Re: [PATCH v4] net: mlx5: Add a missing check on idr_find, free buf

[Saeed Mahameed <saeedm@mellanox.com>]

On Tue, 2019-03-19 at 21:54 -0700, Eric Dumazet wrote:
> 
> On 03/19/2019 02:42 PM, Aditya Pakki wrote:
> > idr_find() can return a NULL value to 'flow' which is used without
> > a
> > check. The patch adds a check to avoid potential NULL pointer
> > dereference.
> > 
> > In case of mlx5_fpga_sbu_conn_sendmsg() failure, free buf allocated
> > using kzalloc.
> > 
> > Fixes: ab412e1dd7db ("net/mlx5: Accel, add TLS rx offload
> > routines")
> > ---
> > v3: Reorder buf allocations and flow check.
> > v2: failure to return in case of flow failure.
> > v1: Failed to free buf in case of flow failure.
> > 
> > Signed-off-by: Aditya Pakki <pakki001@umn.edu>
> > ---
> >  drivers/net/ethernet/mellanox/mlx5/core/fpga/tls.c | 14
> > +++++++++++---
> >  1 file changed, 11 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/net/ethernet/mellanox/mlx5/core/fpga/tls.c
> > b/drivers/net/ethernet/mellanox/mlx5/core/fpga/tls.c
> > index 5cf5f2a9d51f..8de64e88c670 100644
> > --- a/drivers/net/ethernet/mellanox/mlx5/core/fpga/tls.c
> > +++ b/drivers/net/ethernet/mellanox/mlx5/core/fpga/tls.c
> > @@ -217,15 +217,21 @@ int mlx5_fpga_tls_resync_rx(struct
> > mlx5_core_dev *mdev, u32 handle, u32 seq,
> >  	void *cmd;
> >  	int ret;
> >  
> > +	rcu_read_lock();
> > +	flow = idr_find(&mdev->fpga->tls->rx_idr, ntohl(handle));
> > +	rcu_read_unlock();
> 
> This looks suspect (even before your patch)
> 
> What prevents flow from disappearing after this rcu_read_lock() ?
> 
> IMO your patch might prevent a NULL deref, but not use-after-free.

That crossed my mind when i reviewed this patch but since this is an
old issue i just put a todo aside to handle it later

i think the author didn't want to deal with use-after-free here, but
only wanted to keep the data structure safe against add/remove
operations.

Anyway all we need here is
 
rcu_read_lock()
flow = idr_find(..)
mlx5_fpga_tls_flow_to_cmd(flow, cmd); 
rcu_read_unlock()

Will fix it up in a follow up patch,

Thank you Eric !!


> 
> > +
> > +	if (!flow) {
> > +		WARN_ONCE(1, "Received NULL pointer for handle\n");
> > +		return -EINVAL;
> > +	}
> > +
> >  	buf = kzalloc(size, GFP_ATOMIC);
> >  	if (!buf)
> >  		return -ENOMEM;
> >  
> >  	cmd = (buf + 1);
> >  
> > -	rcu_read_lock();
> > -	flow = idr_find(&mdev->fpga->tls->rx_idr, ntohl(handle));
> > -	rcu_read_unlock();
> >  	mlx5_fpga_tls_flow_to_cmd(flow, cmd);
> >  
> >  	MLX5_SET(tls_cmd, cmd, swid, ntohl(handle));
> > @@ -238,6 +244,8 @@ int mlx5_fpga_tls_resync_rx(struct
> > mlx5_core_dev *mdev, u32 handle, u32 seq,
> >  	buf->complete = mlx_tls_kfree_complete;
> >  
> >  	ret = mlx5_fpga_sbu_conn_sendmsg(mdev->fpga->tls->conn, buf);
> > +	if (ret < 0)
> > +		kfree(buf);
> >  
> >  	return ret;
> >  }
> >