Re: [Linux-kernel-mentees] [PATCH] net: rose: Fix Null pointer dereference in rose_send_frame()

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Oct 15, 2020 at 05:47:12AM +0530, Anmol Karn wrote:
> In rose_send_frame(), when comparing two ax.25 addresses, it assigns rose_call to 
> either global ROSE callsign or default port, but when the former block triggers and 
> rose_call is assigned by (ax25_address *)neigh->dev->dev_addr, a NULL pointer is 
> dereferenced by 'neigh' when dereferencing 'dev'.
> 
> - net/rose/rose_link.c
> This bug seems to get triggered in this line:
> 
> rose_call = (ax25_address *)neigh->dev->dev_addr;
> 
> Prevent it by checking NULL condition for neigh->dev before comparing addressed for 
> rose_call initialization.
> 
> Reported-by: syzbot+a1c743815982d9496393@syzkaller.appspotmail.com 
> Link: https://syzkaller.appspot.com/bug?id=9d2a7ca8c7f2e4b682c97578dfa3f236258300b3 
> Signed-off-by: Anmol Karn <anmol.karan123@gmail.com>
> ---
> I am bit sceptical about the error return code, please suggest if anything else is 
> appropriate in place of '-ENODEV'.
> 
>  net/rose/rose_link.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/net/rose/rose_link.c b/net/rose/rose_link.c
> index f6102e6f5161..92ea6a31d575 100644
> --- a/net/rose/rose_link.c
> +++ b/net/rose/rose_link.c
> @@ -97,6 +97,9 @@ static int rose_send_frame(struct sk_buff *skb, struct rose_neigh *neigh)
>  	ax25_address *rose_call;
>  	ax25_cb *ax25s;
>  
> +	if (!neigh->dev)
> +		return -ENODEV;

How can ->dev not be set at this point in time?  Shouldn't that be
fixed, because it could change right after you check this, right?

thanks,

greg k-h