[PATCH] drm/nouveau/dispnv04: Fix a NULL pointer dereference in nv17_tv_get_hd_modes()

[Zhou Qingyang]

In nv17_tv_get_hd_modes(), the return value of drm_mode_duplicate() is
assigned to mode and there is a dereference of it in
nv17_tv_get_hd_modes(), which could lead to a NULL pointer dereference
on failure of drm_mode_duplicate().

Fix this bug add a check of 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_NOUVEAU=m show no new warnings,
and our static analyzer no longer warns about this code.

Fixes: 6ee738610f41 ("drm/nouveau: Add DRM driver for NVIDIA GPUs")
Signed-off-by: Zhou Qingyang <zhou1615@xxxxxxx>
---
 drivers/gpu/drm/nouveau/dispnv04/tvnv17.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/gpu/drm/nouveau/dispnv04/tvnv17.c b/drivers/gpu/drm/nouveau/dispnv04/tvnv17.c
index be28e7bd7490..6fe103fd60e9 100644
--- a/drivers/gpu/drm/nouveau/dispnv04/tvnv17.c
+++ b/drivers/gpu/drm/nouveau/dispnv04/tvnv17.c
@@ -257,6 +257,9 @@ static int nv17_tv_get_hd_modes(struct drm_encoder *encoder,
 		if (modes[i].hdisplay == output_mode->hdisplay &&
 		    modes[i].vdisplay == output_mode->vdisplay) {
 			mode = drm_mode_duplicate(encoder->dev, output_mode);
+			if (!mode)
+				return -ENOMEM;
+
 			mode->type |= DRM_MODE_TYPE_PREFERRED;
 
 		} else {
-- 
2.25.1