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

[Alexey Gladkov]

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.

-- 
Rgrds, legion