Re: General flags to turn things off (getrandom, pid lookup, etc)

From: Eric W. Biederman
Date: Wed Jul 30 2014 - 22:41:50 EST

One Thousand Gnomes <gnomes@xxxxxxxxxxxxxxxxxxx> writes:

> On Wed, 30 Jul 2014 11:41:41 -0700
> ebiederm@xxxxxxxxxxxx (Eric W. Biederman) wrote:
>> One Thousand Gnomes <gnomes@xxxxxxxxxxxxxxxxxxx> writes:
>> >> Andy you seem to be arguing here for two system calls.
>> >> get_urandom() and get_random().
>> >>
>> >> Where get_urandom only blocks if there is not enough starting entropy,
>> >> and get_random(GRND_RANDOM) blocks if there is currently not enough
>> >> entropy.
>> >>
>> >> That would allow -ENOSYS to be the right return value and it would
>> >> simply things for everyone.
>> >
>> > So you replace the "no file handle" special case with the "unsupported or
>> > disabled syscall" special case, which is even harder to test.
>> >
>> > Interfaces have failure modes. People who can't deal with that shouldn't
>> > be writing code that does anything important in languages which don't
>> > handle it for them.
>> Perhaps I misread the earlier conversation but it what I have read of
>> this discussion people want to disable some of get_random() modes with
>> seccomp. Today get_random does not have any failure codes define except
>> get_random(0) succeeding and get_random(GRND_RANDOM) returning -ENOSYS
>> has every chance of causing applications to legitimately assume the
>> get_random system call is not available in any mode.
> Or more likely it'll be used like this
> get_random(foo); /* always works */
> Now the existing failure mode is is
> open(...)
> /* forget the check */
> read()
> /* forget the check */
> and triggered by evil local attacks on file handles. The "improved"
> behaviour is unchecked -ENOSYS returns which are likely to occur
> systemically when users run stuff on old kernels, in vm's with it off etc.
> So you've swapped the odd evil user attack on a single target for the
> likelyhood of mass generation of flawed keys with no error reporting.
> In fact you could do a better job of the whole mess in libc rather than
> the kernel, because in libc you'd write it like this
> if (open(.. ) < 0)
> kill(getpid(), 9);
> if (read(...) < expected)
> kill(getpid(), 9);
> close(fd);
> and
> a) on an older library you'd get a good failure (unable to execute the
> binary)
> b) on a newer system you'd get "do or die" behaviour and can improve its
> robustness as desired

I have said enough about the silliness of disabling this syscall with
seccomp or related infrastructure.

The aspect I like about get_random() is that it will silence the
requests from people to enable binary sysctl support in the kernel.
Just so they can get random numbers when /dev/random and /dev/urandom
are absent in their chroots.

sysctl(2) is finally legitmately going fading away.


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