[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:
> Hi Jani,
>
> Thanks for looking at this.
>
> On Monday, 6 November 2023 10:53:11 CET Jani Nikula wrote:
>> 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. 
>
> Hmm, do you suggest to blame a commit that was actually guilty, or to drop the 
> Fixes: tag completely?  IOW, are we interested in fixing former stable 
> releases that had no single multi-tile platform officially supported?

IMO Fixes: should only point at the actually broken commits.

If you want to get the backport to a specific stable kernel, please use

Cc: stable at vger.kernel.org # v6.6+

or similar.

IIUC in this case you want the fix to go to v6.7 which doesn't exist
yet, so you'll just have to ask the maintainers to pick it up in fixes.


BR,
Jani.



>
>> Also, double Fixes.
>
> Ops, sorry.
>
> Janusz
>
>> 
>> 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