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

[Kees Cook]

On Tue, Aug 1, 2017 at 5:04 AM, Arnd Bergmann <arnd@xxxxxxxx> 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@xxxxxxxx>

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

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

-Kees

-- 
Kees Cook
Pixel Security