linux-hardening - Re: [PATCH][next] wifi: mwifiex: Replace one-element arrays with flexible-array members

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Message-ID: <93b3f91a-baa4-48e1-b3eb-01f738fa8fc1@embeddedor.com>
Date: Wed, 21 Aug 2024 14:59:34 -0600
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Amitkumar Karwar <amitkarwar@...il.com>,
 Ganapathi Bhat <ganapathi017@...il.com>,
 Sharvari Harisangam <sharvari.harisangam@....com>,
 Xinming Hu <huxinming820@...il.com>, Kalle Valo <kvalo@...nel.org>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] wifi: mwifiex: Replace one-element arrays with
 flexible-array members



On 21/08/24 14:26, Andy Shevchenko wrote:
> On Thu, Feb 02, 2023 at 07:32:00PM -0600, Gustavo A. R. Silva wrote:
>> One-element arrays are deprecated, and we are replacing them with flexible
>> array members instead. So, replace one-element arrays with flexible-array
>> members in multiple structures.
>>
>> This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
>> routines on memcpy() and help us make progress towards globally
>> enabling -fstrict-flex-arrays=3 [1].
>>
>> This results in no differences in binary output.
> 
> Sorry for blast from the past, but I have a question here.
> 
> This change seems converts many of the flexible arrays in this driver.
> But what's behind this one?
> 
> struct host_cmd_ds_802_11_scan_ext {
>          u32   reserved;
>          u8    tlv_buffer[1];
> } __packed;
> 
> 
> AFAIU this needs also some care. On the real machine I have got this
> 
> elo 16 17:51:58 surfacebook kernel: ------------[ cut here ]------------
> elo 16 17:51:58 surfacebook kernel: memcpy: detected field-spanning write (size 243) of single field "ext_scan->tlv_buffer" at drivers/net/wireless/marvell/mwifiex/scan.c:2239 (size 1)
> elo 16 17:51:58 surfacebook kernel: WARNING: CPU: 0 PID: 498 at drivers/net/wireless/marvell/mwifiex/scan.c:2239 mwifiex_cmd_802_11_scan_ext+0x83/0x90 [mwifiex]
> 
> which leads to
> 
>          memcpy(ext_scan->tlv_buffer, scan_cfg->tlv_buf, scan_cfg->tlv_buf_len);
> 
> but the code allocates 2k or more for the command buffer, so this seems
> quite enough for 243 bytes.
> 

I think this would do it:

diff --git a/drivers/net/wireless/marvell/mwifiex/fw.h b/drivers/net/wireless/marvell/mwifiex/fw.h
index e91def0afa14..d03129d5d24e 100644
--- a/drivers/net/wireless/marvell/mwifiex/fw.h
+++ b/drivers/net/wireless/marvell/mwifiex/fw.h
@@ -1627,7 +1627,7 @@ struct host_cmd_ds_802_11_scan_rsp {

  struct host_cmd_ds_802_11_scan_ext {
         u32   reserved;
-       u8    tlv_buffer[1];
+       u8    tlv_buffer[];
  } __packed;

  struct mwifiex_ie_types_bss_mode {
diff --git a/drivers/net/wireless/marvell/mwifiex/scan.c b/drivers/net/wireless/marvell/mwifiex/scan.c
index e782d652cb93..f7153472e2a2 100644
--- a/drivers/net/wireless/marvell/mwifiex/scan.c
+++ b/drivers/net/wireless/marvell/mwifiex/scan.c
@@ -2536,8 +2536,7 @@ int mwifiex_ret_802_11_scan_ext(struct mwifiex_private *priv,
         ext_scan_resp = &resp->params.ext_scan;

         tlv = (void *)ext_scan_resp->tlv_buffer;
-       buf_left = le16_to_cpu(resp->size) - (sizeof(*ext_scan_resp) + S_DS_GEN
-                                             - 1);
+       buf_left = le16_to_cpu(resp->size) - (sizeof(*ext_scan_resp) + S_DS_GEN);

         while (buf_left >= sizeof(struct mwifiex_ie_types_header)) {
                 type = le16_to_cpu(tlv->type);

--
Gustavo