Date:   Tue, 28 Apr 2020 07:49:36 +0000
From:   Niklas Cassel <Niklas.Cassel@....com>
To:     Javier González <javier@...igon.com>
CC:     Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...com>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        Igor Konopko <igor.j.konopko@...el.com>,
        Matias Bjørling <mb@...htnvm.io>,
        Jens Axboe <axboe@...nel.dk>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] nvme: prevent double free in nvme_alloc_ns() error
 handling

On Tue, Apr 28, 2020 at 09:06:51AM +0200, Javier González wrote:
> CAUTION: This email originated from outside of Western Digital. Do not click on links or open attachments unless you recognize the sender and know that the content is safe.
> 
> 
> On 27.04.2020 18:22, Niklas Cassel wrote:
> > On Mon, Apr 27, 2020 at 08:03:11PM +0200, Javier González wrote:
> > > On 27.04.2020 14:34, Niklas Cassel wrote:
> > > > When jumping to the out_put_disk label, we will call put_disk(), which will
> > > > trigger a call to disk_release(), which calls blk_put_queue().
> > > >
> > > > Later in the cleanup code, we do blk_cleanup_queue(), which will also call
> > > > blk_put_queue().
> > > >
> > > > Putting the queue twice is incorrect, and will generate a KASAN splat.
> > > >
> > > > Set the disk->queue pointer to NULL, before calling put_disk(), so that the
> > > > first call to blk_put_queue() will not free the queue.
> > > >
> > > > The second call to blk_put_queue() uses another pointer to the same queue,
> > > > so this call will still free the queue.
> > > >
> > > > Fixes: 85136c010285 ("lightnvm: simplify geometry enumeration")
> > > > Signed-off-by: Niklas Cassel <niklas.cassel@....com>
> > > > ---
> > > > drivers/nvme/host/core.c | 2 ++
> > > > 1 file changed, 2 insertions(+)
> > > >
> > > > diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> > > > index 91c1bd659947..f2adea96b04c 100644
> > > > --- a/drivers/nvme/host/core.c
> > > > +++ b/drivers/nvme/host/core.c
> > > > @@ -3642,6 +3642,8 @@ static void nvme_alloc_ns(struct nvme_ctrl *ctrl, unsigned nsid)
> > > >
> > > >       return;
> > > >  out_put_disk:
> > > > +      /* prevent double queue cleanup */
> > > > +      ns->disk->queue = NULL;
> > > >       put_disk(ns->disk);
> > > >  out_unlink_ns:
> > > >       mutex_lock(&ctrl->subsys->lock);
> > > > --
> > > > 2.25.3
> > > >
> > > What about delaying the assignment of ns->disk?
> > > 
> > > diff --git i/drivers/nvme/host/core.c w/drivers/nvme/host/core.c
> > > index a4d8c90ee7cc..6da4a9ced945 100644
> > > --- i/drivers/nvme/host/core.c
> > > +++ w/drivers/nvme/host/core.c
> > > @@ -3541,7 +3541,6 @@ static int nvme_alloc_ns(struct nvme_ctrl *ctrl, unsigned nsid)
> > >         disk->queue = ns->queue;
> > >         disk->flags = flags;
> > >         memcpy(disk->disk_name, disk_name, DISK_NAME_LEN);
> > > -       ns->disk = disk;
> > > 
> > >         __nvme_revalidate_disk(disk, id);
> > > 
> > > @@ -3553,6 +3552,8 @@ static int nvme_alloc_ns(struct nvme_ctrl *ctrl, unsigned nsid)
> > >                 }
> > >         }
> > > 
> > > +       ns->disk = disk;
> > > +
> > 
> > Hello Javier!
> > 
> > 
> > The only case where we jump to the out_put_disk label, is if the
> > nvme_nvm_register() call failed.
> > 
> > In that case, we want to undo the alloc_disk_node() operation, i.e.,
> > decrease the refcount.
> > 
> > If we don't set "ns->disk = disk;" before the call to nvme_nvm_register(),
> > then, if register fails, and we jump to the put_disk(ns->disk) label,
> > ns->disk will be NULL, so the recount will not be decreased, so I assume
> > that this memory would then be a memory leak.
> > 
> > 
> > I think that the problem is that the block functions are a bit messy.
> > Most drivers seem to do blk_cleanup_queue() first and then do put_disk(),
> > but some drivers do it in the opposite way, so I think that we might have
> > some more use-after-free bugs in some of these drivers that do it in the
> > opposite way.
> > 
> 
> Hi Niklas,
> 
> Yes, the out_put_disk label was introduced at the same time as the
> LightNVM entry point. We can do a better job at separating the cleanup
> functions, but as far as I can see ns->disk is not used in the LightNVM
> initialization, so delaying the initialization should be ok. Part of
> this should be also changing the out_put_disk to put_disk(disk).

Hello Javier,


If I understand correctly, you are suggesting this:

--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -3616,7 +3616,6 @@ static void nvme_alloc_ns(struct nvme_ctrl *ctrl, unsigned nsid)
        disk->queue = ns->queue;
        disk->flags = flags;
        memcpy(disk->disk_name, disk_name, DISK_NAME_LEN);
-       ns->disk = disk;

        __nvme_revalidate_disk(disk, id);

@@ -3628,6 +3627,8 @@ static void nvme_alloc_ns(struct nvme_ctrl *ctrl, unsigned nsid)
                }
        }

+       ns->disk = disk;
+
        down_write(&ctrl->namespaces_rwsem);
        list_add_tail(&ns->list, &ctrl->namespaces);
        up_write(&ctrl->namespaces_rwsem);
@@ -3642,7 +3643,7 @@ static void nvme_alloc_ns(struct nvme_ctrl *ctrl, unsigned nsid)

        return;
  out_put_disk:
-       put_disk(ns->disk);
+       put_disk(disk);
  out_unlink_ns:
        mutex_lock(&ctrl->subsys->lock);
        list_del_rcu(&ns->siblings);


That would not solve the double free error when the registration fails,
since disk->queue is still set, so this call will still free the queue.

Later in the cleanup code blk_cleanup_queue(ns->queue);
is called, which will still try to free what is already freed.

To remove the double free by delaying assignments, we would need to
delay the assignment of disk->queue until after the LightNVM entry point,
but I don't think that is possible, since __nvme_revalidate_disk(), which
is called before the LightNVM entry point uses disk->queue.

Christoph said that he would clean up this mess with better block layer
APIs. So perhaps the fix that is already queued is good enough until then.


Kind regards,
Niklas

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