diff options
Diffstat (limited to 'lib')
| -rw-r--r-- | lib/Kconfig.debug | 13 | ||||
| -rw-r--r-- | lib/Makefile | 2 | ||||
| -rw-r--r-- | lib/refcount.c | 267 |
3 files changed, 268 insertions, 14 deletions
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug index 55735c9bdb75..97d62c2da6c2 100644 --- a/lib/Kconfig.debug +++ b/lib/Kconfig.debug @@ -729,19 +729,6 @@ source "lib/Kconfig.kmemcheck" source "lib/Kconfig.kasan" -config DEBUG_REFCOUNT - bool "Verbose refcount checks" - help - Say Y here if you want reference counters (refcount_t and kref) to - generate WARNs on dubious usage. Without this refcount_t will still - be a saturating counter and avoid Use-After-Free by turning it into - a resource leak Denial-Of-Service. - - Use of this option will increase kernel text size but will alert the - admin of potential abuse. - - If in doubt, say "N". - endmenu # "Memory Debugging" config ARCH_HAS_KCOV diff --git a/lib/Makefile b/lib/Makefile index c9023efbd4ca..469b2392893a 100644 --- a/lib/Makefile +++ b/lib/Makefile @@ -38,7 +38,7 @@ obj-y += bcd.o div64.o sort.o parser.o debug_locks.o random32.o \ gcd.o lcm.o list_sort.o uuid.o flex_array.o iov_iter.o clz_ctz.o \ bsearch.o find_bit.o llist.o memweight.o kfifo.o \ percpu-refcount.o percpu_ida.o rhashtable.o reciprocal_div.o \ - once.o + once.o refcount.o obj-y += string_helpers.o obj-$(CONFIG_TEST_STRING_HELPERS) += test-string_helpers.o obj-y += hexdump.o diff --git a/lib/refcount.c b/lib/refcount.c new file mode 100644 index 000000000000..1d33366189d1 --- /dev/null +++ b/lib/refcount.c @@ -0,0 +1,267 @@ +/* + * Variant of atomic_t specialized for reference counts. + * + * The interface matches the atomic_t interface (to aid in porting) but only + * provides the few functions one should use for reference counting. + * + * It differs in that the counter saturates at UINT_MAX and will not move once + * there. This avoids wrapping the counter and causing 'spurious' + * use-after-free issues. + * + * Memory ordering rules are slightly relaxed wrt regular atomic_t functions + * and provide only what is strictly required for refcounts. + * + * The increments are fully relaxed; these will not provide ordering. The + * rationale is that whatever is used to obtain the object we're increasing the + * reference count on will provide the ordering. For locked data structures, + * its the lock acquire, for RCU/lockless data structures its the dependent + * load. + * + * Do note that inc_not_zero() provides a control dependency which will order + * future stores against the inc, this ensures we'll never modify the object + * if we did not in fact acquire a reference. + * + * The decrements will provide release order, such that all the prior loads and + * stores will be issued before, it also provides a control dependency, which + * will order us against the subsequent free(). + * + * The control dependency is against the load of the cmpxchg (ll/sc) that + * succeeded. This means the stores aren't fully ordered, but this is fine + * because the 1->0 transition indicates no concurrency. + * + * Note that the allocator is responsible for ordering things between free() + * and alloc(). + * + */ + +#include <linux/refcount.h> +#include <linux/bug.h> + +bool refcount_add_not_zero(unsigned int i, refcount_t *r) +{ + unsigned int old, new, val = atomic_read(&r->refs); + + for (;;) { + if (!val) + return false; + + if (unlikely(val == UINT_MAX)) + return true; + + new = val + i; + if (new < val) + new = UINT_MAX; + old = atomic_cmpxchg_relaxed(&r->refs, val, new); + if (old == val) + break; + + val = old; + } + + WARN(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n"); + + return true; +} +EXPORT_SYMBOL_GPL(refcount_add_not_zero); + +void refcount_add(unsigned int i, refcount_t *r) +{ + WARN(!refcount_add_not_zero(i, r), "refcount_t: addition on 0; use-after-free.