[PATCH v9 4/8] Reimplement RLIMIT_NPROC on top of ucounts

Eric W. Biederman ebiederm at xmission.com
Mon Apr 5 16:56:35 UTC 2021


Alexey Gladkov <gladkov.alexey at gmail.com> writes:

> The rlimit counter is tied to uid in the user_namespace. This allows
> rlimit values to be specified in userns even if they are already
> globally exceeded by the user. However, the value of the previous
> user_namespaces cannot be exceeded.
>
> To illustrate the impact of rlimits, let's say there is a program that
> does not fork. Some service-A wants to run this program as user X in
> multiple containers. Since the program never fork the service wants to
> set RLIMIT_NPROC=1.
>
> service-A
>  \- program (uid=1000, container1, rlimit_nproc=1)
>  \- program (uid=1000, container2, rlimit_nproc=1)
>
> The service-A sets RLIMIT_NPROC=1 and runs the program in container1.
> When the service-A tries to run a program with RLIMIT_NPROC=1 in
> container2 it fails since user X already has one running process.
>
> We cannot use existing inc_ucounts / dec_ucounts because they do not
> allow us to exceed the maximum for the counter. Some rlimits can be
> overlimited by root or if the user has the appropriate capability.

In general this looks good.

My comments are going to be on the infrastructure this patch
introduces rather than the change to RLIMIT_NPROC.  Perhaps
these would fare better as separate patches.

To preserve the existing semantics of incrementing and decrementing
rlimits in places separate from where the limits are checked you
correctly introduce inc_rlimit_ucounts and dec_rlimit_ucounts.

This separation means that for a short period of time the values
in the counts can overflow.  Which is something that the current
ucount helpers don't allow.  Taking advantage of the entire
negative range for the counts, just like we take advantage
of the entire negative range in get_ucounts seems a reasonable thing
to do.

On 32bit using "atomic_long_t" instead of "unsigned long" to hold the
counts has the downside that RLIMIT_MSGQUEUE and RLIMIT_MEMLOCK are
limited to 2GiB instead of 4GiB.  I don't think anyone cares but
it should be mentioned in case someone does.

The functions inc_rlimit_ucounts and inc_rlimit_ucounts_test need
to return overflow in the unlikely event the count values becomes
negative.

Also when setting ns->ucount_max[] in create_user_ns because one value
is signed and the other is unsigned.  Care should be taken so that
rlimit_infinity is translated into the largest positive value the
type can hold.

