Re: [PATCH, RFC] random: introduce getrandom(2) system call

On Sun, Jul 20, 2014 at 12:26:22PM -0400, George Spelvin wrote:

One basic question... why limit this to /dev/random?

If we're trying to avoid fd exhaustion attacks, wouldn't an "atomically
read a file into a buffer" system call (that could be used on
/dev/urandom, or /etc/hostname, or /proc/foo, or...) be more useful?

E.g.

ssize_t readat(int dirfd, char const *path, struct stat *st,
	char *buf, size_t len, int flags);

It's basically equivalent to openat(), optional fstat() (if st is non-NULL),
read(), close(), but it doesn't allocate an fd number.

Is it necessary to have a system call just for entropy?

If you want a "urandom that blocks until seeded", you can always create
another device node for the purpose.

I'd really rather not go down this path.  Your readat(2) proposal is
interesting, but it adds a whole lot of complications.  For example,
just simply booting a new kernel doesn't guarantee that a new device
node for "blocks until seeded" will exist.  So that means a lot of
applications will just either continue to use /dev/urandom, or have to
put in fallback code to first try the new device name, and then fall
back to /dev/urandom.  (And of course, they have to deal with what to
do if /dev/urandom doesn't exist --- which presumably would be
raise(SIGKILL), but we're now talking about a number of lines of codes
that application writers would have to get right.)

Readat(2) would also have to get linked into auditing, and LSM, and
honestly, it's a lot more work that I'm not all that interested in
doing and trying to get right.

If you do stick with a random-specific call, specifying the entropy
in bits (with some specified convention for the last fractional byte)
is anothet interesting idea.  Perhaps too prone to bugs, though.
(People thinking it's bytes and producing low-entropy keys.)

Definitely not worth the complexity.

							- Ted