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    SubjectRe: [CFT][PATCH] ucounts: Fix signal ucount refcounting
    Hillf Danton <hdanton@sina.com> writes:

    > On Fri, 15 Oct 2021 17:10:58 -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);
    >> + }
    >
    > Given kfree in put_ucounts(), this has difficulty surviving tests like
    > kasan if the put pairs with the get in the below
    > inc_rlimit_get_ucounts().

    I don't know if this is what you are thinking about but there is indeed
    a bug in that loop caused by kfree.

    The problem is that iter->ns->ucounts is read after put_ucounts. It
    just needs to be read before hand.


    >> +}
    >> +
    >> +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))
    >> + goto dec_unwind;
    >
    > Add a line of comment for get to ease readers.

    /* you are not expected to understand this */

    I think that is the classic comment from unix source. Seriously I can't
    think of any comment that will make the situation more comprehensible.


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

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