Eric

> Signed-off-by: Alexey Gladkov <gladkov.alexey at gmail.com>
> ---
>  fs/exec.c                      |  2 +-
>  include/linux/cred.h           |  2 ++
>  include/linux/sched/user.h     |  1 -
>  include/linux/user_namespace.h | 13 ++++++++
>  kernel/cred.c                  | 10 +++---
>  kernel/exit.c                  |  2 +-
>  kernel/fork.c                  |  9 ++---
>  kernel/sys.c                   |  2 +-
>  kernel/ucount.c                | 61 ++++++++++++++++++++++++++++++++++
>  kernel/user.c                  |  1 -
>  kernel/user_namespace.c        |  3 +-
>  11 files changed, 91 insertions(+), 15 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index d7c4187ca023..f2bcdbeb3afb 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1878,7 +1878,7 @@ static int do_execveat_common(int fd, struct filename *filename,
>  	 * whether NPROC limit is still exceeded.
>  	 */
>  	if ((current->flags & PF_NPROC_EXCEEDED) &&
> -	    atomic_read(&current_user()->processes) > rlimit(RLIMIT_NPROC)) {
> +	    is_ucounts_overlimit(current_ucounts(), UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC))) {
>  		retval = -EAGAIN;
>  		goto out_ret;
>  	}
> diff --git a/include/linux/cred.h b/include/linux/cred.h
> index 66436e655032..5ca1e8a1d035 100644
> --- a/include/linux/cred.h
> +++ b/include/linux/cred.h
> @@ -372,6 +372,7 @@ static inline void put_cred(const struct cred *_cred)
>  
>  #define task_uid(task)		(task_cred_xxx((task), uid))
>  #define task_euid(task)		(task_cred_xxx((task), euid))
> +#define task_ucounts(task)	(task_cred_xxx((task), ucounts))
>  
>  #define current_cred_xxx(xxx)			\
>  ({						\
> @@ -388,6 +389,7 @@ static inline void put_cred(const struct cred *_cred)
>  #define current_fsgid() 	(current_cred_xxx(fsgid))
>  #define current_cap()		(current_cred_xxx(cap_effective))
>  #define current_user()		(current_cred_xxx(user))
> +#define current_ucounts()	(current_cred_xxx(ucounts))
>  
>  extern struct user_namespace init_user_ns;
>  #ifdef CONFIG_USER_NS
> diff --git a/include/linux/sched/user.h b/include/linux/sched/user.h
> index a8ec3b6093fc..d33d867ad6c1 100644
> --- a/include/linux/sched/user.h
> +++ b/include/linux/sched/user.h
> @@ -12,7 +12,6 @@
>   */
>  struct user_struct {
>  	refcount_t __count;	/* reference count */
> -	atomic_t processes;	/* How many processes does this user have? */
>  	atomic_t sigpending;	/* How many pending signals does this user have? */
>  #ifdef CONFIG_FANOTIFY
>  	atomic_t fanotify_listeners;
> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> index d84cc2c0b443..9d1ca370c201 100644
> --- a/include/linux/user_namespace.h
> +++ b/include/linux/user_namespace.h
> @@ -50,9 +50,12 @@ enum ucount_type {
>  	UCOUNT_INOTIFY_INSTANCES,
>  	UCOUNT_INOTIFY_WATCHES,
>  #endif
> +	UCOUNT_RLIMIT_NPROC,
>  	UCOUNT_COUNTS,
>  };
>  
> +#define MAX_PER_NAMESPACE_UCOUNTS UCOUNT_RLIMIT_NPROC
> +
>  struct user_namespace {
>  	struct uid_gid_map	uid_map;
>  	struct uid_gid_map	gid_map;
> @@ -107,6 +110,16 @@ struct ucounts *alloc_ucounts(struct user_namespace *ns, kuid_t uid);
>  struct ucounts * __must_check get_ucounts(struct ucounts *ucounts);
>  void put_ucounts(struct ucounts *ucounts);
>  
> +static inline long get_ucounts_value(struct ucounts *ucounts, enum ucount_type type)
> +{
> +	return atomic_long_read(&ucounts->ucount[type]);
> +}
> +
> +bool inc_rlimit_ucounts(struct ucounts *ucounts, enum ucount_type type, long v);
> +bool inc_rlimit_ucounts_and_test(struct ucounts *ucounts, enum ucount_type type, long v, long max);
> +void dec_rlimit_ucounts(struct ucounts *ucounts, enum ucount_type type, long v);
> +bool is_ucounts_overlimit(struct ucounts *ucounts, enum ucount_type type, unsigned long max);
> +
>  #ifdef CONFIG_USER_NS
>  
>  static inline struct user_namespace *get_user_ns(struct user_namespace *ns)
> diff --git a/kernel/cred.c b/kernel/cred.c
> index 58a8a9e24347..dcfa30b337c5 100644
> --- a/kernel/cred.c
> +++ b/kernel/cred.c
> @@ -360,7 +360,7 @@ 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));
> -		atomic_inc(&p->cred->user->processes);
> +		inc_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
>  		return 0;
>  	}
>  
> @@ -395,8 +395,8 @@ int copy_creds(struct task_struct *p, unsigned long clone_flags)
