Subject: Re: [Linux-kernel-mentees] [PATCH net] AX.25: Fix out-of-bounds read in ax25_connect()

From: Dan Carpenter <[email-masked]>
Date: Thu, 23 Jul 2020 17:28:15 +0300

On Wed, Jul 22, 2020 at 11:19:01AM -0400, Peilin Ye wrote:
> Checks on `addr_len` and `fsa->fsa_ax25.sax25_ndigis` are insufficient.
> ax25_connect() can go out of bounds when `fsa->fsa_ax25.sax25_ndigis`
> equals to 7 or 8. Fix it.
> 
> This issue has been reported as a KMSAN uninit-value bug, because in such
> a case, ax25_connect() reaches into the uninitialized portion of the
> `struct sockaddr_storage` statically allocated in __sys_connect().
> 
> It is safe to remove `fsa->fsa_ax25.sax25_ndigis > AX25_MAX_DIGIS` because
> `addr_len` is guaranteed to be less than or equal to
> `sizeof(struct full_sockaddr_ax25)`.
> 
> Reported-by: [email-masked]
> Link: https://syzkaller.appspot.com/bug?id=55ef9d629f3b3d7d70b69558015b63b48d01af66
> Signed-off-by: Peilin Ye <[email-masked]>
> ---
>  net/ax25/af_ax25.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
> index fd91cd34f25e..ef5bf116157a 100644
> --- a/net/ax25/af_ax25.c
> +++ b/net/ax25/af_ax25.c
> @@ -1187,7 +1187,9 @@ static int __must_check ax25_connect(struct socket *sock,
>  	if (addr_len > sizeof(struct sockaddr_ax25) &&
>  	    fsa->fsa_ax25.sax25_ndigis != 0) {
>  		/* Valid number of digipeaters ? */
> -		if (fsa->fsa_ax25.sax25_ndigis < 1 || fsa->fsa_ax25.sax25_ndigis > AX25_MAX_DIGIS) {
> +		if (fsa->fsa_ax25.sax25_ndigis < 1 ||
> +		    addr_len < sizeof(struct sockaddr_ax25) +
> +		    sizeof(ax25_address) * fsa->fsa_ax25.sax25_ndigis) {

The "sizeof(ax25_address) * fsa->fsa_ax25.sax25_ndigis" can have an
integer overflow so you still need the
"fsa->fsa_ax25.sax25_ndigis > AX25_MAX_DIGIS" check.

regards,
dan carpenter