linux-hardening - Re: [PATCH 4/6] string: Add __realloc

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Date:   Wed, 2 Nov 2022 12:40:31 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Rasmus Villemoes <rasmus.villemoes@...vas.dk>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Guenter Roeck <linux@...ck-us.net>,
        Andy Shevchenko <andriy.shevchenko@...el.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        David Gow <davidgow@...gle.com>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-hardening@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH 4/6] string: Add __realloc_size hint to kmemdup()

On Wed, Nov 02, 2022 at 10:26:40AM +0100, Rasmus Villemoes wrote:
> On 01/11/2022 23.33, Kees Cook wrote:
> > Add __realloc_size() hint to kmemdup() so the compiler can reason about
> > the length of the returned buffer. (These must not use __alloc_size,
> > since those include __malloc which says the contents aren't defined[1]).
> > 
> > [1] https://lore.kernel.org/linux-hardening/d199c2af-06af-8a50-a6a1-00eefa0b67b4@prevas.dk/
> > 
> 
> >  extern char *kstrndup(const char *s, size_t len, gfp_t gfp);
> > -extern void *kmemdup(const void *src, size_t len, gfp_t gfp) __alloc_size(2);
> > +extern void *kmemdup(const void *src, size_t len, gfp_t gfp) __realloc_size(2);
> 
> What tree is this based on? I see that kmemdup() has grown that bogus
> __alloc_size in next-20221101, but in next-20221102 this commit seems to
> DTRT, namely
> 
> -extern void *kmemdup(const void *src, size_t len, gfp_t gfp);
> +extern void *kmemdup(const void *src, size_t len, gfp_t gfp)
> __realloc_size(2);
> 
> (i.e. there should never be an intermediate commit where kmemdup has
> __alloc_size()).

Right -- I fixed in in my -next tree to not use __alloc_size.

-- 
Kees Cook