>  	}
>  #endif
>  
> -	atomic_inc(&new->user->processes);
>  	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;
> @@ -496,12 +496,12 @@ int commit_creds(struct cred *new)
>  	 * in set_user().
>  	 */
>  	alter_cred_subscribers(new, 2);
> -	if (new->user != old->user)
> -		atomic_inc(&new->user->processes);
> +	if (new->user != old->user || new->user_ns != old->user_ns)
> +		inc_rlimit_ucounts(new->ucounts, UCOUNT_RLIMIT_NPROC, 1);
>  	rcu_assign_pointer(task->real_cred, new);
>  	rcu_assign_pointer(task->cred, new);
>  	if (new->user != old->user)
> -		atomic_dec(&old->user->processes);
> +		dec_rlimit_ucounts(old->ucounts, UCOUNT_RLIMIT_NPROC, 1);
>  	alter_cred_subscribers(old, -2);
>  
>  	/* send notifications */
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 04029e35e69a..61c0fe902b50 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -188,7 +188,7 @@ void release_task(struct task_struct *p)
>  	/* don't need to get the RCU readlock here - the process is dead and
>  	 * can't be modifying its own credentials. But shut RCU-lockdep up */
>  	rcu_read_lock();
> -	atomic_dec(&__task_cred(p)->user->processes);
> +	dec_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
>  	rcu_read_unlock();
>  
>  	cgroup_release(p);
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 37498cca6a1d..d8a4956463ae 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -819,9 +819,11 @@ void __init fork_init(void)
>  	init_task.signal->rlim[RLIMIT_SIGPENDING] =
>  		init_task.signal->rlim[RLIMIT_NPROC];
>  
> -	for (i = 0; i < UCOUNT_COUNTS; i++)
> +	for (i = 0; i < MAX_PER_NAMESPACE_UCOUNTS; i++)
>  		init_user_ns.ucount_max[i] = max_threads/2;
>  
> +	init_user_ns.ucount_max[UCOUNT_RLIMIT_NPROC] = task_rlimit(&init_task, RLIMIT_NPROC);
> +
>  #ifdef CONFIG_VMAP_STACK
>  	cpuhp_setup_state(CPUHP_BP_PREPARE_DYN, "fork:vm_stack_cache",
>  			  NULL, free_vm_stack_cache);
> @@ -1972,8 +1974,7 @@ static __latent_entropy struct task_struct *copy_process(
>  	DEBUG_LOCKS_WARN_ON(!p->softirqs_enabled);
>  #endif
>  	retval = -EAGAIN;
> -	if (atomic_read(&p->real_cred->user->processes) >=
> -			task_rlimit(p, RLIMIT_NPROC)) {
> +	if (is_ucounts_overlimit(task_ucounts(p), UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC))) {
>  		if (p->real_cred->user != INIT_USER &&
>  		    !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
>  			goto bad_fork_free;
> @@ -2376,7 +2377,7 @@ static __latent_entropy struct task_struct *copy_process(
>  #endif
>  	delayacct_tsk_free(p);
>  bad_fork_cleanup_count:
> -	atomic_dec(&p->cred->user->processes);
> +	dec_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
>  	exit_creds(p);
>  bad_fork_free:
>  	p->state = TASK_DEAD;
> diff --git a/kernel/sys.c b/kernel/sys.c
> index cabfc5b86175..00266a65a000 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -473,7 +473,7 @@ static int set_user(struct cred *new)
>  	 * for programs doing set*uid()+execve() by harmlessly deferring the
>  	 * failure to the execve() stage.
>  	 */
> -	if (atomic_read(&new_user->processes) >= rlimit(RLIMIT_NPROC) &&
> +	if (is_ucounts_overlimit(new->ucounts, UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC)) &&
>  			new_user != INIT_USER)
>  		current->flags |= PF_NPROC_EXCEEDED;
>  	else
> diff --git a/kernel/ucount.c b/kernel/ucount.c
> index 7bac19bb3f1e..77699231e002 100644
> --- a/kernel/ucount.c
> +++ b/kernel/ucount.c
> @@ -7,6 +7,7 @@
>  #include <linux/hash.h>
>  #include <linux/kmemleak.h>
>  #include <linux/user_namespace.h>
> +#include <linux/security.h>
>  
>  struct ucounts init_ucounts = {
>  	.ns    = &init_user_ns,
> @@ -80,6 +81,7 @@ static struct ctl_table user_table[] = {
>  	UCOUNT_ENTRY("max_inotify_instances"),
>  	UCOUNT_ENTRY("max_inotify_watches"),
>  #endif
> +	{ },
>  	{ }
>  };
>  #endif /* CONFIG_SYSCTL */
> @@ -207,6 +209,19 @@ static inline bool atomic_long_inc_below(atomic_long_t *v, int u)
>  	}
>  }
>  
> +static inline long atomic_long_dec_value(atomic_long_t *v, long n)
> +{
> +	long c, old;
> +	c = atomic_long_read(v);
> +	for (;;) {
> +		old = atomic_long_cmpxchg(v, c, c - n);
> +		if (likely(old == c))
> +			return c;
> +		c = old;
> +	}
> +	return c;
> +}

