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

[Hangyu Hua]

On 26/10/2023 19:53, asmadeus@xxxxxxxxxxxxx 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.

I just found that any attributes that require memory allocation are 
prone to errors when mixed with ordinary attributes.

> This cannot break anything so I'll push this to -next tomorrow and
> submit to Linus next week

Agreed.

> > 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.
>
> > Signed-off-by: Hangyu Hua <hbh25y@xxxxxxxxx>
> > ---
> >   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.

I get it. I will revise it based on your and Christian's comments and 
send a v2.

Thanks,
Hangyu

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