Subject: Re: [PATCH] net: 9p: fix possible memory leak in p9_check_errors()

From: Christian Schoenebeck <[email-masked]>
Date: Thu, 26 Oct 2023 15:18:16 +0200

On Thursday, October 26, 2023 1:53:55 PM CEST [email-masked] wrote:
> 
> Hangyu Hua wrote on Thu, Oct 26, 2023 at 05:23:51PM +0800:
> > When p9pdu_readf is called with "s?d" attribute, it allocates a pointer
> > that will store a string. But when p9pdu_readf() fails while handling "d"
> > then this pointer will not be freed in p9_check_errors.
> 
> Right, that sounds correct to me.
> 
> Out of curiosity how did you notice this? The leak shouldn't happen with
> any valid server.
> 
> This cannot break anything so I'll push this to -next tomorrow and
> submit to Linus next week
> 
> > Fixes: ca41bb3e21d7 ("[net/9p] Handle Zero Copy TREAD/RERROR case in !dotl case.")
> 
> This commit moves this code a bit, but the p9pdu_readf call predates
> it -- in this case the Fixes tag is probably not useful; this affects
> all maintained kernels.

Looks like it exists since introduction of p9_check_errors(), therefore:

Fixes: 51a87c552dfd ("9p: rework client code to use new protocol support functions")

> > Signed-off-by: Hangyu Hua <[email-masked]>
> > ---
> >  net/9p/client.c | 7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/net/9p/client.c b/net/9p/client.c
> > index 86bbc7147fc1..6c7cd765b714 100644
> > --- a/net/9p/client.c
> > +++ b/net/9p/client.c
> > @@ -540,12 +540,15 @@ static int p9_check_errors(struct p9_client *c, struct p9_req_t *req)
> >  		return 0;
> >  
> >  	if (!p9_is_proto_dotl(c)) {
> > -		char *ename;
> > +		char *ename = NULL;
> >  
> >  		err = p9pdu_readf(&req->rc, c->proto_version, "s?d",
> >  				  &ename, &ecode);
> > -		if (err)
> > +		if (err) {
> > +			if (ename != NULL)
> > +				kfree(ename);
> 
> Don't check for NULL before kfree - kfree does it.
> If that's the only remark you get I can fix it when applying the commit
> on my side.

With those two remarks addressed:

Reviewed-by: Christian Schoenebeck <[email-masked]>

> 
> 
> >  			goto out_err;
> > +		}
> >  
> >  		if (p9_is_proto_dotu(c) && ecode < 512)
> >  			err = -ecode;
> 
>