Re: [kernel-hardening] [PATCH] move RLIMIT_NPROC check from set_user() to do_execve_common()

Vasiliy Kulikov
2011-07-14T15:55:22Z

Solar,

On Thu, Jul 14, 2011 at 19:22 +0400, Solar Designer wrote:
> On Tue, Jul 12, 2011 at 05:27:23PM +0400, Vasiliy Kulikov wrote:
> > + const struct cred *cred = current_cred();
> > +
> > + /*
> > + * We check for RLIMIT_NPROC in execve() instead of set_user() because
> > + * too many poorly written programs don't check setuid() return code.
> > + * The check in execve() does the same thing for programs doing
> > + * setuid()+execve(), but without similar security issues.
> > + */
> > + if (atomic_read(&cred->user->processes) > rlimit(RLIMIT_NPROC) &&
> > + cred->user != INIT_USER) {
> > + retval = -EAGAIN;
> > + goto out_ret;
> > + }
>
> Is cred->user == NULL impossible here? Somehow I had a check for NULL
> here in -ow patches (for older kernels), maybe out of paranoia or maybe
> for specific reasons (I don't recall).

It is not checked in copy_process(), which is the same kind of syscalls,
I don't see how it can be NULL here.

> > +++ b/kernel/sys.c
> > @@ -591,12 +591,6 @@ static int set_user(struct cred *new)
> > if (!new_user)
> > return -EAGAIN;
> >
> > - if (atomic_read(&new_user->processes) >= rlimit(RLIMIT_NPROC) &&
> > - new_user != INIT_USER) {
> > - free_uid(new_user);
> > - return -EAGAIN;
> > - }
>
> So you're moving the check almost literally. However, I think a similar
> check on fork() also checked "!capable(CAP_SYS_ADMIN) &&
> !capable(CAP_SYS_RESOURCE)", and I had this additional check/bypass
> included in -ow patches' execve().

Oh, right. I also noted these cap checks while observing -ow and
-grsecurity patches, but somehow missed them now. As you see, there
already was such inconsistency in setuid() case. I don't know what is a
right way - either copy setuid's blind way or immitate fork's pedantic
checks.

However, I really don't see any need in CAP_SYS_ADMIN check - if it is
to be sure some core system process has not suffocated then it simply
should reset RLIMIT_NPROC and that's all.

Thanks,

--
Vasiliy