\n"); +} +EXPORT_SYMBOL_GPL(refcount_add); + +/* + * Similar to atomic_inc_not_zero(), will saturate at UINT_MAX and WARN. + * + * Provides no memory ordering, it is assumed the caller has guaranteed the + * object memory to be stable (RCU, etc.). It does provide a control dependency + * and thereby orders future stores. See the comment on top. + */ +bool refcount_inc_not_zero(refcount_t *r) +{ + unsigned int old, new, val = atomic_read(&r->refs); + + for (;;) { + new = val + 1; + + if (!val) + return false; + + if (unlikely(!new)) + return true; + + old = atomic_cmpxchg_relaxed(&r->refs, val, new); + if (old == val) + break; + + val = old; + } + + WARN(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n"); + + return true; +} +EXPORT_SYMBOL_GPL(refcount_inc_not_zero); + +/* + * Similar to atomic_inc(), will saturate at UINT_MAX and WARN. + * + * Provides no memory ordering, it is assumed the caller already has a + * reference on the object, will WARN when this is not so. + */ +void refcount_inc(refcount_t *r) +{ + WARN(!refcount_inc_not_zero(r), "refcount_t: increment on 0; use-after-free.\n"); +} +EXPORT_SYMBOL_GPL(refcount_inc); + +bool refcount_sub_and_test(unsigned int i, refcount_t *r) +{ + unsigned int old, new, val = atomic_read(&r->refs); + + for (;;) { + if (unlikely(val == UINT_MAX)) + return false; + + new = val - i; + if (new > val) { + WARN(new > val, "refcount_t: underflow; use-after-free.\n"); + return false; + } + + old = atomic_cmpxchg_release(&r->refs, val, new); + if (old == val) + break; + + val = old; + } + + return !new; +} +EXPORT_SYMBOL_GPL(refcount_sub_and_test); + +/* + * Similar to atomic_dec_and_test(), it will WARN on underflow and fail to + * decrement when saturated at UINT_MAX. + * + * Provides release memory ordering, such that prior loads and stores are done + * before, and provides a control dependency such that free() must come after. + * See the comment on top. + */ +bool refcount_dec_and_test(refcount_t *r) +{ + return refcount_sub_and_test(1, r); +} +EXPORT_SYMBOL_GPL(refcount_dec_and_test); + +/* + * Similar to atomic_dec(), it will WARN on underflow and fail to decrement + * when saturated at UINT_MAX. + * + * Provides release memory ordering, such that prior loads and stores are done + * before. + */ + +void refcount_dec(refcount_t *r) +{ + WARN(refcount_dec_and_test(r), "refcount_t: decrement hit 0; leaking memory.\n"); +} +EXPORT_SYMBOL_GPL(refcount_dec); + +/* + * No atomic_t counterpart, it attempts a 1 -> 0 transition and returns the + * success thereof. + * + * Like all decrement operations, it provides release memory order and provides + * a control dependency. + * + * It can be used like a try-delete operator; this explicit case is provided + * and not cmpxchg in generic, because that would allow implementing unsafe + * operations. + */ +bool refcount_dec_if_one(refcount_t *r) +{ + return atomic_cmpxchg_release(&r->refs, 1, 0) == 1; +} +EXPORT_SYMBOL_GPL(refcount_dec_if_one); + +/* + * No atomic_t counterpart, it decrements unless the value is 1, in which case + * it will return false. + * + * Was often done like: atomic_add_unless(&var, -1, 1) + */ +bool refcount_dec_not_one(refcount_t *r) +{ + unsigned int old, new, val = atomic_read(&r->refs); + + for (;;) { + if (unlikely(val == UINT_MAX)) + return true; + + if (val == 1) + return false; + + new = val - 1; + if (new > val) { + WARN(new > val, "refcount_t: underflow; use-after-free.\n"); + return true; + } + + old = atomic_cmpxchg_release(&r->refs, val, new); + if (old == val) + break; + + val = old; + } + + return true; +} +EXPORT_SYMBOL_GPL(refcount_dec_not_one); + +/* + * Similar to atomic_dec_and_mutex_lock(), it will WARN on underflow and fail + * to decrement when saturated at UINT_MAX. + * + * Provides release memory ordering, such that prior loads and stores are done + * before, and provides a control dependency such that free() must come after. + * See the comment on top. + */ +bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock) +{ + if (refcount_dec_not_one(r)) + return false; + + mutex_lock(lock); + if (!refcount_dec_and_test(r)) { + mutex_unlock(lock); + return false; + } + + return true; +} +EXPORT_SYMBOL_GPL(refcount_dec_and_mutex_lock); + +/* + * Similar to atomic_dec_and_lock(), it will WARN on underflow and fail to + * decrement when saturated at UINT_MAX. + * + * Provides release memory ordering, such that prior loads and stores are done + * before, and provides a control dependency such that free() must come after. + * See the comment on top. + */ +bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock) +{ + if (refcount_dec_not_one(r)) + return false; + + spin_lock(lock); + if (!refcount_dec_and_test(r)) { + spin_unlock(lock); + return false; + } + + return true; +} +EXPORT_SYMBOL_GPL(refcount_dec_and_lock); + |