atomic_t variables are currently used to implement reference counters with the following properties: - counter is initialized to 1 using atomic_set() - a resource is freed upon counter reaching zero - once counter reaches zero, its further increments aren't allowed - counter schema uses basic atomic operations (set, inc, inc_not_zero, dec_and_test, etc.) Such atomic variables should be converted to a newly provided refcount_t type and API that prevents accidental counter overflows and underflows. This is important since overflows and underflows can lead to use-after-free situation and be exploitable. The variable cred.usage is used as pure reference counter. Convert it to refcount_t and fix up the operations. **Important note for maintainers: Some functions from refcount_t API defined in lib/refcount.c have different memory ordering guarantees than their atomic counterparts. The full comparison can be seen in https://lkml.org/lkml/2017/11/15/57 and it is hopefully soon in state to be merged to the documentation tree. Normally the differences should not matter since refcount_t provides enough guarantees to satisfy the refcounting use cases, but in some rare cases it might matter. Please double check that you don't have some undocumented memory guarantees for this variable usage. For the cred.usage it might make a difference in following places: - get_task_cred(): increment in refcount_inc_not_zero() only guarantees control dependency on success vs. fully ordered atomic counterpart - put_cred(): decrement in refcount_dec_and_test() only provides RELEASE ordering and control dependency on success vs. fully ordered atomic counterpart Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx> Reviewed-by: David Windsor <dwindsor@xxxxxxxxx> Reviewed-by: Hans Liljestrand <ishkamiel@xxxxxxxxx> Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx> --- include/linux/cred.h | 6 +++--- kernel/cred.c | 44 ++++++++++++++++++++++---------------------- 2 files changed, 25 insertions(+), 25 deletions(-) diff --git a/include/linux/cred.h b/include/linux/cred.h index 00948dd..a9f217b 100644 --- a/include/linux/cred.h +++ b/include/linux/cred.h @@ -109,7 +109,7 @@ extern bool may_setgroups(void); * same context as task->real_cred. */ struct cred { - atomic_t usage; + refcount_t usage; #ifdef CONFIG_DEBUG_CREDENTIALS atomic_t subscribers; /* number of processes subscribed */ void *put_addr; @@ -222,7 +222,7 @@ static inline bool cap_ambient_invariant_ok(const struct cred *cred) */ static inline struct cred *get_new_cred(struct cred *cred) { - atomic_inc(&cred->usage); + refcount_inc(&cred->usage); return cred; } @@ -262,7 +262,7 @@ static inline void put_cred(const struct cred *_cred) struct cred *cred = (struct cred *) _cred; validate_creds(cred); - if (atomic_dec_and_test(&(cred)->usage)) + if (refcount_dec_and_test(&(cred)->usage)) __put_cred(cred); } diff --git a/kernel/cred.c b/kernel/cred.c index 9604c1a..86c039a 100644 --- a/kernel/cred.c +++ b/kernel/cred.c @@ -42,7 +42,7 @@ struct group_info init_groups = { .usage = REFCOUNT_INIT(2) }; * The initial credentials for the initial task */ struct cred init_cred = { - .usage = ATOMIC_INIT(4), + .usage = REFCOUNT_INIT(4), #ifdef CONFIG_DEBUG_CREDENTIALS .subscribers = ATOMIC_INIT(2), .magic = CRED_MAGIC, @@ -101,17 +101,17 @@ static void put_cred_rcu(struct rcu_head *rcu) #ifdef CONFIG_DEBUG_CREDENTIALS if (cred->magic != CRED_MAGIC_DEAD || - atomic_read(&cred->usage) != 0 || + refcount_read(&cred->usage) != 0 || read_cred_subscribers(cred) != 0) panic("CRED: put_cred_rcu() sees %p with" " mag %x, put %p, usage %d, subscr %d\n", cred, cred->magic, cred->put_addr, - atomic_read(&cred->usage), + refcount_read(&cred->usage), read_cred_subscribers(cred)); #else - if (atomic_read(&cred->usage) != 0) + if (refcount_read(&cred->usage) != 0) panic("CRED: put_cred_rcu() sees %p with usage %d\n", - cred, atomic_read(&cred->usage)); + cred, refcount_read(&cred->usage)); #endif security_cred_free(cred); @@ -135,10 +135,10 @@ static void put_cred_rcu(struct rcu_head *rcu) void __put_cred(struct cred *cred) { kdebug("__put_cred(%p{%d,%d})", cred, - atomic_read(&cred->usage), + refcount_read(&cred->usage), read_cred_subscribers(cred)); - BUG_ON(atomic_read(&cred->usage) != 0); + BUG_ON(refcount_read(&cred->usage) != 