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

[Danilo Krummrich]

On 4/5/24 22:05, Lyude Paul wrote:

On Fri, 2024-04-05 at 17:53 +0200, Danilo Krummrich wrote:

On 3/31/24 08:45, Mikhail Kobuk wrote:

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()').

I don't really know much about the semantics of this code.

Looking at fabricate_dcb_output() though I wonder if the intention
was to assign
BIT(or) to entry->or.

@Lyude, can you help here?

This code is definitely a bit before my time as well - but I think
you're completely correct. Especially considering this bit I found in
nouveau_bios.h:

Thanks for confirming.

@Mikhail, I think we should rather fix this assignment then.

- Danilo

enum nouveau_or {
        DCB_OUTPUT_A = (1 << 0),
        DCB_OUTPUT_B = (1 << 1),
        DCB_OUTPUT_C = (1 << 2)
};

Otherwise, for parsing the DCB entries, it seems that the bound
checks are
happening in olddcb_outp_foreach() [1].

[1]
https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/nouveau/nouveau_bios.c#L1331

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));
   }