Subject: Re: [PATCH][next] wifi: mwifiex: Replace one-element array with flexible-array member

From: Kees Cook <[email-masked]>
Date: Fri, 3 Feb 2023 17:57:15 +0000

On Thu, Feb 02, 2023 at 07:34:05PM -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 array with flexible-array
> member in struct mwifiex_ie_types_rates_param_set.
> 
> These are the only binary differences I see after the change:
> 
> mwifiex.o
> _@@ -50154,7 +50154,7 @@
>                         23514: R_X86_64_32S     kmalloc_caches+0x50
>     23518:      call   2351d <mwifiex_scan_networks+0x11d>
>                         23519: R_X86_64_PLT32   __tsan_read8-0x4
> -   2351d:      mov    $0x225,%edx
> +   2351d:      mov    $0x224,%edx
>     23522:      mov    $0xdc0,%esi
>     23527:      mov    0x0(%rip),%rdi        # 2352e <mwifiex_scan_networks+0x12e>
>                         2352a: R_X86_64_PC32    kmalloc_caches+0x4c
> scan.o
> _@@ -5582,7 +5582,7 @@
>                         4394: R_X86_64_32S      kmalloc_caches+0x50
>      4398:      call   439d <mwifiex_scan_networks+0x11d>
>                         4399: R_X86_64_PLT32    __tsan_read8-0x4
> -    439d:      mov    $0x225,%edx
> +    439d:      mov    $0x224,%edx
>      43a2:      mov    $0xdc0,%esi
>      43a7:      mov    0x0(%rip),%rdi        # 43ae <mwifiex_scan_networks+0x12e>
>                         43aa: R_X86_64_PC32     kmalloc_caches+0x4c
> 
> and the reason for that is the following line:
> 
> drivers/net/wireless/marvell/mwifiex/scan.c:
> 1517         scan_cfg_out = kzalloc(sizeof(union mwifiex_scan_cmd_config_tlv),
> 1518                                GFP_KERNEL);
> 
> sizeof(union mwifiex_scan_cmd_config_tlv) is now one-byte smaller due to the
> flex-array transformation:
> 
>   46 union mwifiex_scan_cmd_config_tlv {
>   47         /* Scan configuration (variable length) */
>   48         struct mwifiex_scan_cmd_config config;
>   49         /* Max allocated block */
>   50         u8 config_alloc_buf[MAX_SCAN_CFG_ALLOC];
>   51 };

Interesting! So this looks like it's fixing a minor bug in the original
implementation which was allocation 1 byte too much.

> 
> Notice that MAX_SCAN_CFG_ALLOC is defined in terms of
> sizeof(struct mwifiex_ie_types_rates_param_set), see:
> 
>   26 /* Memory needed to store supported rate */
>   27 #define RATE_TLV_MAX_SIZE   (sizeof(struct mwifiex_ie_types_rates_param_set) \
>   28                                 + HOSTCMD_SUPPORTED_RATES)
> 
>   37 /* Maximum memory needed for a mwifiex_scan_cmd_config with all TLVs at max */
>   38 #define MAX_SCAN_CFG_ALLOC (sizeof(struct mwifiex_scan_cmd_config)        \
>   39                                 + sizeof(struct mwifiex_ie_types_num_probes)   \
>   40                                 + sizeof(struct mwifiex_ie_types_htcap)       \
>   41                                 + CHAN_TLV_MAX_SIZE                 \
>   42                                 + RATE_TLV_MAX_SIZE                 \
>   43                                 + WILDCARD_SSID_TLV_MAX_SIZE)

Yeah, the config_alloc_buf size appears to be very specifically
calculated, so this seems sane to me.

> 
> 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].
> 
> Link: https://github.com/KSPP/linux/issues/79
> Link: https://github.com/KSPP/linux/issues/252
> Link: https://gcc.gnu.org/pipermail/gcc-patches/2022-October/602902.html [1]
> Signed-off-by: Gustavo A. R. Silva <[email-masked]>

Reviewed-by: Kees Cook <[email-masked]>

-- 
Kees Cook