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 >> -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 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. Eric -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
