Re: [PATCH v13 6/7] random: introduce generic vDSO getrandom() implementation

Eric Biggers
2022-12-21T22:23:42Z

On Wed, Dec 21, 2022 at 03:23:26PM +0100, Jason A. Donenfeld wrote:
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 6425f5f838e0..660cd15b6228 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -60,6 +60,7 @@
> #include <crypto/blake2s.h>
> #ifdef CONFIG_VDSO_GETRANDOM
> #include <vdso/getrandom.h>
> +#include <vdso/datapage.h>
> #endif
> #include <asm/archrandom.h>
> #include <asm/processor.h>
> @@ -407,6 +408,9 @@ static void crng_reseed(struct work_struct *work)
> /*
> * We copy the new key into the base_crng, overwriting the old one,
> * and update the generation counter. We avoid hitting ULONG_MAX,
> * because the per-cpu crngs are initialized to ULONG_MAX, so this
> * forces new CPUs that come online to always initialize.
> */
> spin_lock_irqsave(&base_crng.lock, flags);
> memcpy(base_crng.key, key, sizeof(base_crng.key));
> next_gen = base_crng.generation + 1;
> if (next_gen == ULONG_MAX)
> ++next_gen;
> WRITE_ONCE(base_crng.generation, next_gen);
> +#ifdef CONFIG_VDSO_GETRANDOM
> + smp_store_release(&_vdso_rng_data.generation, next_gen + 1);
> +#endif

It's confusing that "uninitialized generation" is ULONG_MAX in the per-cpu
crngs, but 0 in the vdso_rng_data. That results in a weird off-by one thing,
where the vdso_rng_data generation number has to be 1 higher.

Would it be possible to use 0 for both?

- Eric