[PATCH libXdmcp] Use getrandom() syscall if available

[Benjamin Tissoires]

On Mon, Apr 3, 2017 at 4:02 PM, Alan Coopersmith
<alan.coopersmith at oracle.com> wrote:
> On 04/ 3/17 05:52 AM, Benjamin Tissoires wrote:
>>
>> 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>
>
>
> This is dangerous - Solaris <sys/syscall.h> defines SYS_getrandom, but
> I don't know if our syscall arguments/semantics are the same, and we
> only support applications calling the libc getrandom() function, not the
> raw syscall.

I see. In that case, would it help to use unconditionally the libc
getrandom() function, and in case it's not there, provide a stub for
it through the syscall or other emulation layer?

Matthieu, I am not 100% sure, but do you have the getrandom() libc
call in BSD, and would it be OK to use it instead of arc4random_buf()
when available?

>
>> 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])
>>
>
> Could you move that check up into the case $host_os section above it
> under a new case for *linux* perhaps?

Sure.

Cheers,
Benjamin