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

[Shuah Khan <skhan@linuxfoundation.org>]

On 2/11/22 10:50 AM, Eric W. Biederman wrote:
> Alexey Gladkov <legion@kernel.org> 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@xmission.com>
>>> ---
>>>   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.
> 

Checking on this a bit more on other callers of inc_rlimit_ucounts(),
we might have another issue:

1. mqueue_get_inode() does:
      spin_lock(&mq_lock);
      msgqueue = inc_rlimit_ucounts(info->ucounts, UCOUNT_RLIMIT_MSGQUEUE, mq_bytes);
                         if (msgqueue == LONG_MAX || msgqueue > rlimit(RLIMIT_MSGQUEUE)) {
                                 dec_rlimit_ucounts(info->ucounts, UCOUNT_RLIMIT_MSGQUEUE, mq_bytes);
                                 spin_unlock(&mq_lock);


2. user_shm_lock() &shmlock_user_lock
     spin_lock(&shmlock_user_lock);
         memlock = inc_rlimit_ucounts(ucounts, UCOUNT_RLIMIT_MEMLOCK, locked);

         if (!allowed && (memlock == LONG_MAX || memlock > lock_limit) && !capable(CAP_IPC_LOCK)) {
                 dec_rlimit_ucounts(ucounts, UCOUNT_RLIMIT_MEMLOCK, locked);

3. user_namespace_sysctl_init() Doesn't check for max value.

4. copy_creds() doesn't check for max value in its 3 calls to inc_rlimit_ucounts()

You can see that each of these instances some callers dec_rlimit_ucounts().
They hold different locks. So do we have a window where LONG_MAX could
overflow and go unnoticed?

thanks,
-- Shuah