From: "Jason A. Donenfeld" Jason@zx2c4.com
commit 57826feeedb63b091f807ba8325d736775d39afd upstream.
If we're trusting bootloader randomness, crng_fast_load() is called by add_hwgenerator_randomness(), which sets us to crng_init==1. However, usually it is only called once for an initial 64-byte push, so bootloader entropy will not mix any bytes into the input pool. So it's conceivable that crng_init==1 when crng_initialize_primary() is called later, but then the input pool is empty. When that happens, the crng state key will be overwritten with extracted output from the empty input pool. That's bad.
In contrast, if we're not trusting bootloader randomness, we call crng_slow_load() *and* we call mix_pool_bytes(), so that later crng_initialize_primary() isn't drawing on nothing.
In order to prevent crng_initialize_primary() from extracting an empty pool, have the trusted bootloader case mirror that of the untrusted bootloader case, mixing the input into the pool.
[linux@dominikbrodowski.net: rewrite commit message] Signed-off-by: Dominik Brodowski linux@dominikbrodowski.net Signed-off-by: Jason A. Donenfeld Jason@zx2c4.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/char/random.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-)
--- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -2295,8 +2295,12 @@ void add_hwgenerator_randomness(const ch struct entropy_store *poolp = &input_pool;
if (unlikely(crng_init == 0)) { - crng_fast_load(buffer, count); - return; + size_t ret = crng_fast_load(buffer, count); + mix_pool_bytes(poolp, buffer, ret); + count -= ret; + buffer += ret; + if (!count || crng_init == 0) + return; }
/* Suspend writing if we're above the trickle threshold.