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

[Yu Zhao <yuzhao@google.com>]

On Sat, Oct 16, 2021 at 11:35 AM Eric W. Biederman
<ebiederm@xmission.com> wrote:
>
> Alexey Gladkov <legion@kernel.org> 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@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?
>
> 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.

Or just atomic_long_dec_return().

> >> +    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