From: Eric W. Biederman ebiederm@xmission.com
commit 15bc01effefe97757ef02ca09e9d1b927ab22725 upstream.
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.
The new function inc_rlimit_get_ucounts returns 0 on failure to increment the ucount. This is different than inc_rlimit_ucounts which increments the ucounts and returns LONG_MAX if the ucount counter has exceeded it's maximum or it wrapped (to indicate the counter needs to decremented).
I wish we had a single user to account all pending signals to across all of the threads of a process so this complexity was not necessary
Cc: stable@vger.kernel.org Fixes: d64696905554 ("Reimplement RLIMIT_SIGPENDING on top of ucounts") v1: https://lkml.kernel.org/r/87mtnavszx.fsf_-_@disp2133 Link: https://lkml.kernel.org/r/87fssytizw.fsf_-_@disp2133 Reviewed-by: Alexey Gladkov legion@kernel.org Tested-by: Rune Kleveland rune.kleveland@infomedia.dk Tested-by: Yu Zhao yuzhao@google.com Tested-by: Jordan Glover Golden_Miller83@protonmail.ch Signed-off-by: "Eric W. Biederman" ebiederm@xmission.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- include/linux/user_namespace.h | 2 + kernel/signal.c | 25 +++++--------------- kernel/ucount.c | 49 +++++++++++++++++++++++++++++++++++++++++ 3 files changed, 57 insertions(+), 19 deletions(-)
--- a/include/linux/user_namespace.h +++ b/include/linux/user_namespace.h @@ -127,6 +127,8 @@ static inline long get_ucounts_value(str
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, --- a/kernel/signal.c +++ b/kernel/signal.c @@ -425,22 +425,10 @@ __sigqueue_alloc(int sig, struct task_st */ 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) + 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_st }
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 sigqu { 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); --- a/kernel/ucount.c +++ b/kernel/ucount.c @@ -284,6 +284,55 @@ bool dec_rlimit_ucounts(struct ucounts * return (new == 0); }
+static void do_dec_rlimit_put_ucounts(struct ucounts *ucounts, + struct ucounts *last, enum ucount_type type) +{ + struct ucounts *iter, *next; + for (iter = ucounts; iter != last; iter = next) { + long dec = atomic_long_add_return(-1, &iter->ucount[type]); + WARN_ON_ONCE(dec < 0); + next = iter->ns->ucounts; + 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) +{ + /* Caller must hold a reference to ucounts */ + 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; + if (iter == ucounts) + ret = new; + /* + * Grab an extra ucount reference for the caller when + * the rlimit count was previously 0. + */ + if (new != 1) + continue; + if (!get_ucounts(iter)) + goto dec_unwind; + } + 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 0; +} + bool is_ucounts_overlimit(struct ucounts *ucounts, enum ucount_type type, unsigned long max) { struct ucounts *iter;