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Date: Wed, 9 May 2018 11:49:33 -0700
From: Kees Cook <keescook@...omium.org>
To: Rasmus Villemoes <linux@...musvillemoes.dk>
Cc: Matthew Wilcox <mawilcox@...rosoft.com>, LKML <linux-kernel@...r.kernel.org>, 
	Linux-MM <linux-mm@...ck.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 03/13] overflow.h: Add allocation size calculation helpers

On Wed, May 9, 2018 at 11:27 AM, Rasmus Villemoes
<linux@...musvillemoes.dk> wrote:
> On 2018-05-09 02:42, Kees Cook wrote:
>> In preparation for replacing unchecked overflows for memory allocations,
>> this creates helpers for the 3 most common calculations:
>>
>> array_size(a, b): 2-dimensional array
>> array3_size(a, b, c): 2-dimensional array
>
> yeah... complete confusion...
>
>> +/**
>> + * array_size() - Calculate size of 2-dimensional array.
>> + *
>> + * @a: dimension one
>> + * @b: dimension two
>> + *
>> + * Calculates size of 2-dimensional array: @a * @b.
>> + *
>> + * Returns: number of bytes needed to represent the array or SIZE_MAX on
>> + * overflow.
>> + */
>> +static inline __must_check size_t array_size(size_t a, size_t b)
>> +{
>> +     size_t bytes;
>> +
>> +     if (check_mul_overflow(a, b, &bytes))
>> +             return SIZE_MAX;
>> +
>> +     return bytes;
>> +}
>> +
>> +/**
>> + * array3_size() - Calculate size of 3-dimensional array.
>> + *
>
> ...IDGI. array_size is/will most often be used to calculate the size of
> a one-dimensional array, count*elemsize, accessed as foo[i]. Won't a
> three-factor product usually be dim1*dim2*elemsize, i.e. 2-dimensional,
> accessed (because C is lame) as foo[i*dim2 + j]?

I was thinking of byte addressing, not object addressing. I can
rewrite this to be less confusing.

>> +/**
>> + * struct_size() - Calculate size of structure with trailing array.
>> + * @p: Pointer to the structure.
>> + * @member: Name of the array member.
>> + * @n: Number of elements in the array.
>> + *
>> + * Calculates size of memory needed for structure @p followed by an
>> + * array of @n @member elements.
>> + *
>> + * Return: number of bytes needed or SIZE_MAX on overflow.
>> + */
>> +#define struct_size(p, member, n)                                    \
>> +     __ab_c_size(n,                                                  \
>> +                 sizeof(*(p)->member) + __must_be_array((p)->member),\
>> +                 offsetof(typeof(*(p)), member))
>> +
>> +
>
> struct s { int a; char b; char c[]; } has sizeof > offsetof(c), so for
> small enough n, we end up allocating less than sizeof(struct s). And the
> caller might reasonably do memset(s, 0, sizeof(*s)) to initialize all
> members before c. In practice our allocators round up to a multiple of
> 8, so I don't think it's a big problem, but some sanitizer might
> complain. But I do think you should make that addend sizeof() instead of
> offsetof().

Erg. Yeah, I think we'd best "round up". Besides the "< sizeof()" vs
memset() case you mention, another pattern I've seen is doing stuff
like:

array = (array_type *)(thing + 1);

So if padding somehow caused us to under-allocate, we'll get it wrong there too.

I'll change this to be strictly sizeof(*(p)).

(Though it might be nice to enforce that "member" is at the end of the
structure, though, otherwise this could be misused for struct s { int
a; char c[2]; char b[]; } ... )

-Kees

-- 
Kees Cook
Pixel Security

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