linux-hardening - Re: [PATCH v2 4/7] slab: Add __alloc_size attributes for better bounds checking

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Message-ID: <a54715b4-3d95-293c-6b18-6d84299bd58f@rasmusvillemoes.dk>
Date:   Thu, 19 Aug 2021 10:27:33 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org
Cc:     Daniel Micay <danielmicay@...il.com>,
        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>,
        Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        Joe Perches <joe@...ches.com>, Miguel Ojeda <ojeda@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Andy Whitcroft <apw@...onical.com>,
        Dwaipayan Ray <dwaipayanray1@...il.com>,
        Lukas Bulwahn <lukas.bulwahn@...il.com>,
        Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        clang-built-linux@...glegroups.com, linux-kbuild@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2 4/7] slab: Add __alloc_size attributes for better
 bounds checking

On 18/08/2021 23.40, Kees Cook wrote:
> As already done in GrapheneOS, add the __alloc_size attribute for
> regular kmalloc interfaces, to provide additional hinting for better
> bounds checking, assisting CONFIG_FORTIFY_SOURCE and other compiler
> optimizations.
> 

>  #ifdef CONFIG_NUMA
> +__alloc_size(1)
>  void *__kmalloc_node(size_t size, gfp_t flags, int node) __assume_slab_alignment __malloc;

Eh, can we keep all the attributes together instead of having some
before, some after?

I don't necessarily think this is a good idea, but just throwing it out
there: __alloc_size almost always goes along with __malloc, so one could
define __alloc_size in such a way that it implies __malloc, then just
have a "raw" ____alloc_size version to use for krealloc() and similar.
But I guess it's cleaner to keep it this way.

While declared in string.h, kmemdup() is also eligible for alloc_size(2).

Which brings me to an old wishlist item of mine [it's almost christmas]:
that alloc_size could understand more general expressions for the size
of the returned memory, not just the primitive one based on
malloc()/calloc() prototypes. So e.g. kmemdup_nul() returns something of
size $2+1, while it is also very common to have a alloc_foo(void) helper
which returns something of size sizeof(struct foo). Unfortunately I
don't think gcc's attribute parsing machinery can easily be tweaked into
accepting

struct bar *alloc_bars(unsigned count) __new_a_s(count * sizeof(struct bar))

but maybe clang could. If a compiler could understand that kind of
attribute, it would also pave the way for implementing
__attribute__((__buffer_size__(param, size, access)))

e.g.

memchr(src, c, size) __buffer_size(src, size, "r")

clk_bulk_get(struct device *dev, int num_clks, struct clk_bulk_data
*clks) __buffer_size(clks, num_clks * sizeof(*clks), "rw")

which could be used for both static analysis and optional run-time
instrumentation.

Rasmus