Subject: Re: [PATCH] net: sunrpc: svcauth_gss: use BUG_ON instead of if condition followed by BUG

From: Jeff Layton <[email-masked]>
Date: Wed, 25 Oct 2017 13:04:50 -0400

On Tue, 2017-10-24 at 15:12 -0500, Gustavo A. R. Silva wrote:
> Quoting "J. Bruce Fields" <[email-masked]>:
> 
> > On Tue, Oct 24, 2017 at 02:18:52PM -0400, Jeff Layton wrote:
> > > On Tue, 2017-10-24 at 13:53 -0400, J. Bruce Fields wrote:
> > > > On Tue, Oct 24, 2017 at 01:26:49PM -0400, Weston Andros Adamson wrote:
> > > > > Is there a reason to BUG() in these places? Couldn't we  
> > > 
> > > WARN_ON_ONCE and return an error?
> > > > 
> > > > I think the BUG() will just kill an nfsd thread that isn't holding any
> > > > interesting locks.
> > > > 
> > > 
> > > Not necessarily. If panic_on_oops is set (and it usually is in
> > > "production" setups), it'll crash the box there.
> > 
> > Maybe they're getting what they asked for?
> > 
> > > > The failures look unlikely.  (Except for that read_u32... return, I
> > > > wonder if we're missing a check there.)
> > > 
> > > Agreed, looks like you only hit an error if the read attempts to go out
> > > of bounds. In principle that shouldn't ever happen (and I haven't seen
> > > any reports of it).
> > > 
> > > Still...I agree with Dros that it's better to handle this without
> > > oopsing if we can. We can return an error from either of those
> > > functions. A sane error and a WARN_ONCE would be better here.
> > 
> > OK, OK, OK.
> > 
> > There are also some more BUGs that could use looking into if anyone
> > wants to.
> > 
> > --b.
> > 
> > commit eb754930662f
> > Author: J. Bruce Fields <[email-masked]>
> > Date:   Tue Oct 24 14:58:11 2017 -0400
> > 
> >     rpc: remove some BUG()s
> > 
> >     It would be kinder to WARN() and recover in several spots here instead
> >     of BUG()ing.
> > 
> >     Also, it looks like the read_u32_from_xdr_buf() call could actually
> >     fail, though it might require a broken (or malicious) client, so convert
> >     that to just an error return.
> > 
> >     Reported-by: Weston Andros Adamson <[email-masked]>
> >     Signed-off-by: J. Bruce Fields <[email-masked]>
> > 
> > diff --git a/net/sunrpc/auth_gss/svcauth_gss.c  
> > b/net/sunrpc/auth_gss/svcauth_gss.c
> > index 7b1ee5a0b03c..73165e9ca5bf 100644
> > --- a/net/sunrpc/auth_gss/svcauth_gss.c
> > +++ b/net/sunrpc/auth_gss/svcauth_gss.c
> > @@ -855,11 +855,13 @@ unwrap_integ_data(struct svc_rqst *rqstp,  
> > struct xdr_buf *buf, u32 seq, struct g
> >  		return stat;
> >  	if (integ_len > buf->len)
> >  		return stat;
> > -	if (xdr_buf_subsegment(buf, &integ_buf, 0, integ_len))
> > -		BUG();
> > +	if (xdr_buf_subsegment(buf, &integ_buf, 0, integ_len)) {
> > +		WARN_ON_ONCE(1);
> > +		return stat;
> > +	}
> >  	/* copy out mic... */
> >  	if (read_u32_from_xdr_buf(buf, integ_len, &mic.len))
> > -		BUG();
> > +		return stat;
> >  	if (mic.len > RPC_MAX_AUTH_SIZE)
> >  		return stat;
> >  	mic.data = kmalloc(mic.len, GFP_KERNEL);
> > @@ -1611,8 +1613,10 @@ svcauth_gss_wrap_resp_integ(struct svc_rqst *rqstp)
> >  	BUG_ON(integ_len % 4);
> >  	*p++ = htonl(integ_len);
> >  	*p++ = htonl(gc->gc_seq);
> > -	if (xdr_buf_subsegment(resbuf, &integ_buf, integ_offset, integ_len))
> > -		BUG();
> > +	if (xdr_buf_subsegment(resbuf, &integ_buf, integ_offset, integ_len)) {
> > +		WARN_ON_ONCE(1);
> > +		goto out_err;
> > +	}
> >  	if (resbuf->tail[0].iov_base == NULL) {
> >  		if (resbuf->head[0].iov_len + RPC_MAX_AUTH_SIZE > PAGE_SIZE)
> >  			goto out_err;
> 
> What about the following BUG() at net/sunrpc/svc_xprt.c:1058:
> 
> /*
>   * Remove a dead transport
>   */
> static void svc_delete_xprt(struct svc_xprt *xprt)
> {
>          struct svc_serv *serv = xprt->xpt_server;
>          struct svc_deferred_req *dr;
> 
>          /* Only do this once */
>          if (test_and_set_bit(XPT_DEAD, &xprt->xpt_flags))
>                  BUG();
> 
>          dprintk("svc: svc_delete_xprt(%p)\n", xprt);
>          xprt->xpt_ops->xpo_detach(xprt);
> 
>          spin_lock_bh(&serv->sv_lock);
>          list_del_init(&xprt->xpt_list);
>          WARN_ON_ONCE(!list_empty(&xprt->xpt_ready));
>          if (test_bit(XPT_TEMP, &xprt->xpt_flags))
>                  serv->sv_tmpcnt--;
>          spin_unlock_bh(&serv->sv_lock);
> 
>          while ((dr = svc_deferred_dequeue(xprt)) != NULL)
>                  kfree(dr);
> 
>          call_xpt_users(xprt);
>          svc_xprt_put(xprt);
> }
> 
> I'm suspicious about that comment above the _if_ condition: /* Only do  
> this once */
> 
> Would it help to replace that BUG with a WARN_ON_ONCE?
> 

Maybe. How safe is it to run all of that more than once after XPT_DEAD
is set? You'd probably just end up crashing at some later point down the
line anyway...

I suppose you could do a WARN() there and then return without actually
doing anything. That might let you limp along if you did hit this.
-- 
Jeff Layton <[email-masked]>