Remove comments about overread for entropy generation (#1551)

AWS-LC does not overread default entropy sources in FIPS mode. This commit is to
clarify this and remove/rephrase comments mentioning overread.

crypto/fipsmodule/FIPS.md

@@ -40,7 +40,7 @@ The DRBG state is kept in a thread-local structure and is seeded using the confi
 
 The CTR-DRBG is reseeded every 4096 calls to `RAND_bytes`. Thus the process will randomly crash about every 2¹³⁵ calls.
 
-The FIPS PRNGs allow “additional input” to be fed into a given call. We use this feature to be as robust as possible to state duplication from process forks and VM copies: for every call we read 32 bytes of “additional data” from the entropy source (without overread) which means that cloned states will diverge at the next call to `RAND_bytes`. This is called “prediction resistance” by FIPS, but we do *not* claim this property in a FIPS context because we don't implement it the way they want.
+The FIPS PRNGs allow “additional input” to be fed into a given call. We use this feature to be as robust as possible to state duplication from process forks and VM copies: on Intel CPUs with RDRAND, for every call we read 32 bytes of “additional data” from RDRAND, which means that cloned states will diverge at the next call to `RAND_bytes`. This is called “prediction resistance” by FIPS, but we do *not* claim this property in a FIPS context because we don't implement it the way they want.
 
 There is a second interface to the RNG which allows the caller to supply bytes that will be XORed into the generated additional data (`RAND_bytes_with_additional_data`). This is used in the ECDSA code to include the message and private key in the generation of *k*, the ECDSA nonce. This allows ECDSA to be robust to entropy failures while still following the FIPS rules.
 
@@ -50,7 +50,7 @@ FIPS requires that RNG state be zeroed when the process exits. In order to imple
 
 By default, entropy is sourced using a "Passive" method where the specific entropy source depends on the OE. Seeding and reseeding material for the DRBG is sourced from the specific entropy source.
 
-In FIPS mode, each of the entropy sources is subject to a 10× overread. That is, when *n* bytes of entropy are needed, *10n* bytes will be read from the entropy source and XORed down to *n* bytes.
+In FIPS mode, when the CPU Jitter entropy source is used, we do a 3x oversampling (the CPU Jitter default setting). Note that the CPU Jitter is not the default entropy source (see subsection below).
 
 ### Modify entropy source - not recommended
 

crypto/fipsmodule/rand/rand.c

@@ -364,8 +364,9 @@ static void rand_get_seed(struct rand_thread_state *state,
 static void rand_get_seed(struct rand_thread_state *state,
                           uint8_t seed[CTR_DRBG_ENTROPY_LEN],
                           int *out_want_additional_input) {
-  // If not in FIPS mode, we don't overread from the system entropy source and
-  // we don't depend only on the hardware RDRAND.
+  // If not in FIPS mode, we use the system entropy source.
+  // We don't source the entropy directly from the CPU.
+  // Therefore, |*out_want_additonal_input| is set to zero.
   CRYPTO_sysrand_for_seed(seed, CTR_DRBG_ENTROPY_LEN);
   *out_want_additional_input = 0;
 }