Re: [CFT][PATCH] ucounts: Fix signal ucount refcounting

[Alexey Gladkov <legion@kernel.org>]

On Fri, Oct 15, 2021 at 05:10:58PM -0500, Eric W. Biederman wrote:
> 
> In commit fda31c50292a ("signal: avoid double atomic counter
> increments for user accounting") Linus made a clever optimization to
> how rlimits and the struct user_struct.  Unfortunately that
> optimization does not work in the obvious way when moved to nested
> rlimits.  The problem is that the last decrement of the per user
> namespace per user sigpending counter might also be the last decrement
> of the sigpending counter in the parent user namespace as well.  Which
> means that simply freeing the leaf ucount in __free_sigqueue is not
> enough.
> 
> Maintain the optimization and handle the tricky cases by introducing
> inc_rlimit_get_ucounts and dec_rlimit_put_ucounts.
> 
> By moving the entire optimization into functions that perform all of
> the work it becomes possible to ensure that every level is handled
> properly.
> 
> I wish we had a single user across all of the threads whose rlimit
> could be charged so we did not need this complexity.
> 
> Cc: stable@vger.kernel.org
> Fixes: d64696905554 ("Reimplement RLIMIT_SIGPENDING on top of ucounts")
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> ---
> 
> With a lot of help from Alex who found a way I could reproduce this
> I believe I have found the issue.
> 
> Could people who are seeing this issue test and verify this solves the
> problem for them?
> 
>  include/linux/user_namespace.h |  2 ++
>  kernel/signal.c                | 25 +++++----------------
>  kernel/ucount.c                | 41 ++++++++++++++++++++++++++++++++++
>  3 files changed, 49 insertions(+), 19 deletions(-)
> 
> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> index eb70cabe6e7f..33a4240e6a6f 100644
> --- a/include/linux/user_namespace.h
> +++ b/include/linux/user_namespace.h
> @@ -127,6 +127,8 @@ static inline long get_ucounts_value(struct ucounts *ucounts, enum ucount_type t
>  
>  long inc_rlimit_ucounts(struct ucounts *ucounts, enum ucount_type type, long v);
>  bool dec_rlimit_ucounts(struct ucounts *ucounts, enum ucount_type type, long v);
> +long inc_rlimit_get_ucounts(struct ucounts *ucounts, enum ucount_type type);
> +void dec_rlimit_put_ucounts(struct ucounts *ucounts, enum ucount_type type);
>  bool is_ucounts_overlimit(struct ucounts *ucounts, enum ucount_type type, unsigned long max);
>  
>  static inline void set_rlimit_ucount_max(struct user_namespace *ns,
> diff --git a/kernel/signal.c b/kernel/signal.c
> index a3229add4455..762de58c6e76 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -425,22 +425,10 @@ __sigqueue_alloc(int sig, struct task_struct *t, gfp_t gfp_flags,
>  	 */
>  	rcu_read_lock();
>  	ucounts = task_ucounts(t);
> -	sigpending = inc_rlimit_ucounts(ucounts, UCOUNT_RLIMIT_SIGPENDING, 1);
> -	switch (sigpending) {
> -	case 1:
> -		if (likely(get_ucounts(ucounts)))
> -			break;
> -		fallthrough;
> -	case LONG_MAX:
> -		/*
