[PATCH][next] drm/xe/guc: Fix dereference before Null check

[Dan Carpenter]

On Wed, Oct 09, 2024 at 12:49:49PM -0600, Everest K.C. wrote:
> The pointer list->list was derefrenced before the Null check
> resulting in possibility of Null pointer derefrencing.
> This patch moves the Null check outside the for loop, so that
> the check is performed before the derefrencing.
> 
> This issue was reported by Coverity Scan.
> 
> Signed-off-by: Everest K.C. <everestkc at everestkc.com.np>

You need to add a Fixes tag.

> ---
>  drivers/gpu/drm/xe/xe_guc_capture.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_guc_capture.c b/drivers/gpu/drm/xe/xe_guc_capture.c
> index 41262bda20ed..de63c622747d 100644
> --- a/drivers/gpu/drm/xe/xe_guc_capture.c
> +++ b/drivers/gpu/drm/xe/xe_guc_capture.c
> @@ -1537,13 +1537,13 @@ read_reg_to_node(struct xe_hw_engine *hwe, const struct __guc_mmio_reg_descr_gro
>  	if (!regs)
>  		return;
>  
> +	if (!list->list)
> +		return;

Could you merge this with the other sanity checks at the start of the function.

-       if (!list || list->num_regs == 0)
+       if (!list || !list->list || list->num_regs == 0)

The list->list pointer can't actually be NULL.  It comes from
guc_capture_get_one_list(), so if the reglists[i].list pointer is NULL it
returns NULL.  However, obviously checking for NULL after a dereference is not
the correct so it's worth fixing and probably deserves a Fixes tag.  Although it
doesn't affect runtime, adding a Fixes tag helps backporters know they can
automatically ignore this one because the commit it's fixing is very recent.

regards,
dan carpenter