linux-kernel - Re: [PATCH][next] drm/i915: Fix dereference of pointer backup before it is null checked

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Date:   Tue, 12 Oct 2021 16:47:24 +0200
From:   Thomas Hellström 
        <thomas.hellstrom@...ux.intel.com>
To:     Colin King <colin.king@...onical.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Matthew Auld <matthew.auld@...el.com>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org
Cc:     kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] drm/i915: Fix dereference of pointer backup before
 it is null checked

Hi,

On 10/12/21 15:25, Colin King wrote:
> From: Colin Ian King <colin.king@...onical.com>
>
> The assignment of pointer backup_bo dereferences pointer backup before
> backup is null checked, this could lead to a null pointer dereference
> issue. Fix this by only assigning backup_bo after backup has been null
> checked.
>
> Addresses-Coverity: ("Dereference before null check")
> Fixes: c56ce9565374 ("drm/i915 Implement LMEM backup and restore for suspend / resume")
> Signed-off-by: Colin Ian King <colin.king@...onical.com>

There's not really a pointer dereference here, just pointer arithmetics, 
so the code should be safe (but admittedly fragile), so to keep Coverity 
happy,

Reviewed-by: Thomas Hellström <thomas.hellstrom@...ux.intel.com>



> ---
>   drivers/gpu/drm/i915/gem/i915_gem_ttm_pm.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_ttm_pm.c b/drivers/gpu/drm/i915/gem/i915_gem_ttm_pm.c
> index 3b6d14b5c604..4ec6c557083a 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_ttm_pm.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_ttm_pm.c
> @@ -149,7 +149,7 @@ static int i915_ttm_restore(struct i915_gem_apply_to_region *apply,
>   	struct i915_gem_ttm_pm_apply *pm_apply =
>   		container_of(apply, typeof(*pm_apply), base);
>   	struct drm_i915_gem_object *backup = obj->ttm.backup;
> -	struct ttm_buffer_object *backup_bo = i915_gem_to_ttm(backup);
> +	struct ttm_buffer_object *backup_bo;
>   	struct ttm_operation_ctx ctx = {};
>   	int err;
>   
> @@ -163,6 +163,8 @@ static int i915_ttm_restore(struct i915_gem_apply_to_region *apply,
>   	if (err)
>   		return err;
>   
> +	backup_bo = i915_gem_to_ttm(backup);
> +
>   	/* Content may have been swapped. */
>   	err = ttm_tt_populate(backup_bo->bdev, backup_bo->ttm, &ctx);
>   	if (!err) {