Re: [PATCH 6/8] ucounts: Handle inc_rlimit_ucounts wrapping in fork

[Alexey Gladkov]

On Fri, Feb 11, 2022 at 11:50:46AM -0600, Eric W. Biederman wrote:
> Alexey Gladkov <legion@xxxxxxxxxx> writes:
> 
> > On Thu, Feb 10, 2022 at 08:13:22PM -0600, Eric W. Biederman wrote:
> >> Move inc_rlimit_ucounts from copy_creds into copy_process immediately
> >> after copy_creds where it can be called exactly once.  Test for and
> >> handle it when inc_rlimit_ucounts returns LONG_MAX indicating the
> >> count has wrapped.
> >> 
> >> This is good hygenine and fixes a theoretical bug.  In practice
> >> PID_MAX_LIMIT is at most 2^22 so there is not a chance the number of
> >> processes would ever wrap even on an architecture with a 32bit long.
> >> 
> >> Fixes: 21d1c5e386bc ("Reimplement RLIMIT_NPROC on top of ucounts")
> >> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> >> ---
> >>  kernel/cred.c | 2 --
> >>  kernel/fork.c | 2 ++
> >>  2 files changed, 2 insertions(+), 2 deletions(-)
> >> 
> >> diff --git a/kernel/cred.c b/kernel/cred.c
> >> index 229cff081167..96d5fd6ff26f 100644
> >> --- a/kernel/cred.c
> >> +++ b/kernel/cred.c
> >> @@ -358,7 +358,6 @@ int copy_creds(struct task_struct *p, unsigned long clone_flags)
> >>  		kdebug("share_creds(%p{%d,%d})",
> >>  		       p->cred, atomic_read(&p->cred->usage),
> >>  		       read_cred_subscribers(p->cred));
> >> -		inc_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
> >>  		return 0;
> >>  	}
> >>  
> >> @@ -395,7 +394,6 @@ int copy_creds(struct task_struct *p, unsigned long clone_flags)
> >>  #endif
> >>  
> >>  	p->cred = p->real_cred = get_cred(new);
> >> -	inc_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
> >>  	alter_cred_subscribers(new, 2);
> >>  	validate_creds(new);
> >>  	return 0;
> >> diff --git a/kernel/fork.c b/kernel/fork.c
> >> index 6f62d37f3650..69333078259c 100644
> >> --- a/kernel/fork.c
> >> +++ b/kernel/fork.c
> >> @@ -2026,6 +2026,8 @@ static __latent_entropy struct task_struct *copy_process(
> >>  		goto bad_fork_free;
> >>  
> >>  	retval = -EAGAIN;
> >> +	if (inc_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1) == LONG_MAX)
> >> +		goto bad_fork_cleanup_count;
> >>  	if (is_ucounts_overlimit(task_ucounts(p), UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC))) {
> >>  		if ((task_ucounts(p) != &init_ucounts) &&
> >>  		    !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
> >
> > It might make sense to do something like:
> >
> > 	if (inc_rlimit_ucounts_overlimit(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1, rlimit(RLIMIT_NPROC)) == LONG_MAX) {
> > 		if ((task_ucounts(p) != &init_ucounts) &&
> > 		    !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
> >
> > and the new function:
> >
> > long inc_rlimit_ucounts_overlimit(struct ucounts *ucounts, enum ucount_type type, long v, unsigned long rlimit)
> > {
> > 	struct ucounts *iter;
> > 	long ret = 0;
> > 	long max = rlimit;
> > 	if (rlimit > LONG_MAX)
> > 		max = LONG_MAX;
> > 	for (iter = ucounts; iter; iter = iter->ns->ucounts) {
> > 		long new = atomic_long_add_return(v, &iter->ucount[type]);
> > 		if (new < 0 || new > max)
> > 			ret = LONG_MAX;
> > 		else if (iter == ucounts)
> > 			ret = new;
> > 		max = READ_ONCE(iter->ns->ucount_max[type]);
> > 	}
> > 	return ret;
> > }
> >
> > This will avoid double checking the same userns tree.
> >
> > Or even modify inc_rlimit_ucounts. This function is used elsewhere like
> > this:
> >
> >
> > msgqueue = inc_rlimit_ucounts(info->ucounts, UCOUNT_RLIMIT_MSGQUEUE, mq_bytes);
> > if (msgqueue == LONG_MAX || msgqueue > rlimit(RLIMIT_MSGQUEUE)) {
> >
> >
> > memlock = inc_rlimit_ucounts(ucounts, UCOUNT_RLIMIT_MEMLOCK, locked);
> > if (!allowed && (memlock == LONG_MAX || memlock > lock_limit) && !capable(CAP_IPC_LOCK)) {
> >
> >
> > In all cases, we have max value for comparison.
> 
> Good point.   The downside is that it means we can't use the same code
> in exec.  The upside is that the code is more idiomatic.

My suggestion was before I saw the 8/8 patch :)

We can make something like:

static inline bool is_nproc_overlimit(struct task_struct *task)
{
	return (task_ucounts(task) != &init_ucounts) &&
		!has_capability(task, CAP_SYS_RESOURCE) &&
		!has_capability(task, CAP_SYS_ADMIN);
}

In copy_process:

if (inc_rlimit_ucounts_overlimit(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1, rlimit(RLIMIT_NPROC)) == LONG_MAX) {
	if (is_nproc_overlimit(p))
		goto bad_fork_cleanup_count;
}

In do_execveat_common:

if ((current->flags & PF_NPROC_CHECK) &&
    is_ucounts_overlimit(current_ucounts(), UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC)) &&
    is_nproc_overlimit(current)) {
	retval = -EAGAIN;
	goto out_ret;
}

-- 
Rgrds, legion