Re: [Linux-kernel-mentees] [PATCH] hfs, hfsplus: Fix NULL pointer dereference in hfs_find_init()

[Peilin Ye <yepeilin.cs@gmail.com>]

On Wed, Aug 12, 2020 at 10:18:52AM +0200, Greg Kroah-Hartman wrote:
> On Wed, Aug 12, 2020 at 03:13:06AM -0400, Peilin Ye wrote:
> > On Wed, Aug 12, 2020 at 09:08:27AM +0200, Greg Kroah-Hartman wrote:
> > > On Wed, Aug 12, 2020 at 02:55:56AM -0400, Peilin Ye wrote:
> > > > Prevent hfs_find_init() from dereferencing `tree` as NULL.
> > > > 
> > > > Reported-and-tested-by: syzbot+7ca256d0da4af073b2e2@syzkaller.appspotmail.com
> > > > Signed-off-by: Peilin Ye <yepeilin.cs@gmail.com>
> > > > ---
> > > >  fs/hfs/bfind.c     | 3 +++
> > > >  fs/hfsplus/bfind.c | 3 +++
> > > >  2 files changed, 6 insertions(+)
> > > > 
> > > > diff --git a/fs/hfs/bfind.c b/fs/hfs/bfind.c
> > > > index 4af318fbda77..880b7ea2c0fc 100644
> > > > --- a/fs/hfs/bfind.c
> > > > +++ b/fs/hfs/bfind.c
> > > > @@ -16,6 +16,9 @@ int hfs_find_init(struct hfs_btree *tree, struct hfs_find_data *fd)
> > > >  {
> > > >  	void *ptr;
> > > >  
> > > > +	if (!tree)
> > > > +		return -EINVAL;
> > > > +
> > > >  	fd->tree = tree;
> > > >  	fd->bnode = NULL;
> > > >  	ptr = kmalloc(tree->max_key_len * 2 + 4, GFP_KERNEL);
> > > > diff --git a/fs/hfsplus/bfind.c b/fs/hfsplus/bfind.c
> > > > index ca2ba8c9f82e..85bef3e44d7a 100644
> > > > --- a/fs/hfsplus/bfind.c
> > > > +++ b/fs/hfsplus/bfind.c
> > > > @@ -16,6 +16,9 @@ int hfs_find_init(struct hfs_btree *tree, struct hfs_find_data *fd)
> > > >  {
> > > >  	void *ptr;
> > > >  
> > > > +	if (!tree)
> > > > +		return -EINVAL;
> > > > +
> > > 
> > > How can tree ever be NULL in these calls?  Shouldn't that be fixed as
> > > the root problem here?
> > 
> > I see, I will try to figure out what is going on with the reproducer.
> 
> That's good to figure out.  Note, your patch might be the correct thing
> to do, as that might be an allowed way to call the function.  But in
> looking at all the callers, they seem to think they have a valid pointer
> at the moment, so perhaps if this check is added, some other root
> problem is papered over to be only found later on?

That's right - Yesterday I noticed that this function has a number of
callers who don't check `tree` at all, so I thought maybe we just add
the check here...Turned out to be quite the opposite.

Thank you,
Peilin Ye
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