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

[Eric W. Biederman]

Alexey Gladkov <legion@xxxxxxxxxx> writes:

> 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@xxxxxxxxxxxxxxx
>> Fixes: d64696905554 ("Reimplement RLIMIT_SIGPENDING on top of ucounts")
>> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
>> ---
>> 
>> 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?

My only concern would be if we could not run inc_rlimit_get_ucounts
would not be safe to call under rcu_read_lock().  I don't see anything
in get_ucounts or put_ucounts that would not be safe under
rcu_read_lock().

For get_ucounts we do need to test to see if it fails.  Either by
testing for NULL or testing to see if it does not return the expected
ucount.

Does that make sense or do you have another concern?


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

Yes it should.  I will fix and resend.

>> +	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
>> 

Eric