Message-ID: <5c3852e6-4a6a-42d8-85ff-8c1605939454@suse.cz>
Date: Tue, 27 Aug 2024 23:32:14 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Kees Cook <kees@...nel.org>,
 Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>,
 David Rientjes <rientjes@...gle.com>, Joonsoo Kim <iamjoonsoo.kim@....com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 Hyeonggon Yoo <42.hyeyoo@...il.com>,
 "Gustavo A . R . Silva" <gustavoars@...nel.org>,
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>,
 Jann Horn <jannh@...gle.com>, Przemek Kitszel
 <przemyslaw.kitszel@...el.com>, Marco Elver <elver@...gle.com>,
 linux-mm@...ck.org, Nathan Chancellor <nathan@...nel.org>,
 Nick Desaulniers <ndesaulniers@...gle.com>, linux-kernel@...r.kernel.org,
 llvm@...ts.linux.dev, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v3] slab: Introduce kmalloc_obj() and family

+Cc Linus

On 8/23/24 01:13, Kees Cook wrote:
> Introduce type-aware kmalloc-family helpers to replace the common
> idioms for single, array, and flexible object allocations:
> 
> 	ptr = kmalloc(sizeof(*ptr), gfp);
> 	ptr = kzalloc(sizeof(*ptr), gfp);
> 	ptr = kmalloc_array(count, sizeof(*ptr), gfp);
> 	ptr = kcalloc(count, sizeof(*ptr), gfp);
> 	ptr = kmalloc(struct_size(ptr, flex_member, count), gfp);
> 
> These become, respectively:
> 
> 	kmalloc_obj(ptr, gfp);
> 	kzalloc_obj(ptr, gfp);
> 	kmalloc_objs(ptr, count, gfp);
> 	kzalloc_objs(ptr, count, gfp);
> 	kmalloc_flex(ptr, flex_member, count, gfp);

This is indeed better than the previous version. The hidden assignment to
ptr seems still very counter-intuitive, but if it's the only way to do those
validations, the question is then just whether it's worth the getting used
to it, or not.

> These each return the assigned value of ptr (which may be NULL on
> failure). For cases where the total size of the allocation is needed,
> the kmalloc_obj_sz(), kmalloc_objs_sz(), and kmalloc_flex_sz() family
> of macros can be used. For example:
> 
> 	info->size = struct_size(ptr, flex_member, count);
> 	ptr = kmalloc(info->size, gfp);
> 
> becomes:
> 
> 	kmalloc_flex_sz(ptr, flex_member, count, gfp, &info->size);
> 
> Internal introspection of allocated type now becomes possible, allowing
> for future alignment-aware choices and hardening work. For example,
> adding __alignof(*ptr) as an argument to the internal allocators so that
> appropriate/efficient alignment choices can be made, or being able to
> correctly choose per-allocation offset randomization within a bucket
> that does not break alignment requirements.
> 
> Introduces __flex_count() for when __builtin_get_counted_by() is added

"Also introduce __flex_counter() ..."?

> by GCC[1] and Clang[2]. The internal use of __flex_count() allows for
> automatically setting the counter member of a struct's flexible array

But if it's a to-be-implemented feature, perhaps it would be too early to
include it here? Were you able to even test that part right now?

> member when it has been annotated with __counted_by(), avoiding any
> missed early size initializations while __counted_by() annotations are
> added to the kernel. Additionally, this also checks for "too large"
> allocations based on the type size of the counter variable. For example:
> 
> 	if (count > type_max(ptr->flex_count))
> 		fail...;
> 	info->size = struct_size(ptr, flex_member, count);
> 	ptr = kmalloc(info->size, gfp);
> 	ptr->flex_count = count;
> 
> becomes (i.e. unchanged from earlier example):
> 
> 	kmalloc_flex_sz(ptr, flex_member, count, gfp, &info->size);
> 
> Replacing all existing simple code patterns found via Coccinelle[3]
> shows what could be replaced immediately (saving roughly 1,500 lines):
> 
>  7040 files changed, 14128 insertions(+), 15557 deletions(-)

Since that could be feasible to apply only if Linus ran that directly
himself, including him now. Because doing it any other way would leave us
semi-converted forever and not bring the full benefits?

> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116016 [1]
> Link: https://github.com/llvm/llvm-project/issues/99774 [2]
> Link: https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/kmalloc_obj-assign-size.cocci [3]
> Signed-off-by: Kees Cook <kees@...nel.org>
> ---
> Initial testing looks good. Before I write all the self-tests, I just
> wanted to validate that the new API is reasonable (i.e. it is no longer
> using optional argument counts for choosing the internal API).
> 
>  v3:
>   - Add .rst documentation
>   - Add kern-doc
>   - Return ptr instead of size by default
>   - Add *_sz() variants that provide allocation size output
>   - Implement __flex_counter() logic
>  v2: https://lore.kernel.org/linux-hardening/20240807235433.work.317-kees@kernel.org/
>  v1: https://lore.kernel.org/linux-hardening/20240719192744.work.264-kees@kernel.org/
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: Christoph Lameter <cl@...ux.com>
> Cc: Pekka Enberg <penberg@...nel.org>
> Cc: David Rientjes <rientjes@...gle.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Roman Gushchin <roman.gushchin@...ux.dev>
> Cc: Hyeonggon Yoo <42.hyeyoo@...il.com>
> Cc: Gustavo A. R. Silva <gustavoars@...nel.org>
> Cc: Bill Wendling <morbo@...gle.com>
> Cc: Justin Stitt <justinstitt@...gle.com>
> Cc: Jann Horn <jannh@...gle.com>
> Cc: Przemek Kitszel <przemyslaw.kitszel@...el.com>
> Cc: Marco Elver <elver@...gle.com>
> Cc: linux-mm@...ck.org
> ---
>  Documentation/process/deprecated.rst |  41 +++++++
>  include/linux/compiler_types.h       |  22 ++++
>  include/linux/slab.h                 | 174 +++++++++++++++++++++++++++
>  3 files changed, 237 insertions(+)
> 
> diff --git a/Documentation/process/deprecated.rst b/Documentation/process/deprecated.rst
> index 1f7f3e6c9cda..b22ec088a044 100644
> --- a/Documentation/process/deprecated.rst
> +++ b/Documentation/process/deprecated.rst
> @@ -372,3 +372,44 @@ The helper must be used::
>  			DECLARE_FLEX_ARRAY(struct type2, two);
>  		};
>  	};
> +
> +Open-coded kmalloc assignments
> +------------------------------
> +Performing open-coded kmalloc()-family allocation assignments prevents
> +the kernel (and compiler) from being able to examine the type of the
> +variable being assigned, which limits any related introspection that
> +may help with alignment, wrap-around, or additional hardening. The
> +kmalloc_obj()-family of macros provide this introspection, which can be
> +used for the common code patterns for single, array, and flexible object
> +allocations. For example, these open coded assignments::
> +
> +	ptr = kmalloc(sizeof(*ptr), gfp);
> +	ptr = kzalloc(sizeof(*ptr), gfp);
> +	ptr = kmalloc_array(count, sizeof(*ptr), gfp);
> +	ptr = kcalloc(count, sizeof(*ptr), gfp);
> +	ptr = kmalloc(struct_size(ptr, flex_member, count), gfp);
> +
> +become, respectively::
> +
> +	kmalloc_obj(ptr, gfp);
> +	kzalloc_obj(ptr, gfp);
> +	kmalloc_objs(ptr, count, gfp);
> +	kzalloc_objs(ptr, count, gfp);
> +	kmalloc_flex(ptr, flex_member, count, gfp);
> +
> +For the cases where the total size of the allocation is also needed,
> +the kmalloc_obj_size(), kmalloc_objs_sz(), and kmalloc_flex_sz() family of
> +macros can be used. For example, converting these assignments::
> +
> +	total_size = struct_size(ptr, flex_member, count);
> +	ptr = kmalloc(total_size, gfp);
> +
> +becomes::
> +
> +	kmalloc_flex_sz(ptr, flex_member, count, gfp, &total_size);
> +
> +If `ptr->flex_member` is annotated with __counted_by(), the allocation
> +will automatically fail if `count` is larger than the maximum
> +representable value that can be stored in the counter member associated
> +with `flex_member`. Similarly, the allocation will fail if the total
> +size of the allocation exceeds the maximum value `*total_size` can hold.
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index f14c275950b5..b99deae45210 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -421,6 +421,28 @@ struct ftrace_likely_data {
