Subject: Re: [PATCH v5] net: caif: fix stack out-of-bounds write in cfctrl_link_setup()

From: Simon Horman <[email-masked]>
Date: Sun, 12 Apr 2026 14:57:43 +0100

On Wed, Apr 08, 2026 at 12:53:33PM +0000, Kangzheng Gu wrote:
> cfctrl_link_setup() copies the RFM volume name from a received control
> packet into linkparam.u.rfm.volume until a '\0' is found. A malformed
> packet can omit the terminator and make the copy run past the 20-byte
> stack buffer.
> 
> Stop copying once the buffer is full and mark the frame as failed by
> setting CFCTRL_ERR_BIT so the link setup is rejected.
> 
> Fixes: b482cd2053e3 ("net-caif: add CAIF core protocol stack")
> Cc: [email-masked]
> Signed-off-by: Kangzheng Gu <[email-masked]>
> ---
>  v5:
>  - remove the Reported-by.
>  - print a warn message and reject link setup by setting CFCTRL_ERR_BIT.
>  - using %zu to adapt the compilation of 32-bit kernel.
>  - add rate limit to error message
> 
>  net/caif/cfctrl.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/net/caif/cfctrl.c b/net/caif/cfctrl.c
> index c6cc2bfed65d..373ab1dc67a7 100644
> --- a/net/caif/cfctrl.c
> +++ b/net/caif/cfctrl.c
> @@ -416,8 +416,16 @@ static int cfctrl_link_setup(struct cfctrl *cfctrl, struct cfpkt *pkt, u8 cmdrsp
>  		cp = (u8 *) linkparam.u.rfm.volume;
>  		for (tmp = cfpkt_extr_head_u8(pkt);
>  		     cfpkt_more(pkt) && tmp != '\0';
> -		     tmp = cfpkt_extr_head_u8(pkt))
> +		     tmp = cfpkt_extr_head_u8(pkt)) {
> +			if (cp >= (u8 *)linkparam.u.rfm.volume +
> +			    sizeof(linkparam.u.rfm.volume) - 1) {
> +				pr_warn_ratelimited("Request reject, volume name length exceeds %zu\n",
> +						    sizeof(linkparam.u.rfm.volume));
> +				cmdrsp |= CFCTRL_ERR_BIT;
> +				break;
> +			}
>  			*cp++ = tmp;
> +		}
>  		*cp = '\0';
>  
>  		if (CFCTRL_ERR_BIT & cmdrsp)

I am wondering if it would be best to follow the pattern for
writing linkparam.u.utility.name elsewhere in this function.
That:
1. Uses a somewhat more succinct loop control structure
2. Silently truncates input without updating cmdrsp if overrun would occur

Something like this (compile tested only!):

diff --git a/net/caif/cfctrl.c b/net/caif/cfctrl.c
index c6cc2bfed65d..ba184c11386e 100644
--- a/net/caif/cfctrl.c
+++ b/net/caif/cfctrl.c
@@ -15,6 +15,7 @@
 #include <net/caif/cfctrl.h>
 
 #define container_obj(layr) container_of(layr, struct cfctrl, serv.layer)
+#define RFM_VOLUME_LEN 20
 #define UTILITY_NAME_LENGTH 16
 #define CFPKT_CTRL_PKT_LEN 20
 
@@ -414,10 +415,11 @@ static int cfctrl_link_setup(struct cfctrl *cfctrl, struct cfpkt *pkt, u8 cmdrsp
 		 */
 		linkparam.u.rfm.connid = cfpkt_extr_head_u32(pkt);
 		cp = (u8 *) linkparam.u.rfm.volume;
-		for (tmp = cfpkt_extr_head_u8(pkt);
-		     cfpkt_more(pkt) && tmp != '\0';
-		     tmp = cfpkt_extr_head_u8(pkt))
+		caif_assert(sizeof(linkparam.u.rfm.volume) >= RFM_VOLUME_LEN);
+		for(i = 0; i < RFM_VOLUME_LEN - 1 && cfpkt_more(pkt); i++) {
+			tmp = cfpkt_extr_head_u8(pkt);
 			*cp++ = tmp;
+		}
 		*cp = '\0';
 
 		if (CFCTRL_ERR_BIT & cmdrsp)

Also, it seems that writing linkparam.u.utility.paramlen elsewhere
in this function also has a potential buffer overrun (by one byte).