Messages in this thread Patch in this message | | | From | Dmitry Safonov <> | Subject | [PATCH v5 1/5] jump_label: Prevent key->enabled int overflow | Date | Tue, 22 Nov 2022 18:55:30 +0000 |
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1. With CONFIG_JUMP_LABEL=n static_key_slow_inc() doesn't have any protection against key->enabled refcounter overflow. 2. With CONFIG_JUMP_LABEL=y static_key_slow_inc_cpuslocked() still may turn the refcounter negative as (v + 1) may overflow.
key->enabled is indeed a ref-counter as it's documented in multiple places: top comment in jump_label.h, Documentation/staging/static-keys.rst, etc.
As -1 is reserved for static key that's in process of being enabled, functions would break with negative key->enabled refcount: - for CONFIG_JUMP_LABEL=n negative return of static_key_count() breaks static_key_false(), static_key_true() - the ref counter may become 0 from negative side by too many static_key_slow_inc() calls and lead to use-after-free issues.
These flaws result in that some users have to introduce an additional mutex and prevent the reference counter from overflowing themselves, see bpf_enable_runtime_stats() checking the counter against INT_MAX / 2.
Prevent the reference counter overflow by checking if (v + 1) > 0. Change functions API to return whether the increment was successful.
Signed-off-by: Dmitry Safonov <dima@arista.com> Acked-by: Jakub Kicinski <kuba@kernel.org> --- include/linux/jump_label.h | 19 +++++++++++--- kernel/jump_label.c | 54 +++++++++++++++++++++++++++++--------- 2 files changed, 57 insertions(+), 16 deletions(-)
diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h index 570831ca9951..c0a02d4c2ea2 100644 --- a/include/linux/jump_label.h +++ b/include/linux/jump_label.h @@ -224,9 +224,9 @@ extern bool arch_jump_label_transform_queue(struct jump_entry *entry, enum jump_label_type type); extern void arch_jump_label_transform_apply(void); extern int jump_label_text_reserved(void *start, void *end); -extern void static_key_slow_inc(struct static_key *key); +extern bool static_key_slow_inc(struct static_key *key); extern void static_key_slow_dec(struct static_key *key); -extern void static_key_slow_inc_cpuslocked(struct static_key *key); +extern bool static_key_slow_inc_cpuslocked(struct static_key *key); extern void static_key_slow_dec_cpuslocked(struct static_key *key); extern int static_key_count(struct static_key *key); extern void static_key_enable(struct static_key *key); @@ -278,10 +278,21 @@ static __always_inline bool static_key_true(struct static_key *key) return false; } -static inline void static_key_slow_inc(struct static_key *key) +static inline bool static_key_slow_inc(struct static_key *key) { + int v; + STATIC_KEY_CHECK_USE(key); - atomic_inc(&key->enabled); + /* + * Prevent key->enabled getting negative to follow the same semantics + * as for CONFIG_JUMP_LABEL=y, see kernel/jump_label.c comment. + */ + v = atomic_read(&key->enabled); + do { + if (v < 0 || (v + 1) < 0) + return false; + } while (!likely(atomic_try_cmpxchg(&key->enabled, &v, v + 1))); + return true; } static inline void static_key_slow_dec(struct static_key *key) diff --git a/kernel/jump_label.c b/kernel/jump_label.c index 4d6c6f5f60db..677a6674c130 100644 --- a/kernel/jump_label.c +++ b/kernel/jump_label.c @@ -113,9 +113,38 @@ int static_key_count(struct static_key *key) } EXPORT_SYMBOL_GPL(static_key_count); -void static_key_slow_inc_cpuslocked(struct static_key *key) +/*** + * static_key_fast_inc_not_negative - adds a user for a static key + * @key: static key that must be already enabled + * + * The caller must make sure that the static key can't get disabled while + * in this function. It doesn't patch jump labels, only adds a user to + * an already enabled static key. + * + * Returns true if the increment was done. + */ +static bool static_key_fast_inc_not_negative(struct static_key *key) { + int v; + STATIC_KEY_CHECK_USE(key); + /* + * Negative key->enabled has a special meaning: it sends + * static_key_slow_inc() down the slow path, and it is non-zero + * so it counts as "enabled" in jump_label_update(). Note that + * atomic_inc_unless_negative() checks >= 0, so roll our own. + */ + v = atomic_read(&key->enabled); + do { + if (v <= 0 || (v + 1) < 0) + return false; + } while (!likely(atomic_try_cmpxchg(&key->enabled, &v, v + 1))); + + return true; +} + +bool static_key_slow_inc_cpuslocked(struct static_key *key) +{ lockdep_assert_cpus_held(); /* @@ -124,15 +153,9 @@ void static_key_slow_inc_cpuslocked(struct static_key *key) * jump_label_update() process. At the same time, however, * the jump_label_update() call below wants to see * static_key_enabled(&key) for jumps to be updated properly. - * - * So give a special meaning to negative key->enabled: it sends - * static_key_slow_inc() down the slow path, and it is non-zero - * so it counts as "enabled" in jump_label_update(). Note that - * atomic_inc_unless_negative() checks >= 0, so roll our own. */ - for (int v = atomic_read(&key->enabled); v > 0; ) - if (likely(atomic_try_cmpxchg(&key->enabled, &v, v + 1))) - return; + if (static_key_fast_inc_not_negative(key)) + return true; jump_label_lock(); if (atomic_read(&key->enabled) == 0) { @@ -144,16 +167,23 @@ void static_key_slow_inc_cpuslocked(struct static_key *key) */ atomic_set_release(&key->enabled, 1); } else { - atomic_inc(&key->enabled); + if (WARN_ON_ONCE(!static_key_fast_inc_not_negative(key))) { + jump_label_unlock(); + return false; + } } jump_label_unlock(); + return true; } -void static_key_slow_inc(struct static_key *key) +bool static_key_slow_inc(struct static_key *key) { + bool ret; + cpus_read_lock(); - static_key_slow_inc_cpuslocked(key); + ret = static_key_slow_inc_cpuslocked(key); cpus_read_unlock(); + return ret; } EXPORT_SYMBOL_GPL(static_key_slow_inc); -- 2.38.1
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