mlock(2)

Synopsis

#include <sys/mman.h>

int mlock(const void addr[size], size_t size);
int mlock2(const void addr[size], size_t size, unsigned int flags);
int munlock(const void addr[size], size_t size);

int mlockall(int flags);
int munlockall(void);

Rod's take

Your private key leaks three ways: swap, core dumps and fork, and mlock only fixes the first. Lock the pages, and if mlockall comes back ENOMEM, die. Logging a warning and carrying on means the key hits swap during some 3am memory crunch and nobody ever notices. Raise RLIMIT_MEMLOCK or hold CAP_IPC_LOCK. Then add MADV_DONTDUMP for crash dumps, MADV_WIPEONFORK so children see zeros, and explicit_bzero when you're done, because the compiler will happily delete your memset. None of it stops root reading your memory. It stops the boring leaks.

Flags and arguments that matter

Errors you will actually hit

Example

unsigned char *alloc_secret(size_t len) {
    unsigned char *buf;
    if (posix_memalign((void **)&buf, getpagesize(), len) != 0)
        return NULL;
    if (mlock(buf, len) != 0) { free(buf); return NULL; }
    madvise(buf, len, MADV_DONTDUMP);
    madvise(buf, len, MADV_WIPEONFORK);
    return buf;
}

void free_secret(unsigned char *buf, size_t len) {
    explicit_bzero(buf, len);
    munlock(buf, len);
    free(buf);
}

Trimmed from the full walkthrough in mlock, MADV_DONTDUMP, and where your secrets actually leak.

BSD equivalent

FreeBSD: mlock(2)/mlockall(2) (mlock since 4.4BSD); madvise(2) MADV_NOCORE for core-dump exclusion. OpenBSD: mlock(2)/mlockall(2) (4.4BSD).