Re: dm-region-hash: fix a missing-check bug in __rh_alloc()

[Gen Zhang <blackgod016574@gmail.com>]

On Sun, May 26, 2019 at 09:49:14PM -0400, Mike Snitzer wrote:
> On Sun, May 26 2019 at  8:50pm -0400,
> Gen Zhang <blackgod016574@gmail.com> wrote:
> 
> > In function __rh_alloc(), the pointer nreg is allocated a memory space
> > via kmalloc(). And it is used in the following codes. However, when 
> > there is a memory allocation error, kmalloc() fails. Thus null pointer
> > dereference may happen. And it will cause the kernel to crash. Therefore,
> > we should check the return value and handle the error.
> > Further, in __rh_find(), we should also check the return value and
> > handle the error.
> > 
> > Signed-off-by: Gen Zhang <blackgod016574@gmail.com>
> > ---
> > diff --git a/drivers/md/dm-region-hash.c b/drivers/md/dm-region-hash.c
> > index 1f76045..2fa1641 100644
> > --- a/drivers/md/dm-region-hash.c
> > +++ b/drivers/md/dm-region-hash.c
> > @@ -290,8 +290,11 @@ static struct dm_region *__rh_alloc(struct dm_region_hash *rh, region_t region)
> >  	struct dm_region *reg, *nreg;
> >  
> >  	nreg = mempool_alloc(&rh->region_pool, GFP_ATOMIC);
> > -	if (unlikely(!nreg))
> > +	if (unlikely(!nreg)) {
> >  		nreg = kmalloc(sizeof(*nreg), GFP_NOIO | __GFP_NOFAIL);
> > +		if (!nreg)
> > +			return NULL;
> > +	}
> >  
> >  	nreg->state = rh->log->type->in_sync(rh->log, region, 1) ?
> >  		      DM_RH_CLEAN : DM_RH_NOSYNC;
> 
> This patch isn't needed.  __GFP_NOFAIL means the allocation won't fail.
> 
> And there are many other users of __GFP_NOFAIL that don't check for
> failure.  
> 
> Mike
Appreciate your reply, Mike.

Thanks
Gen