[PATCH] drm: nv04: Add check to avoid out of bounds access

[Mikhail Kobuk]

Output Resource (dcb->or) value is not guaranteed to be non-zero (i.e.
in drivers/gpu/drm/nouveau/nouveau_bios.c, in 'fabricate_dcb_encoder_table()'
'dcb->or' is assigned value '0' in call to 'fabricate_dcb_output()').

Add check to validate 'dcb->or' before it's used.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 2e5702aff395 ("drm/nouveau: fabricate DCB encoder table for iMac G4")
Signed-off-by: Mikhail Kobuk <m.ko...@ispras.ru>
---
 drivers/gpu/drm/nouveau/dispnv04/dac.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/nouveau/dispnv04/dac.c 
b/drivers/gpu/drm/nouveau/dispnv04/dac.c
index d6b8e0cce2ac..0c8d4fc95ff3 100644
--- a/drivers/gpu/drm/nouveau/dispnv04/dac.c
+++ b/drivers/gpu/drm/nouveau/dispnv04/dac.c
@@ -428,7 +428,7 @@ void nv04_dac_update_dacclk(struct drm_encoder *encoder, 
bool enable)
        struct drm_device *dev = encoder->dev;
        struct dcb_output *dcb = nouveau_encoder(encoder)->dcb;
 
-       if (nv_gf4_disp_arch(dev)) {
+       if (nv_gf4_disp_arch(dev) && ffs(dcb->or)) {
                uint32_t *dac_users = 
&nv04_display(dev)->dac_users[ffs(dcb->or) - 1];
                int dacclk_off = NV_PRAMDAC_DACCLK + 
nv04_dac_output_offset(encoder);
                uint32_t dacclk = NVReadRAMDAC(dev, 0, dacclk_off);
@@ -453,7 +453,7 @@ bool nv04_dac_in_use(struct drm_encoder *encoder)
        struct drm_device *dev = encoder->dev;
        struct dcb_output *dcb = nouveau_encoder(encoder)->dcb;
 
-       return nv_gf4_disp_arch(encoder->dev) &&
+       return nv_gf4_disp_arch(encoder->dev) && ffs(dcb->or) &&
                (nv04_display(dev)->dac_users[ffs(dcb->or) - 1] & ~(1 << 
dcb->index));
 }
 
-- 
2.44.0