linux-hardening - Re: [PATCH] net: prestera: Add flex arrays to some structs

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Date: Mon, 13 May 2024 11:57:04 -0700
From: Kees Cook <keescook@...omium.org>
To: Erick Archer <erick.archer@...look.com>
Cc: Taras Chornyi <taras.chornyi@...ision.eu>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
	llvm@...ts.linux.dev
Subject: Re: [PATCH] net: prestera: Add flex arrays to some structs

On Sun, May 12, 2024 at 06:10:27PM +0200, Erick Archer wrote:
> The "struct prestera_msg_vtcam_rule_add_req" uses a dynamically sized
> set of trailing elements. Specifically, it uses an array of structures
> of type "prestera_msg_acl_action actions_msg".
> 
> The "struct prestera_msg_flood_domain_ports_set_req" also uses a
> dynamically sized set of trailing elements. Specifically, it uses an
> array of structures of type "prestera_msg_acl_action actions_msg".
> 
> So, use the preferred way in the kernel declaring flexible arrays [1].
> 
> At the same time, prepare for the coming implementation by GCC and Clang
> of the __counted_by attribute. Flexible array members annotated with
> __counted_by can have their accesses bounds-checked at run-time via
> CONFIG_UBSAN_BOUNDS (for array indexing) and CONFIG_FORTIFY_SOURCE (for
> strcpy/memcpy-family functions). In this case, it is important to note
> that the attribute used is specifically __counted_by_le since the
> counters are of type __le32.
> 
> The logic does not need to change since the counters for the flexible
> arrays are asigned before any access to the arrays.
> 
> The order in which the structure prestera_msg_vtcam_rule_add_req and the
> structure prestera_msg_flood_domain_ports_set_req are defined must be
> changed to avoid incomplete type errors.
> 
> Also, avoid the open-coded arithmetic in memory allocator functions [2]
> using the "struct_size" macro.
> 
> Moreover, the new structure members also allow us to avoid the open-
> coded arithmetic on pointers. So, take advantage of this refactoring
> accordingly.
> 
> This code was detected with the help of Coccinelle, and audited and
> modified manually.
> 
> Link: https://www.kernel.org/doc/html/next/process/deprecated.html#zero-length-and-one-element-arrays [1]
> Link: https://www.kernel.org/doc/html/next/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments [2]
> Signed-off-by: Erick Archer <erick.archer@...look.com>

This is a really nice cleanup. :) Fewer lines of code, more readable,
and protected by __counted_by!

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook