[PATCH] drm/i915: Avoid potential vm use-after-free

[Tvrtko Ursulin]

+ some more people based on e1a7ab4fca0c

On 19/01/2023 17:32, Rob Clark wrote:
> From: Rob Clark <robdclark at chromium.org>
> 
> Adding the vm to the vm_xa table makes it visible to userspace, which
> could try to race with us to close the vm.  So we need to take our extra
> reference before putting it in the table.
> 
> Signed-off-by: Rob Clark <robdclark at chromium.org>
> ---
> Note, you could list commit e1a7ab4fca0c ("drm/i915: Remove the vm open
> count") as the "fixed" commit, but really the issue seems to go back
> much further (with the fix needing some backporting in the process).

It would probably be rather essential to identify the correct Fixes: tag.

Since Thomas, Matt and Niranjana you were directly involved in the patch 
which changed significantly how this works, perhaps there is something 
still somewhat easily retrievable from your memory lanes to help with this?

Regards,

Tvrtko

> 
>   drivers/gpu/drm/i915/gem/i915_gem_context.c | 14 +++++++++++---
>   1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> index 6250de9b9196..e4b78ab4773b 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> @@ -1861,11 +1861,19 @@ static int get_ppgtt(struct drm_i915_file_private *file_priv,
>   	vm = ctx->vm;
>   	GEM_BUG_ON(!vm);
>   
> +	/*
> +	 * Get a reference for the allocated handle.  Once the handle is
> +	 * visible in the vm_xa table, userspace could try to close it
> +	 * from under our feet, so we need to hold the extra reference
> +	 * first.
> +	 */
> +	i915_vm_get(vm);
> +
>   	err = xa_alloc(&file_priv->vm_xa, &id, vm, xa_limit_32b, GFP_KERNEL);
> -	if (err)
> +	if (err) {
> +		i915_vm_put(vm);
>   		return err;
> -
> -	i915_vm_get(vm);
> +	}
>   
>   	GEM_BUG_ON(id == 0); /* reserved for invalid/unassigned ppgtt */
>   	args->value = id;