KEYS: Fix keyring ref leak in join_session_keyring()

[mischief]

If a thread is asked to join as a session keyring the keyring that's already
set as its session, we leak a keyring reference.

This can be tested with the following program:

#include <stddef.h>
#include <stdio.h>
#include <sys/types.h>
#include <keyutils.h>

int main(int argc, const char *argv[])
{
    int i = 0;
    key_serial_t serial;

    serial = keyctl(KEYCTL_JOIN_SESSION_KEYRING,
            "leaked-keyring");
    if (serial < 0) {
        perror("keyctl");
        return -1;
    }

    if (keyctl(KEYCTL_SETPERM, serial,
           KEY_POS_ALL | KEY_USR_ALL) < 0) {
        perror("keyctl");
        return -1;
    }

    for (i = 0; i < 100; i++) {
        serial = keyctl(KEYCTL_JOIN_SESSION_KEYRING,
                "leaked-keyring");
        if (serial < 0) {
            perror("keyctl");
            return -1;
        }
    }

    return 0;
}

If, after the program has run, there something like the following line in
/proc/keys:

3f3d898f I--Q--- 100 perm 3f3f0000 0 0 keyring leaked-keyring: empty

with a usage count of 100 * the number of times the program has been run,
then the kernel is malfunctioning. If leaked-keyring has zero usages or
has been garbage collected, then the problem is fixed.

Reported-by: Yevgeny Pats yevgeny@perception-point.io
Signed-off-by: David Howells dhowells@redhat.com

[mjg59]

lgtm