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Subject: Re: [PATCH 34/64] fortify: Detect struct member overflows in memcpy()
 at compile-time
To:     Kees Cook <keescook@chromium.org>, linux-hardening@vger.kernel.org
Cc:     "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        Keith Packard <keithpac@amazon.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-wireless@vger.kernel.org,
        netdev@vger.kernel.org, dri-devel@lists.freedesktop.org,
        linux-staging@lists.linux.dev, linux-block@vger.kernel.org,
        linux-kbuild@vger.kernel.org, clang-built-linux@googlegroups.com
References: <20210727205855.411487-1-keescook@chromium.org>
 <20210727205855.411487-35-keescook@chromium.org>
From:   Rasmus Villemoes <linux@rasmusvillemoes.dk>
Message-ID: <d2f9f21c-4d6e-9458-5887-ca5166d07942@rasmusvillemoes.dk>
Date:   Wed, 28 Jul 2021 13:19:59 +0200
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On 27/07/2021 22.58, Kees Cook wrote:

> At its core, FORTIFY_SOURCE uses the compiler's __builtin_object_size()
> internal[0] to determine the available size at a target address based on
> the compile-time known structure layout details. It operates in two
> modes: outer bounds (0) and inner bounds (1). In mode 0, the size of the
> enclosing structure is used. In mode 1, the size of the specific field
> is used. For example:
> 
> 	struct object {
> 		u16 scalar1;	/* 2 bytes */
> 		char array[6];	/* 6 bytes */
> 		u64 scalar2;	/* 8 bytes */
> 		u32 scalar3;	/* 4 bytes */
> 	} instance;
> 
>
> __builtin_object_size(instance.array, 0) == 18, since the remaining size
> of the enclosing structure starting from "array" is 18 bytes (6 + 8 + 4).

I think the compiler would usually end up making that struct size 24,
with 4 bytes of trailing padding (at least when alignof(u64) is 8). In
that case, does __builtin_object_size(instance.array, 0) actually
evaluate to 18, or to 22? A quick test on x86-64 suggests the latter, so
the memcpy(, , 20) would not be a violation.

Perhaps it's better to base the example on something which doesn't have
potential trailing padding - so either add another 4 byte member, or
also make scalar2 u32.

Rasmus