Reimplement RLIMIT_NPROC on top of ucounts

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.

Changelog

v11:
* Change inc_rlimit_ucounts() which now returns top value of ucounts.
* Drop inc_rlimit_ucounts_and_test() because the return code of
  inc_rlimit_ucounts() can be checked.

Signed-off-by: Alexey Gladkov <legion@kernel.org>
Link: https://lkml.kernel.org/r/c5286a8aa16d2d698c222f7532f3d735c82bc6bc.1619094428.git.legion@kernel.org
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

diff --git a/fs/exec.c b/fs/exec.c
index d7c4187..f2bcdbe 100644 (file)
--- 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 66436e6..5ca1e8a 100644 (file)
--- 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 a8ec3b6..d33d867 100644 (file)
--- 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 80b5bf1..4a97acc 100644 (file)
--- 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;
@@ -110,6 +113,15 @@ 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]);
+}
+
+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);
+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 58a8a9e..dcfa30b 100644 (file)
--- 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 04029e3..61c0fe9 100644 (file)
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -188,7 +188,7 @@ repeat:
        /* 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 321a5e3..ed7dfb0 100644 (file)
--- 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);
@@ -1978,8 +1980,7 @@ 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;
@@ -2382,7 +2383,7 @@ bad_fork_cleanup_threadgroup_lock:
 #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 cabfc5b..00266a6 100644 (file)
--- 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 365865f..6caa56f 100644 (file)
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -80,6 +80,7 @@ static struct ctl_table user_table[] = {
        UCOUNT_ENTRY("max_inotify_instances"),
        UCOUNT_ENTRY("max_inotify_watches"),
 #endif
+       { },
        { }
 };
 #endif /* CONFIG_SYSCTL */
@@ -240,6 +241,48 @@ void dec_ucount(struct ucounts *ucounts, enum ucount_type type)
        put_ucounts(ucounts);
 }
 
+long inc_rlimit_ucounts(struct ucounts *ucounts, enum ucount_type type, long v)
+{
+       struct ucounts *iter;
+       long 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(v, &iter->ucount[type]);
+               if (new < 0 || new > max)
+                       ret = LONG_MAX;
+               else if (iter == ucounts)
+                       ret = new;
+       }
+       return ret;
+}
+
+bool dec_rlimit_ucounts(struct ucounts *ucounts, enum ucount_type type, long v)
+{
+       struct ucounts *iter;
+       long new;
+       for (iter = ucounts; iter; iter = iter->ns->ucounts) {
+               long dec = atomic_long_add_return(-v, &iter->ucount[type]);
+               WARN_ON_ONCE(dec < 0);
+               if (iter == ucounts)
+                       new = dec;
+       }
+       return (new == 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 +299,7 @@ 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 a2478cd..7f5ff49 100644 (file)
--- 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 f1b7b4b..e6577c8 100644 (file)
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -119,9 +119,10 @@ 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);
        ns->ucounts = ucounts;
 
        /* Inherit USERNS_SETGROUPS_ALLOWED from our parent */