>  #define __member_size(p)	__builtin_object_size(p, 1)
>  #endif
>  
> +#if __has_builtin(__builtin_get_counted_by)
> +/**
> + * __flex_counter - Get pointer to counter member for the given
> + *		    flexible array, if it was annotated with __counted_by()
> + * @flex: Pointer to flexible array member of an addressable struct instance
> + *
> + * For example, with:
> + *
> + *	struct foo {
> + *		int counter;
> + *		short array[] __counted_by(counter);
> + *	} *p;
> + *
> + * __flex_counter(p->array) will resolve to &p->counter.
> + *
> + * If p->array is unannotated, this returns (void *)NULL.
> + */
> +#define __flex_counter(flex)	__builtin_get_counted_by(flex)
> +#else
> +#define __flex_counter(flex)	((void *)NULL)
> +#endif
> +
>  /*
>   * Some versions of gcc do not mark 'asm goto' volatile:
>   *
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index eb2bf4629157..c37606b9e248 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -686,6 +686,180 @@ static __always_inline __alloc_size(1) void *kmalloc_noprof(size_t size, gfp_t f
>  }
>  #define kmalloc(...)				alloc_hooks(kmalloc_noprof(__VA_ARGS__))
>  
> +#define __alloc_objs(ALLOC, P, COUNT, FLAGS, SIZE)		\
> +({								\
> +	size_t __obj_size = size_mul(sizeof(*P), COUNT);	\
> +	const typeof(_Generic(SIZE,				\
> +			void *: (size_t *)NULL,			\
> +			default: SIZE)) __size_ptr = (SIZE);	\
> +	typeof(P) __obj_ptr = NULL;				\
> +	/* Does the total size fit in the *SIZE variable? */	\
> +	if (!__size_ptr || __obj_size <= type_max(*__size_ptr))	\
> +		__obj_ptr = ALLOC(__obj_size, FLAGS);		\
> +	if (!__obj_ptr)						\
> +		__obj_size = 0;					\
> +	if (__size_ptr)						\
> +		*__size_ptr = __obj_size;			\
> +	(P) = __obj_ptr;					\
> +})
> +
> +#define __alloc_flex(ALLOC, P, FAM, COUNT, FLAGS, SIZE)			\
> +({									\
> +	size_t __count = (COUNT);					\
> +	size_t __obj_size = struct_size(P, FAM, __count);		\
> +	const typeof(_Generic(SIZE,					\
> +			void *: (size_t *)NULL,				\
> +			default: SIZE)) __size_ptr = (SIZE);		\
> +	typeof(P) __obj_ptr = NULL;					\
> +	/* Just query the counter type for type_max checking. */	\
> +	typeof(_Generic(__flex_counter(__obj_ptr->FAM),			\
> +			void *: (size_t *)NULL,				\
> +			default: __flex_counter(__obj_ptr->FAM)))	\
> +		__counter_type_ptr = NULL;				\
> +	/* Does the count fit in the __counted_by counter member? */	\
> +	if ((__count <= type_max(*__counter_type_ptr)) &&		\
> +	    /* Does the total size fit in the *SIZE variable? */	\
> +	    (!__size_ptr || __obj_size <= type_max(*__size_ptr)))	\
> +		__obj_ptr = ALLOC(__obj_size, FLAGS);			\
> +	if (__obj_ptr) {						\
> +		/* __obj_ptr now allocated so get real counter ptr. */	\
> +		typeof(_Generic(__flex_counter(__obj_ptr->FAM),		\
> +				void *: (size_t *)NULL,			\
> +				default: __flex_counter(__obj_ptr->FAM))) \
> +			__counter_ptr = __flex_counter(__obj_ptr->FAM);	\
> +		if (__counter_ptr)					\
> +			*__counter_ptr = __count;			\
> +	} else {							\
> +		__obj_size = 0;						\
> +	}								\
> +	if (__size_ptr)							\
> +		*__size_ptr = __obj_size;				\
> +	(P) = __obj_ptr;						\
> +})
> +
> +/**
> + * kmalloc_obj - Allocate a single instance of the given structure
> + * @P: Pointer to hold allocation of the structure
> + * @FLAGS: GFP flags for the allocation
> + *
> + * Returns the newly allocated value of @P on success, NULL on failure.
> + * @P is assigned the result, either way.
> + */
> +#define kmalloc_obj(P, FLAGS)				\
> +	__alloc_objs(kmalloc, P, 1, FLAGS, NULL)
> +/**
> + * kmalloc_obj_sz - Allocate a single instance of the given structure and
> + *		 store total size
> + * @P: Pointer to hold allocation of the structure
> + * @FLAGS: GFP flags for the allocation
> + * @SIZE: Pointer to variable to hold the total allocation size
> + *
> + * Returns the newly allocated value of @P on success, NULL on failure.
