linux-kernel - Re: [patch 1/3] NFS: fix potential NULL pointer dereference

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Date:	Thu, 17 Apr 2008 00:19:36 +0400
From:	"Cyrill Gorcunov" <gorcunov@...il.com>
To:	"Trond Myklebust" <trond.myklebust@....uio.no>
Cc:	bfields@...ldses.org, neilb@...e.de, ibm-acpi@....eng.br,
	len.brown@...el.com, kkeil@...e.de, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 1/3] NFS: fix potential NULL pointer dereference

Trond, I've just pointed the problem and its solution (which is seems
to be a bit ugly, according to the rest nfs coding principle). So if
you prefer to have such a check in 'walk_path' function - just say me
that. You choose :) Thanks for comments

On 4/16/08, Trond Myklebust <trond.myklebust@....uio.no> wrote:
>
> On Wed, 2008-04-16 at 22:13 +0400, Cyrill Gorcunov wrote:
> > [Trond Myklebust - Wed, Apr 16, 2008 at 02:11:31PM -0400]
> > |
> > | On Wed, 2008-04-16 at 21:44 +0400, Cyrill Gorcunov wrote:
> > | > plain text document attachment (nfs-kstrdup-nul-fix)
> > | > It's possible to get NULL pointer dereference
> > | > if kstrndup failed
> > | >
> > | > Here is a possible scenario
> > | >
> > | > nfs4_get_sb
> > | >   nfs4_validate_mount_data
> > | >     o kstrndup failed so args->nfs_server.export_path = NULL
> > | >   nfs4_create_server
> > | >     nfs4_path_walk(..., NULL) -> Oops!
> > | >
> > | > Signed-off-by: Cyrill Gorcunov <gorcunov@...il.com>
> > |
> > | Why fix only the one case? What about the other kstrdup/kstrndup cases
> > | in super.c that appear to be unchecked?
> > |
> > | Trond
> > |
> > | > ---
> > | >
> > | > Index: linux-2.6.git/fs/nfs/super.c
> > | > ===================================================================
> > | > --- linux-2.6.git.orig/fs/nfs/super.c	2008-04-15 23:01:30.000000000
> +0400
> > | > +++ linux-2.6.git/fs/nfs/super.c	2008-04-16 20:01:44.000000000 +0400
> > | > @@ -1858,6 +1858,8 @@ static int nfs4_validate_mount_data(void
> > | >  		if (len > NFS4_MAXPATHLEN)
> > | >  			return -ENAMETOOLONG;
> > | >  		args->nfs_server.export_path = kstrndup(c, len, GFP_KERNEL);
> > | > +		if (!args->nfs_server.export_path)
> > | > +			return -ENOMEM;
> > | >
> > | >  		dprintk("NFS: MNTPATH: '%s'\n", args->nfs_server.export_path);
> > | >
> > |
> >
> > This one is leading to NULL deref, others - don't
>
> So? The defensive coding principle is that you perform validity checks
> when the pointer is created. Otherwise, we could equally well have added
> the NULL deref check to nfs4_path_walk()...
>
>   Trond
>
>
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