>
> 


-- 
Cheers,
Carlos.