Hey Zhou,
Thanks for submitting this patch.
On Tue, 30 Nov 2021 at 14:11, Zhou Qingyang <zhou1615@xxxxxxx> wrote:
>
> In cdns_mhdp_atomic_enable(), the return value of drm_mode_duplicate()
> is assigned to mhdp_state->current_mode and used in drm_mode_set_name().
> There is a dereference of it in drm_mode_set_name(), which could lead
> to a NULL pointer dereference on failure of drm_mode_duplicate().
>
> Fix this bug by adding a check of mhdp_state->current_mode.
>
> This bug was found by a static analyzer. The analysis employs
> differential checking to identify inconsistent security operations
> (e.g., checks or kfrees) between two code paths and confirms that the
> inconsistent operations are not recovered in the current function or
> the callers, so they constitute bugs.
>
> Note that, as a bug found by static analysis, it can be a false
> positive or hard to trigger. Multiple researchers have cross-reviewed
> the bug.
>
> Builds with CONFIG_DRM_CDNS_MHDP8546=m show no new warnings,
> and our static analyzer no longer warns about this code.
>
> Fixes: fb43aa0acdfd ("drm: bridge: Add support for Cadence MHDP8546 DPI/DP bridge")
> Signed-off-by: Zhou Qingyang <zhou1615@xxxxxxx>
> ---
> drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c b/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
> index 5530fbf64f1e..347fbecf76a4 100644
> --- a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
> +++ b/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
> @@ -2040,6 +2040,11 @@ static void cdns_mhdp_atomic_enable(struct drm_bridge *bridge,
> mhdp_state = to_cdns_mhdp_bridge_state(new_state);
>
> mhdp_state->current_mode = drm_mode_duplicate(bridge->dev, mode);
> + if (!mhdp_state->current_mode) {
> + ret = -ENOMEM;
> + goto out;
> + }
> +
This appears to be a problem that is handled in other drivers, but the
solution here does strike me as good.
The out-label will schedule modeset_retry_work to be executed if
ret==-ENOMEM. If drm_mode_duplicate() fails, we've had a memory
allocation issue, and failing is probably the correct solution here.
However cdns_mhdp_atomic_enable() does allow for signalling failures.
> drm_mode_set_name(mhdp_state->current_mode);
>
> dev_dbg(mhdp->dev, "%s: Enabling mode %s\n", __func__, mode->name);
> --
> 2.25.1
>
