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

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Message-Id: <785391F0-C381-47FE-89E7-6265F7761208@linux.dev>
Date: Fri, 7 Mar 2025 12:32:07 +0100
From: Thorsten Blum <thorsten.blum@...ux.dev>
To: Kees Cook <kees@...nel.org>
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 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?

Thanks,
Thorsten

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119132
[2] https://github.com/llvm/llvm-project/issues/129951