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

On Wed, 30 Jul 2014 11:41:41 -0700
ebiederm@xmission.com (Eric W. Biederman) wrote:

One Thousand Gnomes [off-list ref] writes:

quoted

quoted

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
-ENOSYS.

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

Alan