> -		 * we need to decrease the ucount in the userns tree on any
> -		 * failure to avoid counts leaking.
> -		 */
> -		dec_rlimit_ucounts(ucounts, UCOUNT_RLIMIT_SIGPENDING, 1);
> -		rcu_read_unlock();
> -		return NULL;
> -	}
> +	sigpending = inc_rlimit_get_ucounts(ucounts, UCOUNT_RLIMIT_SIGPENDING);
>  	rcu_read_unlock();
> +	if (sigpending == LONG_MAX)
> +		return NULL;
>  
>  	if (override_rlimit || likely(sigpending <= task_rlimit(t, RLIMIT_SIGPENDING))) {
>  		q = kmem_cache_alloc(sigqueue_cachep, gfp_flags);
> @@ -449,8 +437,7 @@ __sigqueue_alloc(int sig, struct task_struct *t, gfp_t gfp_flags,
>  	}
>  
>  	if (unlikely(q == NULL)) {
> -		if (dec_rlimit_ucounts(ucounts, UCOUNT_RLIMIT_SIGPENDING, 1))
> -			put_ucounts(ucounts);
> +		dec_rlimit_put_ucounts(ucounts, UCOUNT_RLIMIT_SIGPENDING);
>  	} else {
>  		INIT_LIST_HEAD(&q->list);
>  		q->flags = sigqueue_flags;
> @@ -463,8 +450,8 @@ static void __sigqueue_free(struct sigqueue *q)
>  {
>  	if (q->flags & SIGQUEUE_PREALLOC)
>  		return;
> -	if (q->ucounts && dec_rlimit_ucounts(q->ucounts, UCOUNT_RLIMIT_SIGPENDING, 1)) {
> -		put_ucounts(q->ucounts);
> +	if (q->ucounts) {
> +		dec_rlimit_put_ucounts(q->ucounts, UCOUNT_RLIMIT_SIGPENDING);
>  		q->ucounts = NULL;
>  	}
>  	kmem_cache_free(sigqueue_cachep, q);
> diff --git a/kernel/ucount.c b/kernel/ucount.c
> index 3b7e176cf7a2..687d77aa66bb 100644
> --- a/kernel/ucount.c
> +++ b/kernel/ucount.c
> @@ -285,6 +285,47 @@ bool dec_rlimit_ucounts(struct ucounts *ucounts, enum ucount_type type, long v)
>  	return (new == 0);
>  }
>  
> +static void do_dec_rlimit_put_ucounts(struct ucounts *ucounts,
> +				struct ucounts *last, enum ucount_type type)
> +{
> +	struct ucounts *iter;
> +	for (iter = ucounts; iter != last; iter = iter->ns->ucounts) {
> +		long dec = atomic_long_add_return(-1, &iter->ucount[type]);
> +		WARN_ON_ONCE(dec < 0);
> +		if (dec == 0)
> +			put_ucounts(iter);
> +	}
> +}
> +
> +void dec_rlimit_put_ucounts(struct ucounts *ucounts, enum ucount_type type)
> +{
> +	do_dec_rlimit_put_ucounts(ucounts, NULL, type);
> +}
> +
> +long inc_rlimit_get_ucounts(struct ucounts *ucounts, enum ucount_type type)
> +{
> +	struct ucounts *iter;
> +	long dec, ret = 0;
> +
> +	for (iter = ucounts; iter; iter = iter->ns->ucounts) {
> +		long max = READ_ONCE(iter->ns->ucount_max[type]);
> +		long new = atomic_long_add_return(1, &iter->ucount[type]);
> +		if (new < 0 || new > max)
> +			goto unwind;
> +		else if (iter == ucounts)
> +			ret = new;
> +		if ((new == 1) && (get_ucounts(iter) != iter))

get_ucounts can do put_ucounts. Are you sure it's correct to use
get_ucounts here?

> +			goto dec_unwind;
> +	}
> +	return ret;
> +dec_unwind:
> +	dec = atomic_long_add_return(1, &iter->ucount[type]);

Should be -1 ?

> +	WARN_ON_ONCE(dec < 0);
> +unwind:
> +	do_dec_rlimit_put_ucounts(ucounts, iter, type);
> +	return LONG_MAX;
> +}
> +
>  bool is_ucounts_overlimit(struct ucounts *ucounts, enum ucount_type type, unsigned long max)
>  {
>  	struct ucounts *iter;
> -- 
> 2.20.1
> 

-- 
Rgrds, legion