> + * @P is assigned the result, either way. If @SIZE is non-NULL, the
> + * allocation will immediately fail if the total allocation size is larger
> + * than what the type of *@...E can represent.
> + */
> +#define kmalloc_obj_sz(P, FLAGS, SIZE)			\
> +	__alloc_objs(kmalloc, P, 1, FLAGS, SIZE)
> +/**
> + * kmalloc_objs - Allocate an array of the given structure
> + * @P: Pointer to hold allocation of the structure array
> + * @COUNT: How many elements in the array
> + * @FLAGS: GFP flags for the allocation
> + *
> + * Returns the newly allocated value of @P on success, NULL on failure.
> + * @P is assigned the result, either way.
> + */
> +#define kmalloc_objs(P, COUNT, FLAGS)			\
> +	__alloc_objs(kmalloc, P, COUNT, FLAGS, NULL)
> +/**
> + * kmalloc_objs_sz - Allocate an array of the given structure and store
> + *		     total size
> + * @P: Pointer to hold allocation of the structure array
> + * @COUNT: How many elements in the array
> + * @FLAGS: GFP flags for the allocation
> + * @SIZE: Pointer to variable to hold the total allocation size
> + *
> + * Returns the newly allocated value of @P on success, NULL on failure.
> + * @P is assigned the result, either way. If @SIZE is non-NULL, the
> + * allocation will immediately fail if the total allocation size is larger
> + * than what the type of *@...E can represent.
> + */
> +#define kmalloc_objs_sz(P, COUNT, FLAGS, SIZE)		\
> +	__alloc_objs(kmalloc, P, COUNT, FLAGS, SIZE)
> +/**
> + * kmalloc_flex - Allocate a single instance of the given flexible structure
> + * @P: Pointer to hold allocation of the structure
> + * @FAM: The name of the flexible array member of the structure
> + * @COUNT: How many flexible array member elements are desired
> + * @FLAGS: GFP flags for the allocation
> + *
> + * Returns the newly allocated value of @P on success, NULL on failure.
> + * @P is assigned the result, either way. If @FAM has been annotated with
> + * __counted_by(), the allocation will immediately fail if @COUNT is larger
> + * than what the type of the struct's counter variable can represent.
> + */
> +#define kmalloc_flex(P, FAM, COUNT, FLAGS)		\
> +	__alloc_flex(kmalloc, P, FAM, COUNT, FLAGS, NULL)
> +
> +/**
> + * kmalloc_flex_sz - Allocate a single instance of the given flexible
> + *		     structure and store total size
> + * @P: Pointer to hold allocation of the structure
> + * @FAM: The name of the flexible array member of the structure
> + * @COUNT: How many flexible array member elements are desired
> + * @FLAGS: GFP flags for the allocation
> + * @SIZE: Pointer to variable to hold the total allocation size
> + *
> + * Returns the newly allocated value of @P on success, NULL on failure.
> + * @P is assigned the result, either way. If @FAM has been annotated with
> + * __counted_by(), the allocation will immediately fail if @COUNT is larger
> + * than what the type of the struct's counter variable can represent. If
> + * @SIZE is non-NULL, the allocation will immediately fail if the total
> + * allocation size is larger than what the type of *@...E can represent.
> + */
> +#define kmalloc_flex_sz(P, FAM, COUNT, FLAGS, SIZE)	\
> +	__alloc_flex(kmalloc, P, FAM, COUNT, FLAGS, SIZE)
> +
> +#define kzalloc_obj(P, FLAGS)				\
> +	__alloc_objs(kzalloc, P, 1, FLAGS, NULL)
> +#define kzalloc_obj_sz(P, FLAGS, SIZE)			\
> +	__alloc_objs(kzalloc, P, 1, FLAGS, SIZE)
> +#define kzalloc_objs(P, COUNT, FLAGS)			\
> +	__alloc_objs(kzalloc, P, COUNT, FLAGS, NULL)
> +#define kzalloc_objs_sz(P, COUNT, FLAGS, SIZE)		\
> +	__alloc_objs(kzalloc, P, COUNT, FLAGS, SIZE)
> +#define kzalloc_flex(P, FAM, COUNT, FLAGS)		\
> +	__alloc_flex(kzalloc, P, FAM, COUNT, FLAGS, NULL)
> +#define kzalloc_flex_sz(P, FAM, COUNT, FLAGS, SIZE)	\
> +	__alloc_flex(kzalloc, P, FAM, COUNT, FLAGS, SIZE)
> +
> +#define kvmalloc_obj(P, FLAGS)				\
> +	__alloc_objs(kvmalloc, P, 1, FLAGS, NULL)

