linux-kernel - Re: [PATCH 2/2] adfs: use 'unsigned' types for memcpy length

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Message-ID: <CAGXu5jJZRDX6Vz+Y_gA0cKjawshY_-scvF4sNBDTmjk-hSRVyA@mail.gmail.com>
Date:   Tue, 1 Aug 2017 11:20:26 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Arnd Bergmann <arnd@...db.de>,
        Stephen Rothwell <sfr@...b.auug.org.au>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] adfs: use 'unsigned' types for memcpy length

On Tue, Aug 1, 2017 at 5:04 AM, Arnd Bergmann <arnd@...db.de> wrote:
> After commit 62d1034f53e3 ("fortify: use WARN instead of BUG for now"), we
> get a warning in adfs about a possible buffer overflow:
>
> In function 'memcpy',
>     inlined from '__adfs_dir_put' at fs/adfs/dir_f.c:318:2,
>     inlined from 'adfs_f_update' at fs/adfs/dir_f.c:403:2:
> include/linux/string.h:305:4: error: call to '__read_overflow2' declared with attribute error: detected read beyond size of object passed as 2nd parameter
>     __read_overflow2();
>     ^~~~~~~~~~~~~~~~~~
>
> The warning is correct in the sense that a negative 'pos' argument
> to the function would have that result. However, this is not a bug,
> as we know the position is always positive (in fact, between 5
> and 2007, inclusive) when the function gets called.
>
> Changing the variable to a unsigned type avoids the problem. I decided
> to use 'unsigned int' for the position in the directory and the block
> number, as they are both counting things, but use size_t for the
> offset and length that get passed into memcpy. This shuts up the
> warning.
>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

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

Thanks for the fix! (Added sfr to Cc since he noticed this too.)

-Kees

-- 
Kees Cook
Pixel Security