linux-kernel - Re: [PATCH] mm: Handle ksize() vs __alloc

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Date:   Mon, 28 Feb 2022 15:16:45 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     llvm@...ts.linux.dev, Marco Elver <elver@...gle.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        stable@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Daniel Micay <danielmicay@...il.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] mm: Handle ksize() vs __alloc_size by forgetting size

On Fri, Feb 25, 2022 at 03:45:18PM -0800, Andrew Morton wrote:
> On Fri, 25 Feb 2022 14:16:25 -0800 Kees Cook <keescook@...omium.org> wrote:
> 
> > If ksize() is used on an allocation, the compiler cannot make any
> > assumptions about its size any more (as hinted by __alloc_size). Force
> > it to forget.
> > 
> > One caller was using a container_of() construction that needed to be
> > worked around.
> 
> Please, when fixing something do fully explain what that thing is.  I,
> for one, simply cannot understand why this change is being proposed.
> 
> Especially when proposing a -stable backport!  Tell readers what was
> the end-user impact of the bug.
> 
> > Link: https://github.com/ClangBuiltLinux/linux/issues/1599
> 
> Even that didn't tell me.  Is it just a clang warning?  Does the kernel
> post your private keys on reddit then scribble all over your disk
> drive?  I dunno.

Yup, sorry. I tend to get so deep changes like this that I forget to
give an appropriately detailed summary. As others have mentioned, this
is trying to fix a miscompilation issue, triggered by what can be
considered either a mis-application of __alloc_size, or a failure to
correctly disable compiler optimizations in the face of ksize().

-- 
Kees Cook