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Merge pull request #12507 from poettering/random-fixlets

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three minor fixes to random-util.c
This commit is contained in:
Zbigniew Jędrzejewski-Szmek 2019-05-08 12:02:02 +02:00 committed by GitHub
parent 61b824c561 328f850e36
commit ad3b2109c3
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GPG key ID: 4AEE18F83AFDEB23
2 changed files with 43 additions and 30 deletions
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69
src/basic/random-util.c
View file

@ -28,13 +28,14 @@
#include "io-util.h"
#include "missing.h"
#include "random-util.h"
#include "siphash24.h"
#include "time-util.h"
int rdrand(unsigned long *ret) {
#if defined(__i386__) || defined(__x86_64__)
static int have_rdrand = -1;
unsigned char err;
uint8_t success;
if (have_rdrand < 0) {
uint32_t eax, ebx, ecx, edx;
@ -45,7 +46,12 @@ int rdrand(unsigned long *ret) {
return -EOPNOTSUPP;
}
have_rdrand = !!(ecx & (1U << 30));
/* Compat with old gcc where bit_RDRND didn't exist yet */
#ifndef bit_RDRND
#define bit_RDRND (1U << 30)
#endif
have_rdrand = !!(ecx & bit_RDRND);
}
if (have_rdrand == 0)
@ -54,9 +60,9 @@ int rdrand(unsigned long *ret) {
asm volatile("rdrand %0;"
"setc %1"
: "=r" (*ret),
"=qm" (err));
msan_unpoison(&err, sizeof(err));
if (!err)
"=qm" (success));
msan_unpoison(&success, sizeof(sucess));
if (!success)
return -EAGAIN;
return 0;
@ -71,21 +77,22 @@ int genuine_random_bytes(void *p, size_t n, RandomFlags flags) {
bool got_some = false;
int r;
/* Gathers some randomness from the kernel (or the CPU if the RANDOM_ALLOW_RDRAND flag is set). This call won't
* block, unless the RANDOM_BLOCK flag is set. If RANDOM_DONT_DRAIN is set, an error is returned if the random
* pool is not initialized. Otherwise it will always return some data from the kernel, regardless of whether
* the random pool is fully initialized or not. */
/* Gathers some randomness from the kernel (or the CPU if the RANDOM_ALLOW_RDRAND flag is set). This
* call won't block, unless the RANDOM_BLOCK flag is set. If RANDOM_MAY_FAIL is set, an error is
* returned if the random pool is not initialized. Otherwise it will always return some data from the
* kernel, regardless of whether the random pool is fully initialized or not. */
if (n == 0)
return 0;
if (FLAGS_SET(flags, RANDOM_ALLOW_RDRAND))
/* Try x86-64' RDRAND intrinsic if we have it. We only use it if high quality randomness is not
* required, as we don't trust it (who does?). Note that we only do a single iteration of RDRAND here,
* even though the Intel docs suggest calling this in a tight loop of 10 invocations or so. That's
* because we don't really care about the quality here. We generally prefer using RDRAND if the caller
* allows us too, since this way we won't drain the kernel randomness pool if we don't need it, as the
* pool's entropy is scarce. */
/* Try x86-64' RDRAND intrinsic if we have it. We only use it if high quality randomness is
* not required, as we don't trust it (who does?). Note that we only do a single iteration of
* RDRAND here, even though the Intel docs suggest calling this in a tight loop of 10
* invocations or so. That's because we don't really care about the quality here. We
* generally prefer using RDRAND if the caller allows us to, since this way we won't upset
* the kernel's random subsystem by accessing it before the pool is initialized (after all it
* will kmsg log about every attempt to do so)..*/
for (;;) {
unsigned long u;
size_t m;
@ -153,12 +160,13 @@ int genuine_random_bytes(void *p, size_t n, RandomFlags flags) {
break;
} else if (errno == EAGAIN) {
/* The kernel has no entropy whatsoever. Let's remember to use the syscall the next
* time again though.
/* The kernel has no entropy whatsoever. Let's remember to use the syscall
* the next time again though.
*
* If RANDOM_DONT_DRAIN is set, return an error so that random_bytes() can produce some
* pseudo-random bytes instead. Otherwise, fall back to /dev/urandom, which we know is empty,
* but the kernel will produce some bytes for us on a best-effort basis. */
* If RANDOM_MAY_FAIL is set, return an error so that random_bytes() can
* produce some pseudo-random bytes instead. Otherwise, fall back to
* /dev/urandom, which we know is empty, but the kernel will produce some
* bytes for us on a best-effort basis. */
have_syscall = true;
if (got_some && FLAGS_SET(flags, RANDOM_EXTEND_WITH_PSEUDO)) {
@ -167,7 +175,7 @@ int genuine_random_bytes(void *p, size_t n, RandomFlags flags) {
return 0;
}
if (FLAGS_SET(flags, RANDOM_DONT_DRAIN))
if (FLAGS_SET(flags, RANDOM_MAY_FAIL))
return -ENODATA;
/* Use /dev/urandom instead */
@ -196,14 +204,19 @@ void initialize_srand(void) {
return;
#if HAVE_SYS_AUXV_H
/* The kernel provides us with 16 bytes of entropy in auxv, so let's
* try to make use of that to seed the pseudo-random generator. It's
* better than nothing... */
/* The kernel provides us with 16 bytes of entropy in auxv, so let's try to make use of that to seed
* the pseudo-random generator. It's better than nothing... But let's first hash it to make it harder
* to recover the original value by watching any pseudo-random bits we generate. After all the
* AT_RANDOM data might be used by other stuff too (in particular: ASLR), and we probably shouldn't
* leak the seed for that. */
auxv = (const void*) getauxval(AT_RANDOM);
auxv = ULONG_TO_PTR(getauxval(AT_RANDOM));
if (auxv) {
assert_cc(sizeof(x) <= 16);
memcpy(&x, auxv, sizeof(x));
static const uint8_t auxval_hash_key[16] = {
0x92, 0x6e, 0xfe, 0x1b, 0xcf, 0x00, 0x52, 0x9c, 0xcc, 0x42, 0xcf, 0xdc, 0x94, 0x1f, 0x81, 0x0f
};
x = (unsigned) siphash24(auxv, 16, auxval_hash_key);
} else
#endif
x = 0;
@ -250,7 +263,7 @@ void pseudo_random_bytes(void *p, size_t n) {
void random_bytes(void *p, size_t n) {
if (genuine_random_bytes(p, n, RANDOM_EXTEND_WITH_PSEUDO|RANDOM_DONT_DRAIN|RANDOM_ALLOW_RDRAND) >= 0)
if (genuine_random_bytes(p, n, RANDOM_EXTEND_WITH_PSEUDO|RANDOM_MAY_FAIL|RANDOM_ALLOW_RDRAND) >= 0)
return;
/* If for some reason some user made /dev/urandom unavailable to us, or the kernel has no entropy, use a PRNG instead. */

