[PATCH v5 0/3] drm/i915: Fix VMA UAF on destroy against deactivate race

[Nirmoy Das]

Hi Janusz,

There seems to be a regression in CI related to this:

https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_129026v2/bat-dg1-7/igt@gem_lmem_swapping@random-engines@lmem0.html#dmesg-warnings1053

Please have a look.


Regards,

Nirmoy

On 1/24/2024 6:13 PM, Janusz Krzysztofik wrote:
> Object debugging tools were sporadically reporting illegal attempts to
> free a still active i915 VMA object when parking a GT 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 is
> deactivating the VMA inside __active_retire() helper, after the VMA's
> active counter has been already decremented to 0, but before deactivation
> of the VMA's object is reported to the object debugging tool.
>
> We could prevent from that race by serializing i915_active_fini() with
> __active_retire() via ref->tree_lock, but that wouldn't stop the VMA from
> being used, e.g. from __i915_vma_retire() called at the end of
> __active_retire(), after that VMA has been already freed by a concurrent
> i915_vma_destroy() on return from the i915_active_fini().  Then, we should
> rather fix the issue at the VMA level, not in i915_active.
>
> Since __i915_vma_parked() is called from __gt_park() on last put of the
> GT's wakeref, the issue could be addressed by holding the GT wakeref long
> enough for __active_retire() to complete before that wakeref is released
> and the GT parked.
>
> A VMA associated with a request doesn't acquire a GT wakeref by itself.
> Instead, it depends on a wakeref held directly by the request's active
> intel_context for a GT associated with its VM, and indirectly on that
> intel_context's engine wakeref if the engine belongs to the same GT as the
> VMA's VM.  Those wakerefs are released asynchronously to VMA deactivation.
>
> In case of single-GT platforms, at least one of those wakerefs is usually
> held long enough for the request's VMA to be deactivated on time, before
> it is destroyed on last put of its VM GT wakeref.  However, on multi-GT
> platforms, a request may use a VMA from a GT other than the one that hosts
> the request's engine, then it is protected only with the intel_context's
> VM GT wakeref.
>
> There was an attempt to fix the issue on 2-GT Meteor Lake by acquiring an
> extra wakeref for a Primary GT from i915_gem_do_execbuffer() -- see commit
> f56fe3e91787 ("drm/i915: Fix a VMA UAF for multi-gt platform").  However,
> that fix occurred insufficient -- the issue was still reported by CI.
> That wakeref was released on exit from i915_gem_do_execbuffer(), then
> potentially before completion of the request and deactivation of its
> associated VMAs.  Moreover, CI reports indicated that single-GT platforms
> also suffered sporadically from the same race.
>
> I believe the issue was introduced by commit d93939730347 ("drm/i915:
> Remove the vma refcount") which moved a call to i915_active_fini() from
> a dropped i915_vma_release(), called on last put of the removed VMA kref,
> to i915_vma_parked() processing path called on last put of a GT wakeref.
> However, its visibility to the object debugging tool was suppressed by a
> bug in i915_active that was fixed two weeks later with commit e92eb246feb9
> ("drm/i915/active: Fix missing debug object activation").
>
> Fix the issue by getting a wakeref for the VMA's GT when activating it,
> and putting that wakeref only after the VMA is deactivated.  However,
> exclude global GTT 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.  Also, to avoid circular locking dependency,
> take care of acquiring the wakeref before VM mutex when both are needed.
>
> Having that fixed, stop explicitly acquiring the extra GT0 wakeref from
> inside i915_gem_do_execbuffer(), and also drop an extra call to
> i915_active_wait(), introduced by commit 7a2280e8dcd2 ("drm/i915: Wait for
> active retire before i915_active_fini()") as another insufficient fix for
> this UAF race.
>
> v5: Replace "tile" with "GT" across commit descriptions (Rodrigo),
>    - reword commit message and description of patch 2 reusing relevant
>      chunks moved there from commit description of patch 1 (Rodrigo),
>    - explain why we take a temporary wakeref unconditionally inside
>      i915_vma_pin_ww() (Rodrigo).
> v4: Refresh on top of commit 5e4e06e4087e ("drm/i915: Track gt pm
>      wakerefs") (Andi),
>    - for more easy backporting, split out removal of former insufficient
>      workarounds and move them to separate patches (Nirmoy).
>    - clean up commit message and description a bit.
> v3: Identify root cause more precisely, and a commit to blame,
>    - identify and drop former workarounds,
>    - update commit message and description.
> v2: Get the wakeref before VM mutex to avoid circular locking dependency,
>    - drop questionable Fixes: tag.
>
> Janusz Krzysztofik (3):
>    drm/i915/vma: Fix UAF on destroy against retire race
>    drm/i915: Remove extra multi-gt pm-references
>    Revert "drm/i915: Wait for active retire before i915_active_fini()"
>
>   .../gpu/drm/i915/gem/i915_gem_execbuffer.c    | 17 -----------
>   drivers/gpu/drm/i915/i915_vma.c               | 28 +++++++++++++------
>   drivers/gpu/drm/i915/i915_vma_types.h         |  1 +
>   3 files changed, 20 insertions(+), 26 deletions(-)
>