grantpt report error after unshare

I have a small program, which tries to create a pseudoterminal after unshare. the output is:

uid before unshare:5000
uid after unshare:0
Grant pt Error: : Permission denied

The Code:

#define _GNU_SOURCE

#include <sys/mount.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
#include <errno.h>
#include <sched.h>

void set_uid_map(pid_t pid, int inside_id, int outside_id, int length) {
    char path[256];
    sprintf(path, "/proc/%d/uid_map", getpid());
    FILE* uid_map = fopen(path, "w");
    fprintf(uid_map, "%d %d %d", inside_id, outside_id, length);
    fclose(uid_map);
}
void set_gid_map(pid_t pid, int inside_id, int outside_id, int length) {
    char path[256];
    sprintf(path, "/proc/%d/gid_map", getpid());
    FILE* gid_map = fopen(path, "w");
    fprintf(gid_map, "%d %d %d", inside_id, outside_id, length);
    fclose(gid_map);
}

int main(void)
{
int master;
int flag = 0;

 flag |= CLONE_NEWUSER;
 flag |= CLONE_NEWNS;
 flag |= CLONE_NEWIPC;
 flag |= CLONE_NEWNET;
 flag |= CLONE_NEWUTS;
 flag |= CLONE_NEWPID;

 printf("uid before unshare:%d n", (int) getuid());
 unshare(flag);

 set_uid_map(getpid(), 0, 5000, 1);
 set_gid_map(getpid(), 0, 5000, 1);

 printf("uid after unshare:%d n", (int) getuid());

 if ( ( master = posix_openpt(O_RDWR | O_NOCTTY) ) < 0)
      perror("Openpt Error: ");
 if ( grantpt(master) < 0 )
      perror("Grant pt Error: ");
 unlockpt(master);


return 0;
} // main

If I remove flag |= CLONE_NEWUSER;, there is not error reported. Can you help to explain why this happens? thanks in advance!

Answer

Since I’ve had the same issue I have also looked into this. Here are my findings:

grantpt(3) tries to ensure that the slave pseudo terminal has its group set to the special tty group (or whatever TTY_GROUP is when compiling glibc):

static int tty_gid = -1;
if (__glibc_unlikely (tty_gid == -1))
  {
    char *grtmpbuf;
    struct group grbuf;
    size_t grbuflen = __sysconf (_SC_GETGR_R_SIZE_MAX);
    struct group *p;

    /* Get the group ID of the special `tty' group.  */
    if (grbuflen == (size_t) -1L)
      /* `sysconf' does not support _SC_GETGR_R_SIZE_MAX.
         Try a moderate value.  */
      grbuflen = 1024;
    grtmpbuf = (char *) __alloca (grbuflen);
    __getgrnam_r (TTY_GROUP, &grbuf, grtmpbuf, grbuflen, &p);
    if (p != NULL)
      tty_gid = p->gr_gid;
  }
gid_t gid = tty_gid == -1 ? __getgid () : tty_gid;

/* Make sure the group of the device is that special group.  */
if (st.st_gid != gid)
  {
    if (__chown (buf, uid, gid) < 0)
      goto helper;
  }

See https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/grantpt.c;h=c04c85d450f9296efa506121bcee022afda3e2dd;hb=HEAD#l137.

On my system, the tty group is 5. However, that group isn’t mapped into your user namespace and the chown(2) fails because the GID 5 doesn’t exist. glibc then falls back to executing the pt_chown helper, which also fails. I haven’t looked into the details of why it fails, but I assume it’s because it’s setuid nobody unless you mapped the root user to your user namespace. Here’s strace output that shows the failing operation:

[pid    30] chown("/dev/pts/36", 1000, 5) = -1 EINVAL (Invalid argument)

The gives you a couple of methods to work around this problem:

• Map the required groups (i.e. tty), which may not be possible without CAP_SYS_ADMIN in the binary that opens the user namespace
• Use subuids and subgids together with newuidmap(1) and newgidmap(1) to make these groups available (this might work, but I haven’t tested it).
• Make changes that avoid the failure of the chown(2) call, e.g. by using a mount namespace and changing the GID of the tty group in /etc/groups to your user’s GID.
• Avoid the chown(2) call, e.g. by making the st.st_gid != gid check false; this should be possible by deleting the tty group from your target mount namespace’s /etc/groups. Of course, that may cause other problems.