Deny an access to shared memory from non-forked processes

Question

I need to create a shared memory segment which contains some secret data. I use shmget and shmat functions to access the segment with 0600 rights. I want to share this segment of memory with forked processes only. I tried to create another application which tried to access this segment and it was unsuccessful, so it looks like it's working as I want.

But when I run the application which created the segment again, it can access the segment. How is it possible? Is a good idea to store secret data into shared memory?

el.pescado el.pescado · Accepted Answer · 2016-01-12T09:08:23

You can mmap() shared and anonymous memory region by providing MAP_SHARED and MAP_ANONYMOUS flags in parent process. That memory will be accessible only to that process and its children. As memory segment is anonymous, no other processes will be able to refer to it, let alone access/map it:

void *shared_mem = mmap(NULL, n_bytes, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0);

Parent process should create shared memory segment using mmap(). That memory segment is inherited by any child process created by fork(). Child process can simply use shared_mem pointer inherited from parent to refer to that memory segment:

#include <sys/mman.h>
#include <sys/types.h>
#include <unistd.h>

int main()
{
    void *shared_mem = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0);

    pid_t pid = fork();
    if (pid > 0) {
        // parent
        // use shared_mem here

    } else if (pid == 0) {
        // child
        // use shared_mem here

    } else {
        // error
    }
    return 0;
}

Deny an access to shared memory from non-forked processes

2 Answers