Re: [PATCH] ublk: fix NULL pointer dereference in ublk_ctrl_set_size()

[Ming Lei <ming.lei@redhat.com>]

On Thu, Mar 05, 2026 at 02:31:46PM -0500, Mehul Rao wrote:
> ublk_ctrl_set_size() unconditionally dereferences ub->ub_disk via
> set_capacity_and_notify() without checking if it is NULL.
> 
> ub->ub_disk is NULL before UBLK_CMD_START_DEV completes (it is only
> assigned in ublk_ctrl_start_dev()) and after UBLK_CMD_STOP_DEV runs
> (ublk_detach_disk() sets it to NULL). Since the UBLK_CMD_UPDATE_SIZE
> handler performs no state validation, a user can trigger a NULL pointer
> dereference by sending UPDATE_SIZE to a device that has been added but
> not yet started, or one that has been stopped.
> 
> Fix this by checking ub->ub_disk under ub->mutex before dereferencing
> it, and returning -ENODEV if the disk is not available.
> 
> Fixes: 98b995660bff ("ublk: Add UBLK_U_CMD_UPDATE_SIZE")
> Cc: stable@vger.kernel.org
> Signed-off-by: Mehul Rao <mehulrao@gmail.com>

Looks fine given ublk_detach_disk() is called with ub->mutex grabbed:

Reviewed-by: Ming Lei <ming.lei@redhat.com>


Thanks,
Ming