Just use atomic_add_long_return.


> +
>  struct ucounts *inc_ucount(struct user_namespace *ns, kuid_t uid,
>  			   enum ucount_type type)
>  {
> @@ -240,6 +255,51 @@ void dec_ucount(struct ucounts *ucounts, enum ucount_type type)
>  	put_ucounts(ucounts);
>  }
>  
> +bool inc_rlimit_ucounts(struct ucounts *ucounts, enum ucount_type type, long v)
> +{
> +	struct ucounts *iter;
> +	bool overlimit = false;
> +
> +	for (iter = ucounts; iter; iter = iter->ns->ucounts) {
> +		long max = READ_ONCE(iter->ns->ucount_max[type]);
> +		if (atomic_long_add_return(v, &iter->ucount[type]) > max)
> +			overlimit = true;

Please check for negative to detect counter overflow.
At least the mq code already checks for overflow.

> +	}
> +
> +	return overlimit;
> +}
> +
> +bool inc_rlimit_ucounts_and_test(struct ucounts *ucounts, enum ucount_type type,
> +		long v, long max)
> +{
> +	bool overlimit = inc_rlimit_ucounts(ucounts, type, v);
> +	if (!overlimit && get_ucounts_value(ucounts, type) > max)
> +		overlimit = true;

The return of get_ucounts_value also needs to be checked for negative.

> +	return overlimit;
> +}
> +
> +void dec_rlimit_ucounts(struct ucounts *ucounts, enum ucount_type type, long v)
> +{
> +	struct ucounts *iter;
> +	for (iter = ucounts; iter; iter = iter->ns->ucounts) {
> +		long dec = atomic_long_dec_value(&iter->ucount[type], v);
                           atomic_long_add_return(&iter->ucount[type], -v);
> +		WARN_ON_ONCE(dec < 0);
> +	}
> +}
> +
> +bool is_ucounts_overlimit(struct ucounts *ucounts, enum ucount_type type, unsigned long max)
> +{
> +	struct ucounts *iter;
> +	if (get_ucounts_value(ucounts, type) > max)
> +		return true;
> +	for (iter = ucounts; iter; iter = iter->ns->ucounts) {
> +		max = READ_ONCE(iter->ns->ucount_max[type]);
> +		if (get_ucounts_value(iter, type) > max)
> +			return true;
> +	}
> +	return false;
> +}
> +
>  static __init int user_namespace_sysctl_init(void)
>  {
>  #ifdef CONFIG_SYSCTL
> @@ -256,6 +316,7 @@ static __init int user_namespace_sysctl_init(void)
>  	BUG_ON(!setup_userns_sysctls(&init_user_ns));
>  #endif
>  	hlist_add_ucounts(&init_ucounts);
> +	inc_rlimit_ucounts(&init_ucounts, UCOUNT_RLIMIT_NPROC, 1);
>  	return 0;
>  }
>  subsys_initcall(user_namespace_sysctl_init);
> diff --git a/kernel/user.c b/kernel/user.c
> index a2478cddf536..7f5ff498207a 100644
> --- a/kernel/user.c
> +++ b/kernel/user.c
> @@ -98,7 +98,6 @@ static DEFINE_SPINLOCK(uidhash_lock);
>  /* root_user.__count is 1, for init task cred */
>  struct user_struct root_user = {
>  	.__count	= REFCOUNT_INIT(1),
> -	.processes	= ATOMIC_INIT(1),
>  	.sigpending	= ATOMIC_INIT(0),
>  	.locked_shm     = 0,
>  	.uid		= GLOBAL_ROOT_UID,
> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> index 516db53166ab..2434b13b02e5 100644
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -118,9 +118,10 @@ int create_user_ns(struct cred *new)
>  	ns->owner = owner;
>  	ns->group = group;
>  	INIT_WORK(&ns->work, free_user_ns);
> -	for (i = 0; i < UCOUNT_COUNTS; i++) {
> +	for (i = 0; i < MAX_PER_NAMESPACE_UCOUNTS; i++) {
>  		ns->ucount_max[i] = INT_MAX;
>  	}
> +	ns->ucount_max[UCOUNT_RLIMIT_NPROC] = rlimit(RLIMIT_NPROC);

This should have a special case that translates RLIMIT_INFINITY
to LONG_MAX.

>  	ns->ucounts = ucounts;
>  
>  	/* Inherit USERNS_SETGROUPS_ALLOWED from our parent */


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