0); #ifdef CONFIG_DEBUG_CREDENTIALS BUG_ON(read_cred_subscribers(cred) != 0); cred->magic = CRED_MAGIC_DEAD; @@ -159,7 +159,7 @@ void exit_creds(struct task_struct *tsk) struct cred *cred; kdebug("exit_creds(%u,%p,%p,{%d,%d})", tsk->pid, tsk->real_cred, tsk->cred, - atomic_read(&tsk->cred->usage), + refcount_read(&tsk->cred->usage), read_cred_subscribers(tsk->cred)); cred = (struct cred *) tsk->real_cred; @@ -194,7 +194,7 @@ const struct cred *get_task_cred(struct task_struct *task) do { cred = __task_cred((task)); BUG_ON(!cred); - } while (!atomic_inc_not_zero(&((struct cred *)cred)->usage)); + } while (!refcount_inc_not_zero(&((struct cred *)cred)->usage)); rcu_read_unlock(); return cred; @@ -212,7 +212,7 @@ struct cred *cred_alloc_blank(void) if (!new) return NULL; - atomic_set(&new->usage, 1); + refcount_set(&new->usage, 1); #ifdef CONFIG_DEBUG_CREDENTIALS new->magic = CRED_MAGIC; #endif @@ -258,7 +258,7 @@ struct cred *prepare_creds(void) old = task->cred; memcpy(new, old, sizeof(struct cred)); - atomic_set(&new->usage, 1); + refcount_set(&new->usage, 1); set_cred_subscribers(new, 0); get_group_info(new->group_info); get_uid(new->user); @@ -335,7 +335,7 @@ int copy_creds(struct task_struct *p, unsigned long clone_flags) get_cred(p->cred); alter_cred_subscribers(p->cred, 2); kdebug("share_creds(%p{%d,%d})", - p->cred, atomic_read(&p->cred->usage), + p->cred, refcount_read(&p->cred->usage), read_cred_subscribers(p->cred)); atomic_inc(&p->cred->user->processes); return 0; @@ -426,7 +426,7 @@ int commit_creds(struct cred *new) const struct cred *old = task->real_cred; kdebug("commit_creds(%p{%d,%d})", new, - atomic_read(&new->usage), + refcount_read(&new->usage), read_cred_subscribers(new)); BUG_ON(task->cred != old); @@ -435,7 +435,7 @@ int commit_creds(struct cred *new) validate_creds(old); validate_creds(new); #endif - BUG_ON(atomic_read(&new->usage) < 1); + BUG_ON(refcount_read(&new->usage) < 1); get_cred(new); /* we will require a ref for the subj creds too */ @@ -501,13 +501,13 @@ EXPORT_SYMBOL(commit_creds); void abort_creds(struct cred *new) { kdebug("abort_creds(%p{%d,%d})", new, - atomic_read(&new->usage), + refcount_read(&new->usage), read_cred_subscribers(new)); #ifdef CONFIG_DEBUG_CREDENTIALS BUG_ON(read_cred_subscribers(new) != 0); #endif - BUG_ON(atomic_read(&new->usage) < 1); + BUG_ON(refcount_read(&new->usage) < 1); put_cred(new); } EXPORT_SYMBOL(abort_creds); @@ -524,7 +524,7 @@ const struct cred *override_creds(const struct cred *new) const struct cred *old = current->cred; kdebug("override_creds(%p{%d,%d})", new, - atomic_read(&new->usage), + refcount_read(&new->usage), read_cred_subscribers(new)); validate_creds(old); @@ -535,7 +535,7 @@ const struct cred *override_creds(const struct cred *new) alter_cred_subscribers(old, -1); kdebug("override_creds() = %p{%d,%d}", old, - atomic_read(&old->usage), + refcount_read(&old->usage), read_cred_subscribers(old)); return old; } @@ -553,7 +553,7 @@ void revert_creds(const struct cred *old) const struct cred *override = current->cred; kdebug("revert_creds(%p{%d,%d})", old, - atomic_read(&old->usage), + refcount_read(&old->usage), read_cred_subscribers(old)); validate_creds(old); @@ -612,7 +612,7 @@ struct cred *prepare_kernel_cred(struct task_struct *daemon) validate_creds(old); *new = *old; - atomic_set(&new->usage, 1); + refcount_set(&new->usage, 1); set_cred_subscribers(new, 0); get_uid(new->user); get_user_ns(new->user_ns); @@ -736,7 +736,7 @@ static void dump_invalid_creds(const struct cred *cred, const char *label, printk(KERN_ERR "CRED: ->magic=%x, put_addr=%p\n", cred->magic, cred->put_addr); printk(KERN_ERR "CRED: ->usage=%d, subscr=%d\n", - atomic_read(&cred->usage), + refcount_read(&cred->usage), read_cred_subscribers(cred)); printk(KERN_ERR "CRED: ->*uid = { %d,%d,%d,%d }\n", from_kuid_munged(&init_user_ns, cred->uid), @@ -810,7 +810,7 @@ void validate_creds_for_do_exit(struct task_struct *tsk) { kdebug("validate_creds_for_do_exit(%p,%p{%d,%d})", tsk->real_cred, tsk->cred, - atomic_read(&tsk->cred->usage), + refcount_read(&tsk->cred->usage), read_cred_subscribers(tsk->cred)); __validate_process_creds(tsk, __FILE__, __LINE__); -- 2.7.4 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. 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