[PATCH libXdmcp] Use getrandom() syscall if available

[Benjamin Tissoires]

On Mon, Apr 3, 2017 at 3:17 PM, walter harms <wharms at bfs.de> wrote:
>
>
> Am 03.04.2017 14:52, schrieb Benjamin Tissoires:
>> This allows to fix CVE-2017-2625 on Linux platforms without pulling in
>> libbsd.
>> The syscall getrandom is available since kernel v3.17. The code first
>> tries to use the syscall on a supported kernel. If the syscall fails,
>> it falls back to the current (vulnerable) code.
>> We do not implement the glibc getrandom() call given that it's only
>> available in glibc 2.25, and the #if dance is already messy here.
>>
>> Signed-off-by: Benjamin Tissoires <benjamin.tissoires at gmail.com>
>> ---
>>  Key.c        | 12 ++++++++++++
>>  configure.ac |  3 +++
>>  2 files changed, 15 insertions(+)
>>
>> diff --git a/Key.c b/Key.c
>> index a09b316..61b07db 100644
>> --- a/Key.c
>> +++ b/Key.c
>> @@ -36,6 +36,10 @@ in this Software without prior written authorization from The Open Group.
>>  #include <bsd/stdlib.h> /* for arc4random_buf() */
>>  #endif
>>
>> +#if HAVE_DECL_SYS_GETRANDOM
>> +#include <sys/syscall.h> /* for SYS_getrandom */
>> +#endif
>> +
>>  #ifndef HAVE_ARC4RANDOM_BUF
>>  static void
>>  getbits (long data, unsigned char *dst)
>> @@ -68,6 +72,14 @@ XdmcpGenerateKey (XdmAuthKeyPtr key)
>>  #ifndef HAVE_ARC4RANDOM_BUF
>>      long    lowbits, highbits;
>>
>> +#if HAVE_DECL_SYS_GETRANDOM
>> +    int ret;
>> +
>> +    ret = syscall(SYS_getrandom, key->data, 8, 0);
>> +    if (ret == 8)
>> +     return;
>> +#endif
>> +
>
> i am not an expert on syscalls but would it help to test for
> SYS_getrandom directly ?

I am not sure I fully understand you. Are you suggesting to remove the
'ret' variable?

Cheers,
Benjamin

>
> re,
>  wh
>
>
>>      srandom ((int)getpid() ^ time((Time_t *)0));
>>      lowbits = random ();
>>      highbits = random ();
>> diff --git a/configure.ac b/configure.ac
>> index 2288502..d0d4d05 100644
>> --- a/configure.ac
>> +++ b/configure.ac
>> @@ -63,6 +63,9 @@ case $host_os in
>>          ;;
>>  esac
>>
>> +# Checks for syscalls
>> +AC_CHECK_DECLS([SYS_getrandom], [], [], [[#include <sys/syscall.h>]])
>> +
>>  # Checks for library functions.
>>  AC_CHECK_LIB([bsd], [arc4random_buf])
>>  AC_CHECK_FUNCS([srand48 lrand48 arc4random_buf])
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