drm/displayid: add displayid_get_header() and check bounds better

Add a helper to get a pointer to struct displayid_header. To be
pedantic, add buffer overflow checks to not touch the base if that
itself would overflow.

Cc: Iaroslav Boliukin <[e-mail]>
Cc: Dmitry Osipenko <[e-mail]>
Signed-off-by: Jani Nikula <[e-mail]>
Tested-by: Dmitry Osipenko <[e-mail]>
Reviewed-by: Dmitry Osipenko <[e-mail]>
Signed-off-by: Dmitry Osipenko <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/4a0[e-mail]

drivers/gpu/drm/drm_displayid.c

···
 #include <drm/drm_edid.h>
 #include <drm/drm_print.h>
 
+static const struct displayid_header *
+displayid_get_header(const u8 *displayid, int length, int index)
+{
+	const struct displayid_header *base;
+
+	if (sizeof(*base) > length - index)
+		return ERR_PTR(-EINVAL);
+
+	base = (const struct displayid_header *)&displayid[index];
+
+	return base;
+}
+
 static int validate_displayid(const u8 *displayid, int length, int idx)
 {
 	int i, dispid_length;
 	u8 csum = 0;
 	const struct displayid_header *base;
 
-	base = (const struct displayid_header *)&displayid[idx];
+	base = displayid_get_header(displayid, length, idx);
+	if (IS_ERR(base))
+		return PTR_ERR(base);
 
 	DRM_DEBUG_KMS("base revision 0x%x, length %d, %d %d\n",
 		      base->rev, base->bytes, base->prod_id, base->ext_count);