drm/vesadrm: Avoid NULL-ptr deref in vesadrm_pmi_cmap_write()

Only set PMI fields if the screen_info's Vesa PM segment has been
set. Vesa PMI is the power-management interface. It also provides
means to set the color palette. The interface is optional, so not
all VESA graphics cards support it. Print vesafb's warning [1] if
the hardware palette cannot be set at all.

If unsupported the field PrimaryPalette in struct vesadrm.pmi is
NULL, which results in a segmentation fault. Happens with qemu's
Cirrus emulation.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 814d270b31d2 ("drm/sysfb: vesadrm: Add gamma correction")
Link: https://elixir.bootlin.com/linux/v6.15/source/drivers/video/fbdev/vesafb.c#L375
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20250617140944.142392-1-tzimmermann@suse.de

diff --git a/drivers/gpu/drm/sysfb/vesadrm.c b/drivers/gpu/drm/sysfb/vesadrm.c
index 4d62c78e7d1e0b04223d592aa2eedd833880d67d..f7532db3831fe3eb8b48d1a121abb834d12da281 100644 (file)
--- a/drivers/gpu/drm/sysfb/vesadrm.c
+++ b/drivers/gpu/drm/sysfb/vesadrm.c
@@ -362,14 +362,19 @@ static struct vesadrm_device *vesadrm_device_create(struct drm_driver *drv,
 
        if (!__screen_info_vbe_mode_nonvga(si)) {
                vesa->cmap_write = vesadrm_vga_cmap_write;
-#if defined(CONFIG_X86_32)
        } else {
+#if defined(CONFIG_X86_32)
                phys_addr_t pmi_base = __screen_info_vesapm_info_base(si);
-               const u16 *pmi_addr = phys_to_virt(pmi_base);
 
-               vesa->pmi.PrimaryPalette = (u8 *)pmi_addr + pmi_addr[2];
-               vesa->cmap_write = vesadrm_pmi_cmap_write;
+               if (pmi_base) {
+                       const u16 *pmi_addr = phys_to_virt(pmi_base);
+
+                       vesa->pmi.PrimaryPalette = (u8 *)pmi_addr + pmi_addr[2];
+                       vesa->cmap_write = vesadrm_pmi_cmap_write;
+               } else
 #endif
+               if (format->is_color_indexed)
+                       drm_warn(dev, "hardware palette is unchangeable, colors may be incorrect\n");
        }
 
 #ifdef CONFIG_X86