Re: [PATCH] printk: Check real user/group id for %pK

["George Spelvin" <linux@horizon.com>]

The basic idea is good, but I'm not sure if this is the correct permission
check to use.

After all, a setuid program might also want to give filtered access to
a /proc file with some %pK values.

The fundamental problem is that %pK is using permissions at the time
of the read(), while the general Unix rule that setuid programs expect
is that permission is checked at open() time.  pppd is an example; its
options_fom_file() function (pppd/options.c:391 in the 2.4.5 release)
does:

    euid = geteuid();
    if (check_prot && seteuid(getuid()) == -1) {
	option_error("unable to drop privileges to open %s: %m", filename);
	return 0;
    }
    f = fopen(filename, "r");
    err = errno;
    if (check_prot && seteuid(euid) == -1)
	fatal("unable to regain privileges");


Now the whole struct cred and capability system is something I don't
really understand, but it is clear from a brief look at the code
that getting the appropriate credential through the seq_file to
lib/vsprintf.c:pointer() would be tricky.

But it also seems like the Right Thing to do; other fixes seem like
ineffective kludges.