linux-kernel - Re: [RESEND PATCH] mux: Convert mux_control_ops to a flex array member in mux

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Message-ID: <202504071119.DB9497A510@keescook>
Date: Mon, 7 Apr 2025 11:20:44 -0700
From: Kees Cook <kees@...nel.org>
To: Thorsten Blum <thorsten.blum@...ux.dev>
Cc: Peter Rosin <peda@...ntia.se>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [RESEND PATCH] mux: Convert mux_control_ops to a flex array
 member in mux_chip

On Fri, Mar 07, 2025 at 12:32:07PM +0100, Thorsten Blum wrote:
> On 3. Mar 2025, at 19:44, Kees Cook wrote:
> > On Mon, Mar 03, 2025 at 12:02:22AM +0100, Thorsten Blum wrote:
> >> Convert mux_control_ops to a flexible array member at the end of the
> >> mux_chip struct and add the __counted_by() compiler attribute to
> >> improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
> >> CONFIG_FORTIFY_SOURCE.
> >> 
> >> Use struct_size() to calculate the number of bytes to allocate for a new
> >> mux chip and to remove the following Coccinelle/coccicheck warning:
> >> 
> >>  WARNING: Use struct_size
> >> 
> >> Use size_add() to safely add any extra bytes.
> >> 
> >> Compile-tested only.
> > 
> > I believe this will fail at runtime. Note that sizeof_priv follows the
> > allocation, so at the very least, you'd need to update:
> > 
> > static inline void *mux_chip_priv(struct mux_chip *mux_chip)
> > {
> >        return &mux_chip->mux[mux_chip->controllers];
> > }
> > 
> > to not use the mux array itself as a location reference because it will
> > be seen as out of bounds.
> > 
> > To deal with this, the location will need to be calculated using
> > mux_chip as the base, not mux_chip->mux as the base. For example, see
> > commit 838ae9f45c4e ("nouveau/gsp: Avoid addressing beyond end of rpc->entries")
> 
> Since this should work and is well-defined C code according to [1][2],
> could you give this patch another look or should I still change it and
> submit a v2?

I think C is wrong here, but it seems it will continue to accidentally
work. I personally would like a v3 that fixes this, but I leave it to
Peter who is the MUX maintainer...

-Kees

-- 
Kees Cook