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

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

[Colin King]

From: Colin Ian King <colin.king@canonical.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@canonical.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) {
-- 
2.32.0

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

[Thomas Hellström]

Hi,

On 10/12/21 15:25, Colin King wrote:
> From: Colin Ian King <colin.king@canonical.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@canonical.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@linux.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) {

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

[Dan Carpenter]

On Tue, Oct 12, 2021 at 04:47:24PM +0200, Thomas Hellström wrote:
> Hi,
> 
> On 10/12/21 15:25, Colin King wrote:
> > From: Colin Ian King <colin.king@canonical.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@canonical.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@linux.intel.com>

Yeah.  I kind of feel like we shouldn't work around static checker bugs.
But when we do then there shouldn't be a Fixes tag.

regards,
dan carpenter