4
src/basic/random-util.h
View file

@ -8,11 +8,11 @@
typedef enum RandomFlags {
RANDOM_EXTEND_WITH_PSEUDO = 1 << 0, /* If we can't get enough genuine randomness, but some, fill up the rest with pseudo-randomness */
RANDOM_BLOCK = 1 << 1, /* Rather block than return crap randomness (only if the kernel supports that) */
RANDOM_DONT_DRAIN = 1 << 2, /* If we can't get any randomness at all, return early with -EAGAIN */
RANDOM_MAY_FAIL = 1 << 2, /* If we can't get any randomness at all, return early with -ENODATA */
RANDOM_ALLOW_RDRAND = 1 << 3, /* Allow usage of the CPU RNG */
} RandomFlags;
int genuine_random_bytes(void *p, size_t n, RandomFlags flags); /* returns "genuine" randomness, optionally filled upwith pseudo random, if not enough is available */
int genuine_random_bytes(void *p, size_t n, RandomFlags flags); /* returns "genuine" randomness, optionally filled up with pseudo random, if not enough is available */
void pseudo_random_bytes(void *p, size_t n); /* returns only pseudo-randommess (but possibly seeded from something better) */
void random_bytes(void *p, size_t n); /* returns genuine randomness if cheaply available, and pseudo randomness if not. */