linux-kernel - Re: [PATCH v2] wifi: iwlwifi: mld: Work around Clang loop unrolling bug

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Message-ID: <20250426000401.GB3584546@ax162>
Date: Fri, 25 Apr 2025 20:04:01 -0400
From: Nathan Chancellor <nathan@...nel.org>
To: Kees Cook <kees@...nel.org>
Cc: Miri Korenblit <miriam.rachel.korenblit@...el.com>,
	Johannes Berg <johannes.berg@...el.com>,
	Yedidya Benshimol <yedidya.ben.shimol@...el.com>,
	Emmanuel Grumbach <emmanuel.grumbach@...el.com>,
	Avraham Stern <avraham.stern@...el.com>,
	linux-wireless@...r.kernel.org,
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Benjamin Berg <benjamin.berg@...el.com>,
	Daniel Gabay <daniel.gabay@...el.com>, linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] wifi: iwlwifi: mld: Work around Clang loop unrolling
 bug

On Fri, Apr 25, 2025 at 11:44:22AM -0700, Kees Cook wrote:
> The nested loop in iwl_mld_send_proto_offload() confuses Clang into
> thinking there could be final loop iteration past the end of the "nsc"
> array (which is only 4 entries). The FORTIFY checking in memcmp()
> (via ipv6_addr_cmp()) notices this (due to the available bytes in the
> out-of-bounds position of &nsc[4] being 0), and errors out, failing
> the build. For some reason (likely due to architectural loop unrolling
> configurations), this is only exposed on ARM builds currently. Due to
> Clang's lack of inline tracking[1], the warning is not very helpful:
> 
> include/linux/fortify-string.h:719:4: error: call to '__read_overflow' declared with 'error' attribute: detected read beyond size of object (1st parameter)
>   719 |                         __read_overflow();
>       |                         ^
> 1 error generated.
> 
> But this was tracked down to iwl_mld_send_proto_offload()'s
> ipv6_addr_cmp() call.
> 
> An upstream Clang bug has been filed[2] to track this, but for now.
> Fix the build by explicitly bounding the inner loop by "n_nsc", which is
> what "c" is already limited to. Additionally do not repeat the ns_config
> and targ_addrs array sizes with their open-coded names since they can
> be determined at compile-time with ARRAY_SIZE().
> 
> Reported-by: Nathan Chancellor <nathan@...nel.org>
> Closes: https://github.com/ClangBuiltLinux/linux/issues/2076
> Link: https://github.com/llvm/llvm-project/pull/73552 [1]
> Link: https://github.com/llvm/llvm-project/issues/136603 [2]
> Signed-off-by: Kees Cook <kees@...nel.org>

Reviewed-by: Nathan Chancellor <nathan@...nel.org>

> ---
>  v2:
>   - move "j < n_nsc" forward to stabilize loop bounds (Nathan)
>   - use ARRAY_SIZE() for robustness
>  v1: https://lore.kernel.org/all/20250421204153.work.935-kees@kernel.org/
> Cc: Miri Korenblit <miriam.rachel.korenblit@...el.com>
> Cc: Johannes Berg <johannes.berg@...el.com>
> Cc: Yedidya Benshimol <yedidya.ben.shimol@...el.com>
> Cc: Emmanuel Grumbach <emmanuel.grumbach@...el.com>
> Cc: Avraham Stern <avraham.stern@...el.com>
> Cc: <linux-wireless@...r.kernel.org>
> ---
>  drivers/net/wireless/intel/iwlwifi/mld/d3.c | 14 +++++---------
>  1 file changed, 5 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/net/wireless/intel/iwlwifi/mld/d3.c b/drivers/net/wireless/intel/iwlwifi/mld/d3.c
> index dc736fdc176d..c51a6596617d 100644
> --- a/drivers/net/wireless/intel/iwlwifi/mld/d3.c
> +++ b/drivers/net/wireless/intel/iwlwifi/mld/d3.c
> @@ -1728,17 +1728,13 @@ iwl_mld_send_proto_offload(struct iwl_mld *mld,
>  #if IS_ENABLED(CONFIG_IPV6)
>  	struct iwl_mld_vif *mld_vif = iwl_mld_vif_from_mac80211(vif);
>  	struct iwl_mld_wowlan_data *wowlan_data = &mld_vif->wowlan_data;
> -	struct iwl_ns_config *nsc;
> -	struct iwl_targ_addr *addrs;
> -	int n_nsc, n_addrs;
> +	const int n_addrs = ARRAY_SIZE(cmd->targ_addrs);
> +	struct iwl_targ_addr *addrs = cmd->targ_addrs;
> +	const int n_nsc = ARRAY_SIZE(cmd->ns_config);
> +	struct iwl_ns_config *nsc = cmd->ns_config;
>  	int i, c;
>  	int num_skipped = 0;
>  
> -	nsc = cmd->ns_config;
> -	n_nsc = IWL_PROTO_OFFLOAD_NUM_NS_CONFIG_V3L;
> -	addrs = cmd->targ_addrs;
> -	n_addrs = IWL_PROTO_OFFLOAD_NUM_IPV6_ADDRS_V3L;
> -
>  	/* For each address we have (and that will fit) fill a target
>  	 * address struct and combine for NS offload structs with the
>  	 * solicited node addresses.
> @@ -1759,7 +1755,7 @@ iwl_mld_send_proto_offload(struct iwl_mld *mld,
>  
>  		addrconf_addr_solict_mult(&wowlan_data->target_ipv6_addrs[i],
>  					  &solicited_addr);
> -		for (j = 0; j < c; j++)
> +		for (j = 0; j < n_nsc && j < c; j++)
>  			if (ipv6_addr_cmp(&nsc[j].dest_ipv6_addr,
>  					  &solicited_addr) == 0)
>  				break;
> -- 
> 2.34.1
>