On Thu, 13 Dec 2018 at 10:18, Louis Collard <louiscollard@xxxxxxxxxxxx> wrote: > > Some systems, for example embedded systems, do not generate > enough entropy on boot through interrupts, and boot may be blocked for > several minutes waiting for a call to getrandom to complete. > > Currently, random data is read from a hwrng when it is registered, > and is loaded into primary_crng. This data is treated in the same > way as data that is device-specific but otherwise unchanging, and > so primary_crng cannot become initialized with the data from the > hwrng. > > This change causes the data initially read from the hwrng to be > treated the same as subsequent data that is read from the hwrng if > it's quality score is non-zero. > > The implications of this are: > > The data read from hwrng can cause primary_crng to become > initialized, therefore avoiding problems of getrandom blocking > on boot. > > Calls to getrandom (with GRND_RANDOM) may be using entropy > exclusively (or in practise, almost exclusively) from the hwrng. > > Regarding the latter point; this behavior is the same as if a > user specified a quality score of 1 (bit of entropy per 1024 bits) > so hopefully this is not too scary a change to make. > > This change is the result of the discussion here: > https://patchwork.kernel.org/patch/10453893/ > > Signed-off-by: Louis Collard <louiscollard@xxxxxxxxxxxx> > Acked-by: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx> > --- > drivers/char/hw_random/core.c | 11 +++++++++-- > 1 file changed, 9 insertions(+), 2 deletions(-) > > diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c > index 95be7228f327..7736e1a96321 100644 > --- a/drivers/char/hw_random/core.c > +++ b/drivers/char/hw_random/core.c > @@ -24,6 +24,7 @@ > #include <linux/sched.h> > #include <linux/slab.h> > #include <linux/uaccess.h> > +#include <crypto/chacha20.h> > > #define RNG_MODULE_NAME "hw_random" > > @@ -64,13 +65,19 @@ static size_t rng_buffer_size(void) > static void add_early_randomness(struct hwrng *rng) > { > int bytes_read; > - size_t size = min_t(size_t, 16, rng_buffer_size()); > + /* Read enough to initialize crng. */ > + size_t size = min_t(size_t, > + 2*CHACHA20_KEY_SIZE, This should be as symbolic constant that retains its meaning even if we move away from ChaCha20 or modify the current implementation > + rng_buffer_size()); > > mutex_lock(&reading_mutex); > bytes_read = rng_get_data(rng, rng_buffer, size, 1); > mutex_unlock(&reading_mutex); > if (bytes_read > 0) > - add_device_randomness(rng_buffer, bytes_read); > + /* Allow crng to become initialized, but do not add > + * entropy to the pool. > + */ > + add_hwgenerator_randomness(rng_buffer, bytes_read, 0); > } > > static inline void cleanup_rng(struct kref *kref) > -- > 2.13.5 >
