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Use secure random generator on macOS

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The implementation of arc4random_buf differs from its documentation. It
is documented as "always successful, and no return value is reserved to
indicate an error" for the sake of FreeBSD compatibility [1]. However,
the actual implementation on macOS invokes function "ccrng_generate" [2]
without validating the error cases. It might fail silently[3], which leads
to unexpected source of entropy.

The original arc4random used the RC4 a.k.a. ARC4 algorithm, and ChaCha20
based implementation was introduced in FreeBSD 12.0. Since macOS 10.12,
it was replaced with the NIST-approved AES cipher, and it may be replaced
again in the future as cryptographic techniques advance. Therefore, we
should not assume that arc4random never fails.

On the contrary, CCRandomGenerateBytes(), part of Cryptographic Services [4],
returns cryptographically strong random bits with explicit status code.
This patch properly calls CCRandomGenerateBytes() and checks the status.

[1] https://www.freebsd.org/cgi/man.cgi?query=arc4random_buf
[2] https://opensource.apple.com/source/CommonCrypto/CommonCrypto-60178.40.2/lib/CommonRandom.c.auto.html
[3] https://opensource.apple.com/source/Libc/Libc-1439.40.11/gen/FreeBSD/arc4random.c.auto.html
[4] https://developer.apple.com/documentation/security
This commit is contained in:
Jim Huang 2021-04-24 06:24:06 +08:00 committed by Jim Huang
parent 71b2c441bb
commit c4947c8879
1 changed files with 32 additions and 2 deletions
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34
src/random.c
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@ -156,7 +156,8 @@ uintptr_t _mi_random_next(mi_random_ctx_t* ctx) {
/* ----------------------------------------------------------------------------
To initialize a fresh random context we rely on the OS:
- Windows : BCryptGenRandom (or RtlGenRandom)
- osX,bsd,wasi: arc4random_buf
- macOS : CCRandomGenerateBytes
- bsd,wasi : arc4random_buf
- Linux : getrandom,/dev/urandom
If we cannot get good randomness, we fall back to weak randomness based on a timer and ASLR.
-----------------------------------------------------------------------------*/
@ -186,7 +187,36 @@ static bool os_random_buf(void* buf, size_t buf_len) {
}
#endif
#elif defined(ANDROID) || defined(XP_DARWIN) || defined(__APPLE__) || defined(__DragonFly__) || \
#elif defined(__APPLE__)
#include <AvailabilityMacros.h>
#if defined(MAC_OS_X_VERSION_10_10) && \
MAC_OS_X_VERSION_MAX_ALLOWED >= MAC_OS_X_VERSION_10_10
#include <CommonCrypto/CommonRandom.h>
#endif
static bool os_random_buf(void* buf, size_t buf_len) {
/* The implementation of arc4random_buf(3) differs from its documentation.
* It is documented as "always successful, and no return value is reserved
* to indicate an error." However, the actual implementation invokes the
* function "ccrng_generate" without validating the error cases. It might
* fail silently, which leads to unexpected source of entropy.
* See:
* https://opensource.apple.com/source/Libc/Libc-1439.40.11/gen/FreeBSD/arc4random.c.auto.html
*
* CCRandomGenerateBytes(), on the contrary, returns cryptographically strong
* random bits with explicit status code.
*/
#if defined(MAC_OS_X_VERSION_10_15) && \
MAC_OS_X_VERSION_MAX_ALLOWED >= MAC_OS_X_VERSION_10_15
return CCRandomGenerateBytes(buf, buf_len) == kCCSuccess;
#else
/* Prior to macOS 10.15, CCRandomGenerateBytes() might take a bit longer time
* to complete, so failback to arc4random_buf().
*/
arc4random_buf(buf, buf_len);
return true;
#endif
}
#elif defined(ANDROID) || defined(__DragonFly__) || \
defined(__FreeBSD__) || defined(__NetBSD__) || defined(__OpenBSD__) || \
defined(__sun) || defined(__wasi__)
#include <stdlib.h>