[PATCH] drm/i915/vma: Fix potential UAF on multi-tile platforms

[Jani Nikula]

On Mon, 06 Nov 2023, Janusz Krzysztofik <janusz.krzysztofik at linux.intel.com> wrote:
> Object debugging tools were sporadically reporting illegal attempts to
> free a still active i915 VMA object from when parking a GPU tile believed
> to be idle.
>
> [161.359441] ODEBUG: free active (active state 0) object: ffff88811643b958 object type: i915_active hint: __i915_vma_active+0x0/0x50 [i915]
> [161.360082] WARNING: CPU: 5 PID: 276 at lib/debugobjects.c:514 debug_print_object+0x80/0xb0
> ...
> [161.360304] CPU: 5 PID: 276 Comm: kworker/5:2 Not tainted 6.5.0-rc1-CI_DRM_13375-g003f860e5577+ #1
> [161.360314] Hardware name: Intel Corporation Rocket Lake Client Platform/RocketLake S UDIMM 6L RVP, BIOS RKLSFWI1.R00.3173.A03.2204210138 04/21/2022
> [161.360322] Workqueue: i915-unordered __intel_wakeref_put_work [i915]
> [161.360592] RIP: 0010:debug_print_object+0x80/0xb0
> ...
> [161.361347] debug_object_free+0xeb/0x110
> [161.361362] i915_active_fini+0x14/0x130 [i915]
> [161.361866] release_references+0xfe/0x1f0 [i915]
> [161.362543] i915_vma_parked+0x1db/0x380 [i915]
> [161.363129] __gt_park+0x121/0x230 [i915]
> [161.363515] ____intel_wakeref_put_last+0x1f/0x70 [i915]
>
> That has been tracked down to be happening when another thread was
> deactivating the VMA inside __active_retire() helper, after the VMA's
> active counter was already decremented to 0, but before deactivation of
> the VMA's object was reported to the object debugging tools.  Root cause
> has been identified as premature release of last wakeref for the GPU tile
> to which the active VMA belonged.
>
> In case of single tile platforms, an engine associated with a request that
> uses the VMA is keeping the tile's wakeref long enough for that VMA to be
> deactivated on time, before it is going to be freed.  However, on multi-
> tile platforms, a request may use a VMA from a tile other than the one
> that hosts the request's engine, then, not protected with the engine's
> wakeref.
>
> Get an extra wakeref for the VMA's tile when activating it, and put that
> wakeref only after the VMA is deactivated.  However, exclude GGTT from
> that processing path, otherwise the GPU never goes idle.  Since
> __i915_vma_retire() may be called from atomic contexts, use async variant
> of wakeref put.
>
> Closes: https://gitlab.freedesktop.org/drm/intel/issues/8875
> Fixes: Fixes: 213c43676beb ("drm/i915/mtl: Remove the 'force_probe' requirement for Meteor Lake")

I get the motivation, but this is hardly true. Also, double Fixes.

BR,
Jani.

> Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik at linux.intel.com>
> ---
>  drivers/gpu/drm/i915/i915_vma.c | 18 ++++++++++++++++--
>  1 file changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_vma.c b/drivers/gpu/drm/i915/i915_vma.c
> index d09aad34ba37f..70c68f614c6db 100644
> --- a/drivers/gpu/drm/i915/i915_vma.c
> +++ b/drivers/gpu/drm/i915/i915_vma.c
> @@ -34,6 +34,7 @@
>  #include "gt/intel_engine.h"
>  #include "gt/intel_engine_heartbeat.h"
>  #include "gt/intel_gt.h"
> +#include "gt/intel_gt_pm.h"
>  #include "gt/intel_gt_requests.h"
>  #include "gt/intel_tlb.h"
>  
> @@ -103,12 +104,25 @@ static inline struct i915_vma *active_to_vma(struct i915_active *ref)
>  
>  static int __i915_vma_active(struct i915_active *ref)
>  {
> -	return i915_vma_tryget(active_to_vma(ref)) ? 0 : -ENOENT;
> +	struct i915_vma *vma = active_to_vma(ref);
> +
> +	if (!i915_vma_tryget(vma))
> +		return -ENOENT;
> +
> +	if (!i915_vma_is_ggtt(vma))
> +		intel_gt_pm_get(vma->vm->gt);
> +
> +	return 0;
>  }
>  
>  static void __i915_vma_retire(struct i915_active *ref)
>  {
> -	i915_vma_put(active_to_vma(ref));
> +	struct i915_vma *vma = active_to_vma(ref);
> +
> +	if (!i915_vma_is_ggtt(vma))
> +		intel_gt_pm_put_async(vma->vm->gt);
> +
> +	i915_vma_put(vma);
>  }
>  
>  static struct i915_vma *

-- 
Jani Nikula, Intel