Kees Cook: keeping your process unprivileged
One of the prerequisites for seccomp filter is the new PR_SET_NO_NEW_PRIVS prctl from Andy Lutomirski.
If you re not interested in digging into creating a seccomp filter for your program, but you know your program should be effectively a leaf node in the process tree, you can call PR_SET_NO_NEW_PRIVS (nnp) to make sure that the current process and its children can not gain new privileges (like through running a setuid binary). This produces some fun results, since things like the ping tool expect to gain enough privileges to open a raw socket. If you set nnp to 1 , suddenly that can t happen any more.
Here s a quick example that sets nnp, and tries to run the command line arguments:
#include <stdio.h>
#include <unistd.h>
#include <sys/prctl.h>
#ifndef PR_SET_NO_NEW_PRIVS
# define PR_SET_NO_NEW_PRIVS 38
#endif
int main(int argc, char * argv[])
if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0))
perror("prctl(NO_NEW_PRIVS)");
return 1;
return execvp(argv[1], &argv[1]);
When it tries to run ping, the setuid-ness just gets ignored:
$ gcc -Wall nnp.c -o nnp $ ./nnp ping -c1 localhost ping: icmp open socket: Operation not permittedSo, if your program has all the privs its going to need, consider using nnp to keep it from being a potential gateway to more trouble. Hopefully we can ship something like this trivial nnp helper as part of coreutils or similar, like nohup, nice, etc.
2012, Kees Cook. This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 License.
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