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/* SPDX-License-Identifier: GPL-2.0 */
/*
* Kernel Electric-Fence (KFENCE). Public interface for allocator and fault
* handler integration. For more info see Documentation/dev-tools/kfence.rst.
*
* Copyright (C) 2020, Google LLC.
*/
#ifndef _LINUX_KFENCE_H
#define _LINUX_KFENCE_H
#include <linux/mm.h>
#include <linux/types.h>
#ifdef CONFIG_KFENCE
#include <linux/atomic.h>
#include <linux/static_key.h>
/*
* We allocate an even number of pages, as it simplifies calculations to map
* address to metadata indices; effectively, the very first page serves as an
* extended guard page, but otherwise has no special purpose.
*/
#define KFENCE_POOL_SIZE ((CONFIG_KFENCE_NUM_OBJECTS + 1) * 2 * PAGE_SIZE)
extern char *__kfence_pool;
DECLARE_STATIC_KEY_FALSE(kfence_allocation_key);
extern atomic_t kfence_allocation_gate;
/**
* is_kfence_address() - check if an address belongs to KFENCE pool
* @addr: address to check
*
* Return: true or false depending on whether the address is within the KFENCE
* object range.
*
* KFENCE objects live in a separate page range and are not to be intermixed
* with regular heap objects (e.g. KFENCE objects must never be added to the
* allocator freelists). Failing to do so may and will result in heap
* corruptions, therefore is_kfence_address() must be used to check whether
* an object requires specific handling.
*
* Note: This function may be used in fast-paths, and is performance critical.
* Future changes should take this into account; for instance, we want to avoid
* introducing another load and therefore need to keep KFENCE_POOL_SIZE a
* constant (until immediate patching support is added to the kernel).
*/
static __always_inline bool is_kfence_address(const void *addr)
{
/*
* The __kfence_pool != NULL check is required to deal with the case
* where __kfence_pool == NULL && addr < KFENCE_POOL_SIZE. Keep it in
* the slow-path after the range-check!
*/
return unlikely((unsigned long)((char *)addr - __kfence_pool) < KFENCE_POOL_SIZE && __kfence_pool);
}
/**
* kfence_alloc_pool() - allocate the KFENCE pool via memblock
*/
void __init kfence_alloc_pool(void);
/**
* kfence_init() - perform KFENCE initialization at boot time
*
* Requires that kfence_alloc_pool() was called before. This sets up the
* allocation gate timer, and requires that workqueues are available.
*/
void __init kfence_init(void);
/**
* kfence_shutdown_cache() - handle shutdown_cache() for KFENCE objects
* @s: cache being shut down
*
* Before shutting down a cache, one must ensure there are no remaining objects
* allocated from it. Because KFENCE objects are not referenced from the cache
* directly, we need to check them here.
*
* Note that shutdown_cache() is internal to SL*B, and kmem_cache_destroy() does
* not return if allocated objects still exist: it prints an error message and
* simply aborts destruction of a cache, leaking memory.
*
* If the only such objects are KFENCE objects, we will not leak the entire
* cache, but instead try to provide more useful debug info by making allocated
* objects "zombie allocations". Objects may then still be used or freed (which
* is handled gracefully), but usage will result in showing KFENCE error reports
* which include stack traces to the user of the object, the original allocation
* site, and caller to shutdown_cache().
*/
void kfence_shutdown_cache(struct kmem_cache *s);
/*
* Allocate a KFENCE object. Allocators must not call this function directly,
* use kfence_alloc() instead.
*/
void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags);
/**
* kfence_alloc() - allocate a KFENCE object with a low probability
* @s: struct kmem_cache with object requirements
* @size: exact size of the object to allocate (can be less than @s->size
* e.g. for kmalloc caches)
* @flags: GFP flags
*
* Return:
* * NULL - must proceed with allocating as usual,
* * non-NULL - pointer to a KFENCE object.
