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

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Date:   Wed, 2 Aug 2017 07:43:02 +1000
From:   Stephen Rothwell <sfr@...b.auug.org.au>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] adfs: use 'unsigned' types for memcpy length

Hi Arnd,

On Tue, 1 Aug 2017 11:20:26 -0700 Kees Cook <keescook@...omium.org> wrote:
>
> 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.)

Can someone please send me the patch so I can use it if Andrew doesn't
get around to updating mmotd today?

-- 
Cheers,
Stephen Rothwell