[Nouveau] [PATCH] drm/nouveau/dispnv04: Fix a NULL pointer dereference in nv17_tv_get_ld_modes()

[Zhou Qingyang <zhou1615@umn.edu>]

In nv17_tv_get_ld_modes(), drm_mode_duplicate() is assigned to mode
and there is a dereference of it in nv17_tv_get_ld_modes(). which could
lead to a NULL pointer dereference on failure of
drm_mode_duplicate().

Fix this bug by adding a check of mode.

As the return value of .get_modes() can not be negative, this patch
does not directly return error code, but uses NV_INFO to report and
returns n.

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: f24342375927 ("drm/nouveau: Break some long lines in the TV-out code.")
Signed-off-by: Zhou Qingyang <zhou1615@umn.edu>
---
 drivers/gpu/drm/nouveau/dispnv04/tvnv17.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/gpu/drm/nouveau/dispnv04/tvnv17.c b/drivers/gpu/drm/nouveau/dispnv04/tvnv17.c
index 6fe103fd60e9..598180a403ab 100644
--- a/drivers/gpu/drm/nouveau/dispnv04/tvnv17.c
+++ b/drivers/gpu/drm/nouveau/dispnv04/tvnv17.c
@@ -208,6 +208,11 @@ static int nv17_tv_get_ld_modes(struct drm_encoder *encoder,
 		struct drm_display_mode *mode;

 		mode = drm_mode_duplicate(encoder->dev, tv_mode);
+		if (!mode) {
+			NV_INFO(nouveau_drm(encoder->dev),
+				"failure in drm_mode_duplicate\n");
+			return n;
+		}

 		mode->clock = tv_norm->tv_enc_mode.vrefresh *
 			mode->htotal / 1000 *
-- 
2.25.1