On Fri, Sep 23, 2022 at 01:28:08PM -0700, Kees Cook wrote: > In the effort to help the compiler reason about buffer sizes, the > __alloc_size attribute was added to allocators. This improves the scope > of the compiler's ability to apply CONFIG_UBSAN_BOUNDS and (in the near > future) CONFIG_FORTIFY_SOURCE. For most allocations, this works well, > as the vast majority of callers are not expecting to use more memory > than what they asked for. > > There is, however, one common exception to this: anticipatory resizing > of kmalloc allocations. These cases all use ksize() to determine the > actual bucket size of a given allocation (e.g. 128 when 126 was asked > for). This comes in two styles in the kernel: > > 1) An allocation has been determined to be too small, and needs to be > resized. Instead of the caller choosing its own next best size, it > wants to minimize the number of calls to krealloc(), so it just uses > ksize() plus some additional bytes, forcing the realloc into the next > bucket size, from which it can learn how large it is now. For example: > > data = krealloc(data, ksize(data) + 1, gfp); > data_len = ksize(data); > > 2) The minimum size of an allocation is calculated, but since it may > grow in the future, just use all the space available in the chosen > bucket immediately, to avoid needing to reallocate later. A good > example of this is skbuff's allocators: > > data = kmalloc_reserve(size, gfp_mask, node, &pfmemalloc); > ... > /* kmalloc(size) might give us more room than requested. > * Put skb_shared_info exactly at the end of allocated zone, > * to allow max possible filling before reallocation. > */ > osize = ksize(data); > size = SKB_WITH_OVERHEAD(osize); > > In both cases, the "how much was actually allocated?" question is answered > _after_ the allocation, where the compiler hinting is not in an easy place > to make the association any more. This mismatch between the compiler's > view of the buffer length and the code's intention about how much it is > going to actually use has already caused problems[1]. It is possible to > fix this by reordering the use of the "actual size" information. > > We can serve the needs of users of ksize() and still have accurate buffer > length hinting for the compiler by doing the bucket size calculation > _before_ the allocation. Code can instead ask "how large an allocation > would I get for a given size?". > > Introduce kmalloc_size_roundup(), to serve this function so we can start > replacing the "anticipatory resizing" uses of ksize(). > > [1] https://github.com/ClangBuiltLinux/linux/issues/1599 > https://github.com/KSPP/linux/issues/183 > > Cc: Vlastimil Babka <vbabka@xxxxxxx> > Cc: Christoph Lameter <cl@xxxxxxxxx> > Cc: Pekka Enberg <penberg@xxxxxxxxxx> > Cc: David Rientjes <rientjes@xxxxxxxxxx> > Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > Cc: linux-mm@xxxxxxxxx > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> > --- > include/linux/slab.h | 31 +++++++++++++++++++++++++++++++ > mm/slab.c | 9 ++++++--- > mm/slab_common.c | 20 ++++++++++++++++++++ > 3 files changed, 57 insertions(+), 3 deletions(-) > > diff --git a/include/linux/slab.h b/include/linux/slab.h > index 41bd036e7551..727640173568 100644 > --- a/include/linux/slab.h > +++ b/include/linux/slab.h > @@ -188,7 +188,21 @@ void * __must_check krealloc(const void *objp, size_t new_size, gfp_t flags) __r > void kfree(const void *objp); > void kfree_sensitive(const void *objp); > size_t __ksize(const void *objp); > + > +/** > + * ksize - Report actual allocation size of associated object > + * > + * @objp: Pointer returned from a prior kmalloc()-family allocation. > + * > + * This should not be used for writing beyond the originally requested > + * allocation size. Either use krealloc() or round up the allocation size > + * with kmalloc_size_roundup() prior to allocation. If this is used to > + * access beyond the originally requested allocation size, UBSAN_BOUNDS > + * and/or FORTIFY_SOURCE may trip, since they only know about the > + * originally allocated size via the __alloc_size attribute. > + */ > size_t ksize(const void *objp); > + With this now we have two conflicting kernel-doc comments about ksize in mm/slab_common.c and include/linux/slab.h. > #ifdef CONFIG_PRINTK > bool kmem_valid_obj(void *object); > void kmem_dump_obj(void *object); > @@ -779,6 +793,23 @@ extern void kvfree(const void *addr); > extern void kvfree_sensitive(const void *addr, size_t len); > > unsigned int kmem_cache_size(struct kmem_cache *s); > + > +/** > + * kmalloc_size_roundup - Report allocation bucket size for the given size > + * > + * @size: Number of bytes to round up from. > + * > + * This returns the number of bytes that would be available in a kmalloc() > + * allocation of @size bytes. For example, a 126 byte request would be > + * rounded up to the next sized kmalloc bucket, 128 bytes. (This is strictly > + * for the general-purpose kmalloc()-based allocations, and is not for the > + * pre-sized kmem_cache_alloc()-based allocations.) > + * > + * Use this to kmalloc() the full bucket size ahead of time instead of using > + * ksize() to query the size after an allocation. > + */ > +size_t kmalloc_size_roundup(size_t size); > + > void __init kmem_cache_init_late(void); > > #if defined(CONFIG_SMP) && defined(CONFIG_SLAB) > diff --git a/mm/slab.c b/mm/slab.c > index 10e96137b44f..2da862bf6226 100644 > --- a/mm/slab.c > +++ b/mm/slab.c > @@ -4192,11 +4192,14 @@ void __check_heap_object(const void *ptr, unsigned long n, > #endif /* CONFIG_HARDENED_USERCOPY */ > > /** > - * __ksize -- Uninstrumented ksize. > + * __ksize -- Report full size of underlying allocation > * @objp: pointer to the object > * > - * Unlike ksize(), __ksize() is uninstrumented, and does not provide the same > - * safety checks as ksize() with KASAN instrumentation enabled. > + * This should only be used internally to query the true size of allocations. > + * It is not meant to be a way to discover the usable size of an allocation > + * after the fact. Instead, use kmalloc_size_roundup(). Using memory beyond > + * the originally requested allocation size may trigger KASAN, UBSAN_BOUNDS, > + * and/or FORTIFY_SOURCE. > * > * Return: size of the actual memory used by @objp in bytes > */ > diff --git a/mm/slab_common.c b/mm/slab_common.c > index 457671ace7eb..d7420cf649f8 100644 > --- a/mm/slab_common.c > +++ b/mm/slab_common.c > @@ -721,6 +721,26 @@ struct kmem_cache *kmalloc_slab(size_t size, gfp_t flags) > return kmalloc_caches[kmalloc_type(flags)][index]; > } > > +size_t kmalloc_size_roundup(size_t size) > +{ > + struct kmem_cache *c; > + > + /* Short-circuit the 0 size case. */ > + if (unlikely(size == 0)) > + return 0; > + /* Short-circuit saturated "too-large" case. */ > + if (unlikely(size == SIZE_MAX)) > + return SIZE_MAX; > + /* Above the smaller buckets, size is a multiple of page size. */ > + if (size > KMALLOC_MAX_CACHE_SIZE) > + return PAGE_SIZE << get_order(size); > + > + /* The flags don't matter since size_index is common to all. */ > + c = kmalloc_slab(size, GFP_KERNEL); > + return c ? c->object_size : 0; > +} > +EXPORT_SYMBOL(kmalloc_size_roundup); > + > #ifdef CONFIG_ZONE_DMA > #define KMALLOC_DMA_NAME(sz) .name[KMALLOC_DMA] = "dma-kmalloc-" #sz, > #else > -- > 2.34.1 Otherwise looks good! -- Thanks, Hyeonggon
