commit 353c71516e80f41dbb97da00ddfc528ec5ce3c40 Author: Yawning Angel <yawning@schwanenlied.me> Date: Thu Nov 26 15:21:50 2015 +0000 Add support for getrandom() and getentropy() when available Implements feature #13696. --- changes/feature13696 | 3 ++ configure.ac | 2 + src/common/crypto.c | 146 +++++++++++++++++++++++++++++++++++++++++++++----- src/common/sandbox.c | 4 ++ 4 files changed, 142 insertions(+), 13 deletions(-) diff --git a/changes/feature13696 b/changes/feature13696 new file mode 100644 index 0000000..21c2188 --- /dev/null +++ b/changes/feature13696 @@ -0,0 +1,3 @@ + o Minor features (security, cryptography): + - Use modern system calls to generate strong entropy on platforms that + provide them. Closes ticket 13696. diff --git a/configure.ac b/configure.ac index 0530d77..b14361f 100644 --- a/configure.ac +++ b/configure.ac @@ -384,6 +384,7 @@ AC_CHECK_FUNCS( flock \ ftime \ getaddrinfo \ + getentropy \ getifaddrs \ getpass \ getrlimit \ @@ -951,6 +952,7 @@ AC_CHECK_HEADERS( sys/select.h \ sys/socket.h \ sys/statvfs.h \ + sys/syscall.h \ sys/sysctl.h \ sys/syslimits.h \ sys/time.h \ diff --git a/src/common/crypto.c b/src/common/crypto.c index baef755..5d8b45e 100644 --- a/src/common/crypto.c +++ b/src/common/crypto.c @@ -43,6 +43,7 @@ #include <ctype.h> #endif #ifdef HAVE_UNISTD_H +#define _GNU_SOURCE #include <unistd.h> #endif #ifdef HAVE_FCNTL_H @@ -51,6 +52,9 @@ #ifdef HAVE_SYS_FCNTL_H #include <sys/fcntl.h> #endif +#ifdef HAVE_SYS_SYSCALL_H +#include <sys/syscall.h> +#endif #include "torlog.h" #include "aes.h" @@ -68,6 +72,9 @@ /** Longest recognized */ #define MAX_DNS_LABEL_SIZE 63 +/** Largest strong entropy request */ +#define MAX_STRONGEST_RAND_SIZE 256 + /** Macro: is k a valid RSA public or private key? */ #define PUBLIC_KEY_OK(k) ((k) && (k)->key && (k)->key->n) /** Macro: is k a valid RSA private key? */ @@ -2344,23 +2351,18 @@ crypto_seed_weak_rng(tor_weak_rng_t *rng) } /** Try to get <b>out_len</b> bytes of the strongest entropy we can generate, - * storing it into <b>out</b>. Return -1 on success, 0 on failure. + * via system calls, storing it into <b>out</b>. Return -1 on success, 0 on + * failure. A maximum request size of 256 bytes is imposed. */ -int -crypto_strongest_rand(uint8_t *out, size_t out_len) +static int +crypto_strongest_rand_syscall(uint8_t *out, size_t out_len) { -#ifdef _WIN32 + tor_assert(out_len <= MAX_STRONGEST_RAND_SIZE); + +#if defined(_WIN32) static int provider_set = 0; static HCRYPTPROV provider; -#else - static const char *filenames[] = { - "/dev/srandom", "/dev/urandom", "/dev/random", NULL - }; - int fd, i; - size_t n; -#endif -#ifdef _WIN32 if (!provider_set) { if (!CryptAcquireContext(&provider, NULL, NULL, PROV_RSA_FULL, CRYPT_VERIFYCONTEXT)) { @@ -2375,7 +2377,79 @@ crypto_strongest_rand(uint8_t *out, size_t out_len) } return 0; +#elif defined(__linux__) && defined(SYS_getrandom) + static int getrandom_works = 1; /* Be optimitic about our chances... */ + + /* getrandom() isn't as straight foward as getentropy(), and has + * no glibc wrapper. + * + * As far as I can tell from getrandom(2) and the source code, the + * requests we issue will always succeed (though it will block on the + * call if /dev/urandom isn't seeded yet), since we are NOT specifying + * GRND_NONBLOCK and the request is <= 256 bytes. + * + * The manpage is unclear on what happens if a signal interrupts the call + * while the request is blocked due to lack of entropy.... + * + * We optimistically assume that getrandom() is available and functional + * because it is the way of the future, and 2 branch mispredicts pale in + * comparision to the overheads involved with failing to open + * /dev/srandom followed by opening and reading from /dev/urandom. + */ + if (PREDICT_LIKELY(getrandom_works)) { + int ret; + do { + /* A flag of '0' here means to read from '/dev/urandom', and to + * block if insufficient entropy is available to service the + * request. + */ + ret = syscall(SYS_getrandom, out, out_len, 0); + } while (ret == -1 && ((errno == EINTR) ||(errno == EAGAIN))); + + if (PREDICT_UNLIKELY(ret == -1)) { + tor_assert(errno != EAGAIN); + tor_assert(errno != EINTR); + + /* Probably ENOSYS. */ + log_warn(LD_CRYPTO, "Can't get entropy from getrandom()."); + getrandom_works = 0; /* Don't bother trying again. */ + return -1; + } + + tor_assert(ret == (int)out_len); + return 0; + } + + return -1; /* getrandom() previously failed unexpectedly. */ +#elif defined(HAVE_GETENTROPY) + /* getentropy() is what Linux's getrandom() wants to be when it grows up. + * the only gotcha is that requests are limited to 256 bytes. + */ + return getentropy(out, out_len); +#endif + + /* This platform doesn't have a supported syscall based random. */ + return -1; +} + +/** Try to get <b>out_len</b> bytes of the strongest entropy we can generate, + * via the per-platform fallback mechanism, storing it into <b>out</b>. + * Return -1 on success, 0 on failure. A maximum request size of 256 bytes + * is imposed. + */ +static int +crypto_strongest_rand_fallback(uint8_t *out, size_t out_len) +{ +#ifdef _WIN32 + /* Windows exclusively uses crypto_strongest_rand_syscall(). */ + return -1; #else + static const char *filenames[] = { + "/dev/srandom", "/dev/urandom", "/dev/random", NULL + }; + int fd, i; + size_t n; + for (i = 0; filenames[i]; ++i) { log_debug(LD_FS, "Opening %s for entropy", filenames[i]); fd = open(sandbox_intern_string(filenames[i]), O_RDONLY, 0); @@ -2393,11 +2467,57 @@ crypto_strongest_rand(uint8_t *out, size_t out_len) return 0; } - log_warn(LD_CRYPTO, "Cannot get strong entropy: no entropy source found."); return -1; #endif } +/** Try to get <b>out_len</b> bytes of the strongest entropy we can generate, + * storing it into <b>out</b>. Return -1 on success, 0 on failure. A maximum + * request size of 256 bytes is imposed. + */ +int +crypto_strongest_rand(uint8_t *out, size_t out_len) +{ + static const size_t sanity_min_size = 16; + static const int max_attempts = 3; + tor_assert(out_len <= MAX_STRONGEST_RAND_SIZE); + + /* For buffers >= 16 bytes (128 bits), we sanity check the output by + * zero filling the buffer and ensuring that it actually was at least + * partially modified. + * + * Checking that any individual byte is non-zero seems like it would + * fail too often (p = out_len * 1/256) for comfort, but this is an + * "adjust according to taste" sort of check. + */ + memwipe(out, 0, out_len); + for (int i = 0; i < max_attempts; i++) { + /* Try to use the syscall/OS favored mechanism to get strong entropy. */ + if (crypto_strongest_rand_syscall(out, out_len) != 0) { + /* Try to use the less-favored mechanism to get strong entropy. */ + if (crypto_strongest_rand_fallback(out, out_len) != 0) { + /* Welp, we tried. Hopefully the calling code terminates the process + * since we're basically boned without good entropy. + */ + log_warn(LD_CRYPTO, + "Cannot get strong entropy: no entropy source found."); + return -1; + } + } + + if ((out_len < sanity_min_size) || !tor_mem_is_zero((char*)out, out_len)) + return 0; + } + + /* We tried max_attempts times to fill a buffer >= 128 bits long, + * and each time it returned all '0's. Either the system entropy + * source is busted, or the user should go out and buy a ticket to + * every lottery on the planet. + */ + log_warn(LD_CRYPTO, "Strong OS entropy returned all zero buffer."); + return -1; +} + /** Seed OpenSSL's random number generator with bytes from the operating * system. Return 0 on success, -1 on failure. */ diff --git a/src/common/sandbox.c b/src/common/sandbox.c index b995762..8198858 100644 --- a/src/common/sandbox.c +++ b/src/common/sandbox.c @@ -199,6 +199,10 @@ static int filter_nopar_gen[] = { SCMP_SYS(stat64), #endif +#ifdef __NR_getrandom + SCMP_SYS(getrandom), +#endif + /* * These socket syscalls are not required on x86_64 and not supported with * some libseccomp versions (eg: 1.0.1)