[raschupkin.ri@gmail.com]

[PATCH] livepatch: support of modifying refcount_t without underflow after unpatch

CVE fixes sometimes add refcount_inc/dec() pairs to the code with existing refcount_t.
Two problems arise when applying live-patch in this case:
1) After refcount_t is being inc() during system is live-patched, after unpatch the counter value will not be valid, as corresponing dec() would never be called.
2) Underflows are possible in runtime in case dec() is called before corresponding inc() in the live-patched code.

Proposed kprefcount_t functions are using following approach to solve these two problems:
1) In addition to original refcount_t, temporary refcount_t is allocated, and after unpatch it is just removed. This way system is safe with correct refcounting while patch is applied, and no underflow would happend after unpatch.
2) For inc/dec() added by live-patch code, one bit in reference-holder structure is used (unsigned char *ref_holder, kprefholder_flag). In case dec() is called first, it is just ignored as ref_holder bit would still not be initialized.


API is defined include/linux/livepatch_refcount.h:

typedef struct kprefcount_struct {
	refcount_t *refcount;
	refcount_t kprefcount;
	spinlock_t lock;
} kprefcount_t;

kprefcount_t *kprefcount_alloc(refcount_t *refcount, gfp_t flags);
void kprefcount_free(kprefcount_t *kp_ref);
int kprefcount_read(kprefcount_t *kp_ref);
void kprefcount_inc(kprefcount_t *kp_ref, unsigned char *ref_holder, int kprefholder_flag);
void kprefcount_dec(kprefcount_t *kp_ref, unsigned char *ref_holder, int kprefholder_flag);
bool kprefcount_dec_and_test(kprefcount_t *kp_ref, unsigned char *ref_holder, int kprefholder_flag);