Subject: Re: [PATCH v2 2/2][next] wl3501_cs: Fix out-of-bounds warning in wl3501_mgmt_join

From: Kees Cook <[email-masked]>
Date: Wed, 7 Apr 2021 12:02:00 -0700

On Wed, Mar 31, 2021 at 04:45:34PM -0500, Gustavo A. R. Silva wrote:
> Fix the following out-of-bounds warning by enclosing
> some structure members into new struct req:
> 
> arch/x86/include/asm/string_32.h:182:25: warning: '__builtin_memcpy' offset [39, 108] from the object at 'sig' is out of the bounds of referenced subobject 'beacon_period' with type 'short unsigned int' at offset 36 [-Warray-bounds]
> 
> Refactor the code, accordingly:
> 
> $ pahole -C wl3501_join_req drivers/net/wireless/wl3501_cs.o
> struct wl3501_join_req {
> 	u16                        next_blk;             /*     0     2 */
> 	u8                         sig_id;               /*     2     1 */
> 	u8                         reserved;             /*     3     1 */
> 	struct iw_mgmt_data_rset   operational_rset;     /*     4    10 */
> 	u16                        reserved2;            /*    14     2 */
> 	u16                        timeout;              /*    16     2 */
> 	u16                        probe_delay;          /*    18     2 */
> 	u8                         timestamp[8];         /*    20     8 */
> 	u8                         local_time[8];        /*    28     8 */
> 	struct {
> 		u16                beacon_period;        /*    36     2 */
> 		u16                dtim_period;          /*    38     2 */
> 		u16                cap_info;             /*    40     2 */
> 		u8                 bss_type;             /*    42     1 */
> 		u8                 bssid[6];             /*    43     6 */
> 		struct iw_mgmt_essid_pset ssid;          /*    49    34 */
> 		/* --- cacheline 1 boundary (64 bytes) was 19 bytes ago --- */
> 		struct iw_mgmt_ds_pset ds_pset;          /*    83     3 */
> 		struct iw_mgmt_cf_pset cf_pset;          /*    86     8 */
> 		struct iw_mgmt_ibss_pset ibss_pset;      /*    94     4 */
> 		struct iw_mgmt_data_rset bss_basic_rset; /*    98    10 */
> 	} req;                                           /*    36    72 */

This section is the same as a large portion of struct wl3501_scan_confirm:

struct wl3501_scan_confirm {
        u16                         next_blk;
        u8                          sig_id;
        u8                          reserved;
        u16                         status;
        char                        timestamp[8];
        char                        localtime[8];

from here
        u16                         beacon_period;
        u16                         dtim_period;
        u16                         cap_info;
        u8                          bss_type;
        u8                          bssid[ETH_ALEN];
        struct iw_mgmt_essid_pset   ssid;
        struct iw_mgmt_ds_pset      ds_pset;
        struct iw_mgmt_cf_pset      cf_pset;
        struct iw_mgmt_ibss_pset    ibss_pset;
        struct iw_mgmt_data_rset    bss_basic_rset;
through here

        u8                          rssi;
};

It seems like maybe extracting that and using it in both structures
would make more sense?

> 
> 	/* size: 108, cachelines: 2, members: 10 */
> 	/* last cacheline: 44 bytes */
> };
> 
> The problem is that the original code is trying to copy data into a
> bunch of struct members adjacent to each other in a single call to
> memcpy(). Now that a new struct _req_ enclosing all those adjacent
> members is introduced, memcpy() doesn't overrun the length of
> &sig.beacon_period, because the address of the new struct object
> _req_ is used as the destination, instead.
> 
> Also, this helps with the ongoing efforts to enable -Warray-bounds and
> avoid confusing the compiler.
> 
> Link: https://github.com/KSPP/linux/issues/109
> Reported-by: kernel test robot <[email-masked]>
> Build-tested-by: kernel test robot <[email-masked]>
> Link: https://lore.kernel.org/lkml/[email-masked]/
> Signed-off-by: Gustavo A. R. Silva <[email-masked]>
> ---
> Changes in v2:
>  - None.
> 
>  drivers/net/wireless/wl3501.h    | 22 ++++++++++++----------
>  drivers/net/wireless/wl3501_cs.c |  4 ++--
>  2 files changed, 14 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/net/wireless/wl3501.h b/drivers/net/wireless/wl3501.h
> index ef9d605d8c88..774d8cac046d 100644
> --- a/drivers/net/wireless/wl3501.h
> +++ b/drivers/net/wireless/wl3501.h
> @@ -389,16 +389,18 @@ struct wl3501_join_req {
>  	u16			    probe_delay;
>  	u8			    timestamp[8];
>  	u8			    local_time[8];
> -	u16			    beacon_period;
> -	u16			    dtim_period;
> -	u16			    cap_info;
> -	u8			    bss_type;
> -	u8			    bssid[ETH_ALEN];
> -	struct iw_mgmt_essid_pset   ssid;
> -	struct iw_mgmt_ds_pset	    ds_pset;
> -	struct iw_mgmt_cf_pset	    cf_pset;
> -	struct iw_mgmt_ibss_pset    ibss_pset;
> -	struct iw_mgmt_data_rset    bss_basic_rset;
> +	struct {
> +		u16			    beacon_period;
> +		u16			    dtim_period;
> +		u16			    cap_info;
> +		u8			    bss_type;
> +		u8			    bssid[ETH_ALEN];
> +		struct iw_mgmt_essid_pset   ssid;
> +		struct iw_mgmt_ds_pset	    ds_pset;
> +		struct iw_mgmt_cf_pset	    cf_pset;
> +		struct iw_mgmt_ibss_pset    ibss_pset;
> +		struct iw_mgmt_data_rset    bss_basic_rset;
> +	} req;
>  };
>  
>  struct wl3501_join_confirm {
> diff --git a/drivers/net/wireless/wl3501_cs.c b/drivers/net/wireless/wl3501_cs.c
> index e149ef81d6cc..399d3bd2ae76 100644
> --- a/drivers/net/wireless/wl3501_cs.c
> +++ b/drivers/net/wireless/wl3501_cs.c
> @@ -590,7 +590,7 @@ static int wl3501_mgmt_join(struct wl3501_card *this, u16 stas)
>  	struct wl3501_join_req sig = {
>  		.sig_id		  = WL3501_SIG_JOIN_REQ,
>  		.timeout	  = 10,
> -		.ds_pset = {
> +		.req.ds_pset = {
>  			.el = {
>  				.id  = IW_MGMT_INFO_ELEMENT_DS_PARAMETER_SET,
>  				.len = 1,
> @@ -599,7 +599,7 @@ static int wl3501_mgmt_join(struct wl3501_card *this, u16 stas)
>  		},
>  	};
>  
> -	memcpy(&sig.beacon_period, &this->bss_set[stas].beacon_period, 72);
> +	memcpy(&sig.req, &this->bss_set[stas].beacon_period, sizeof(sig.req));

If not, then probably something like this should be added to make sure
nothing unexpected happens to change structure sizes:

BUILD_BUG_ON(sizeof(sig.req) != 72);

>  	return wl3501_esbq_exec(this, &sig, sizeof(sig));
>  }
>  
> -- 
> 2.27.0
> 

-- 
Kees Cook