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

[Trond Myklebust <trond.myklebust@fys.uio.no>]

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@gmail.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