*
* kfence_alloc() should be inserted into the heap allocation fast path,
* allowing it to transparently return KFENCE-allocated objects with a low
* probability using a static branch (the probability is controlled by the
* kfence.sample_interval boot parameter).
*/
static __always_inline void *kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
{
#if defined(CONFIG_KFENCE_STATIC_KEYS) || CONFIG_KFENCE_SAMPLE_INTERVAL == 0
if (!static_branch_unlikely(&kfence_allocation_key))
return NULL;
#else
if (!static_branch_likely(&kfence_allocation_key))
return NULL;
#endif
if (likely(atomic_read(&kfence_allocation_gate)))
return NULL;
return __kfence_alloc(s, size, flags);
}
/**
* kfence_ksize() - get actual amount of memory allocated for a KFENCE object
* @addr: pointer to a heap object
*
* Return:
* * 0 - not a KFENCE object, must call __ksize() instead,
* * non-0 - this many bytes can be accessed without causing a memory error.
*
* kfence_ksize() returns the number of bytes requested for a KFENCE object at
* allocation time. This number may be less than the object size of the
* corresponding struct kmem_cache.
*/
size_t kfence_ksize(const void *addr);
/**
* kfence_object_start() - find the beginning of a KFENCE object
* @addr: address within a KFENCE-allocated object
*
* Return: address of the beginning of the object.
*
* SL[AU]B-allocated objects are laid out within a page one by one, so it is
* easy to calculate the beginning of an object given a pointer inside it and
* the object size. The same is not true for KFENCE, which places a single
* object at either end of the page. This helper function is used to find the
* beginning of a KFENCE-allocated object.
*/
void *kfence_object_start(const void *addr);
/**
* __kfence_free() - release a KFENCE heap object to KFENCE pool
* @addr: object to be freed
*
* Requires: is_kfence_address(addr)
*
* Release a KFENCE object and mark it as freed.
*/
void __kfence_free(void *addr);
/**
* kfence_free() - try to release an arbitrary heap object to KFENCE pool
* @addr: object to be freed
*
* Return:
* * false - object doesn't belong to KFENCE pool and was ignored,
* * true - object was released to KFENCE pool.
*
* Release a KFENCE object and mark it as freed. May be called on any object,
* even non-KFENCE objects, to simplify integration of the hooks into the
* allocator's free codepath. The allocator must check the return value to
* determine if it was a KFENCE object or not.
*/
static __always_inline __must_check bool kfence_free(void *addr)
{
if (!is_kfence_address(addr))
return false;
__kfence_free(addr);
return true;
}
/**
* kfence_handle_page_fault() - perform page fault handling for KFENCE pages
* @addr: faulting address
* @is_write: is access a write
* @regs: current struct pt_regs (can be NULL, but shows full stack trace)
*
* Return:
* * false - address outside KFENCE pool,
* * true - page fault handled by KFENCE, no additional handling required.
*
* A page fault inside KFENCE pool indicates a memory error, such as an
* out-of-bounds access, a use-after-free or an invalid memory access. In these
* cases KFENCE prints an error message and marks the offending page as
* present, so that the kernel can proceed.
*/
bool __must_check kfence_handle_page_fault(unsigned long addr, bool is_write, struct pt_regs *regs);
#else /* CONFIG_KFENCE */
static inline bool is_kfence_address(const void *addr) { return false; }
static inline void kfence_alloc_pool(void) { }
static inline void kfence_init(void) { }
static inline void kfence_shutdown_cache(struct kmem_cache *s) { }
static inline void *kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags) { return NULL; }
static inline size_t kfence_ksize(const void *addr) { return 0; }
static inline void *kfence_object_start(const void *addr) { return NULL; }
static inline void __kfence_free(void *addr) { }
static inline bool __must_check kfence_free(void *addr) { return false; }
static inline bool __must_check kfence_handle_page_fault(unsigned long addr, bool is_write,
struct pt_regs *regs)
{
return false;
}
#endif
#endif /* _LINUX_KFENCE_H */
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