drm/nouveau/kms/nv50-: atom: fix an incorrect NULL check on list iterator

commit 6ce4431c7ba7954c4fa6a96ce16ca1b2943e1a83 upstream.

The bug is here:
	return encoder;

The list iterator value 'encoder' will *always* be set and non-NULL
by drm_for_each_encoder_mask(), so it is incorrect to assume that the
iterator value will be NULL if the list is empty or no element found.
Otherwise it will bypass some NULL checks and lead to invalid memory
access passing the check.

To fix this bug, just return 'encoder' when found, otherwise return
NULL.

Cc: stable@vger.kernel.org
Fixes: 12885ecb ("drm/nouveau/kms/nvd9-: Add CRC support")
Signed-off-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
[Changed commit title]
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220327073925.11121-1-xiam0nd.tong@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/gpu/drm/nouveau/dispnv50/atom.h

@@ -160,14 +160,14 @@ nv50_head_atom_get(struct drm_atomic_state *state, struct drm_crtc *crtc)
 static inline struct drm_encoder *
 nv50_head_atom_get_encoder(struct nv50_head_atom *atom)
 {
-	struct drm_encoder *encoder = NULL;
+	struct drm_encoder *encoder;
 
 	/* We only ever have a single encoder */
 	drm_for_each_encoder_mask(encoder, atom->state.crtc->dev,
 				  atom->state.encoder_mask)
-		break;
+		return encoder;
 
-	return encoder;
+	return NULL;
 }
 
 #define nv50_wndw_atom(p) container_of((p), struct nv50_wndw_atom, state)

drivers/gpu/drm/nouveau/dispnv50/crc.c

@@ -411,9 +411,18 @@ void nv50_crc_atomic_check_outp(struct nv50_atom *atom)
 		struct nv50_head_atom *armh = nv50_head_atom(old_crtc_state);
 		struct nv50_head_atom *asyh = nv50_head_atom(new_crtc_state);
 		struct nv50_outp_atom *outp_atom;
-		struct nouveau_encoder *outp =
-			nv50_real_outp(nv50_head_atom_get_encoder(armh));
-		struct drm_encoder *encoder = &outp->base.base;
+		struct nouveau_encoder *outp;
+		struct drm_encoder *encoder, *enc;
+
+		enc = nv50_head_atom_get_encoder(armh);
+		if (!enc)
+			continue;
+
+		outp = nv50_real_outp(enc);
+		if (!outp)
+			continue;
+
+		encoder = &outp->base.base;
 
 		if (!asyh->clr.crc)
 			continue;
@@ -464,8 +473,16 @@ void nv50_crc_atomic_set(struct nv50_head *head,
 	struct drm_device *dev = crtc->dev;
 	struct nv50_crc *crc = &head->crc;
 	const struct nv50_crc_func *func = nv50_disp(dev)->core->func->crc;
-	struct nouveau_encoder *outp =
-		nv50_real_outp(nv50_head_atom_get_encoder(asyh));
+	struct nouveau_encoder *outp;
+	struct drm_encoder *encoder;
+
+	encoder = nv50_head_atom_get_encoder(asyh);
+	if (!encoder)
+		return;
+
+	outp = nv50_real_outp(encoder);
+	if (!outp)
+		return;
 
 	func->set_src(head, outp->or,
 		      nv50_crc_source_type(outp, asyh->crc.src),