[Intel-gfx] [PATCH] drm/i915/gem: Prevent NULL pointer dereference on missing ctx->vm

[Joonas Lahtinen]

Quoting Chris Wilson (2020-01-23 17:26:02)
> If the ctx->vm is freed before we can acquire a local reference to it,
> we proceed to call i915_vm_put(NULL), which is invalid.
> 
> Reported-by: Colin Ian King <colin.king at canonical.com>
> Fixes: 5dbd2b7be61e ("drm/i915/gem: Convert vm idr to xarray")
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Colin Ian King <colin.king at canonical.com>
> ---
>  drivers/gpu/drm/i915/gem/i915_gem_context.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> index 5d4157e1ccf7..8873db5be123 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> @@ -1002,13 +1002,14 @@ static int get_ppgtt(struct drm_i915_file_private *file_priv,
>         if (!rcu_access_pointer(ctx->vm))
>                 return -ENODEV;
>  
> -       err = -ENODEV;
>         rcu_read_lock();
>         vm = context_get_vm_rcu(ctx);
> -       if (vm)
> -               err = xa_alloc(&file_priv->vm_xa, &id, vm,
> -                              xa_limit_32b, GFP_KERNEL);
>         rcu_read_unlock();
> +       if (!vm)
> +               return -ENODEV;
> +
> +       err = xa_alloc(&file_priv->vm_xa, &id, vm,
> +                      xa_limit_32b, GFP_KERNEL);

Reviewed-by: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>

Regards, Joonas