Wonder if there is really a single struct (not array) with no flex array
that could need kvmalloc? :)

> +#define kvmalloc_obj_sz(P, FLAGS, SIZE)			\
> +	__alloc_objs(kvmalloc, P, 1, FLAGS, SIZE)
> +#define kvmalloc_objs(P, COUNT, FLAGS)			\
> +	__alloc_objs(kvmalloc, P, COUNT, FLAGS, NULL)
> +#define kvmalloc_objs_sz(P, COUNT, FLAGS, SIZE)		\
> +	__alloc_objs(kvmalloc, P, COUNT, FLAGS, SIZE)
> +#define kvmalloc_flex(P, FAM, COUNT, FLAGS)		\
> +	__alloc_flex(kvmalloc, P, FAM, COUNT, FLAGS, NULL)
> +#define kvmalloc_flex_sz(P, FAM, COUNT, FLAGS, SIZE)	\
> +	__alloc_flex(kvmalloc, P, FAM, COUNT, FLAGS, SIZE)
> +
> +#define kvzalloc_obj(P, FLAGS)				\
> +	__alloc_objs(kvzalloc, P, 1, FLAGS, NULL)
> +#define kvzalloc_obj_sz(P, FLAGS, SIZE)			\
> +	__alloc_objs(kvzalloc, P, 1, FLAGS, SIZE)
> +#define kvzalloc_objs(P, COUNT, FLAGS)			\
> +	__alloc_objs(kvzalloc, P, COUNT, FLAGS, NULL)
> +#define kvzalloc_objs_sz(P, COUNT, FLAGS, SIZE)		\
> +	__alloc_objs(kvzalloc, P, COUNT, FLAGS, SIZE)
> +#define kvzalloc_flex(P, FAM, COUNT, FLAGS)		\
> +	__alloc_flex(kvzalloc, P, FAM, COUNT, FLAGS, NULL)
> +#define kvzalloc_flex_sz(P, FAM, COUNT, FLAGS, SIZE)	\
> +	__alloc_flex(kvzalloc, P, FAM, COUNT, FLAGS, SIZE)
> +
>  #define kmem_buckets_alloc(_b, _size, _flags)	\
>  	alloc_hooks(__kmalloc_node_noprof(PASS_BUCKET_PARAMS(_size, _b), _flags, NUMA_NO_NODE))
>  

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