linux-kernel - Re: [PATCH] ISDN: Avoid a potential NULL ptr deref in ippp

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Date:	Tue, 21 Nov 2006 23:53:45 +0100
From:	"Jesper Juhl" <jesper.juhl@...il.com>
To:	"David Rientjes" <rientjes@...washington.edu>
Cc:	linux-kernel@...r.kernel.org,
	"Michael Hipp" <Michael.Hipp@...dent.uni-tuebingen.de>,
	"Karsten Keil" <kkeil@...e.de>,
	"Kai Germaschewski" <kai.germaschewski@....de>,
	isdn4linux@...tserv.isdn4linux.de, starvik@...s.com,
	dev-etrax@...s.com
Subject: Re: [PATCH] ISDN: Avoid a potential NULL ptr deref in ippp

On 21/11/06, David Rientjes <rientjes@...washington.edu> wrote:
> On Tue, 21 Nov 2006, Jesper Juhl wrote:
>
> > Ok, I see your point. There may not be an actual bug here, but
> > couldn't it still be considered an improvement, given that with my
> > patch we'll  a) print a warning that we ran into a memory shortage
> > problem, and  b) we save a call to ipc->decompress() and some switch
> > logic in the failing case.   ???
> >
>
> No, because there is duplication of code.  This error condition is already
> addressed:
>
>         skb_out = dev_alloc_skb(is->mru + PPP_HDRLEN);
>         len = ipc->decompress(stat, skb, skb_out, &rsparm);
>         kfree_skb(skb);
>         if (len <= 0) {
>                 switch (len) {
>                 case DECOMP_ERROR:
>                         ...
>                         break;
>                 case DECOMP_FATALERROR:
>                         ...
>                         break;
>                 }
>                 kfree_skb(skb_out);
>                 return NULL;
>         }
>
> Since neither DECOMP_ERROR or DECOMP_FATALERROR represent a NULL return
> from ipc->decompress(), the switch clause is a no-op and skb_out is freed
> and NULL is returned.
>
> The only thing your patch addresses is moving this before the
> ipc->decompress() call and _duplicating_ both the skb free and the return
> code, as well as adding a warning.  The warning is unnecessary because OOM
> killer will be called soon anyway if this condition is ever reached so the
> fact that it was this allocation that could not be satisfied doesn't
> matter.  Likewise, we need the return code from ipc->decompress() to do
> the other error checking involved.
>

You are right. I concede your point.

-- 
Jesper Juhl <jesper.juhl@...il.com>
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