On Friday 18 July 2014 08:56:06 Theodore Ts'o wrote:
>
> The change in the v3 version of the commit was to eliminate potential
> short reads and EINTR returns when reading from urandom (once the
> urandom pool is initialized). This was based on comments and requests
> from Theo de Raadt. See the NOTES section in the suggested man page for
> a more in-depth discussion of the issues involved.
I think there is a problem with the completion:
> @@ -657,6 +661,7 @@ retry:
> r->entropy_total = 0;
> if (r == &nonblocking_pool) {
> prandom_reseed_late();
> + complete_all(&urandom_initialized);
> pr_notice("random: %s pool is initialized\n", r->name);
> }
> }
complete_all() sets the 'done' part of the completion to 'UINT_MAX/2',
which is enough to ensure that everyone waiting for the completion
at a given time will wake up, but a completion should not
be reused without calling init_completion again after a call to
complete_all().
> +SYSCALL_DEFINE3(getrandom, char __user *, buf, size_t, count,
> + unsigned int, flags)
> +{
> + int r;
> +
> + if (flags & ~(GRND_NONBLOCK|GRND_RANDOM))
> + return -EINVAL;
> +
> + if (count > INT_MAX)
> + count = INT_MAX;
> +
> + if (flags & GRND_RANDOM)
> + return _random_read(flags & GRND_NONBLOCK, buf, count);
> + if (flags & GRND_NONBLOCK) {
> + if (!completion_done(&urandom_initialized))
> + return -EAGAIN;
> + } else {
> + r = wait_for_completion_interruptible(&urandom_initialized);
> + if (r)
> + return r;
> + }
> + return urandom_read(NULL, buf, count, NULL);
> +}
> +
However, here you can get called an arbitrary number of times.
It seems entirely possible than an attacker can manage to call
this function 2 billion times. Assuming a latency of 1 microsecond
per syscall, that would take about half an hour. After that, you
never again get any urandom data out of the syscall.
I think you are better off using a plain wait_event() here.
Arnd
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