RE: [PATCH v1 3/4] iommu: Unpublish the old attach handle before the group replace callback

[Tian, Kevin]

> From: Nicolin Chen <nicolinc@xxxxxxxxxx>
> Sent: Wednesday, September 30, 2026 5:52 AM
> 
> iommu_replace_group_handle() looks like it hides the detaching handle ahead
> of the driver callback. But in fact, xa_reserve() calls xa_cmpxchg(), which
> stores only when the current entry matches @old=NULL. For a replace, there
> must be an entry sitting there by definition (i.e. @old cannot be NULL), so
> the compare fails, XA_ZERO_ENTRY is never written, and the call decays into
> a plain read.
> 
> Therefore, the old handle stays published during
> __iommu_group_set_domain()
> and only gets replaced after the call. This creates a window, during which
> an asynchronous fault path might hit UAF:
> 
>   core, holding group->mutex          fault path, no group->mutex
>   ==========================          ===========================
> 
>   xa_reserve()                    ,-- old handle stays published
>     no-op, the slot is occupied   |
>   __iommu_group_set_domain()      |
>     driver attach/detach ops      |
>         iopf_queue_flush_dev()    |
>                                   |   iommu_attach_handle_get()
>                                   |     reads old_handle
>   xa_store(new entry)             `-- window closes
>   mutex_unlock()
>   kfree(old_handle)  [caller]
>                                   UAF: old_handle->domain->iopf_handler()
> 
> Replace the xa_reserve() with xa_store(XA_ZERO_ENTRY) to unpublish the old
> handle.
> 
> Fixes: 8519e689834a ("iommu: Extend domain attach group with handle
> support")
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: LLM
> Signed-off-by: Nicolin Chen <nicolinc@xxxxxxxxxx>

Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>

btw I just realized that neither old code nor this fix can actually support
replacement with lossless fault handling, but I don't recall whether it
was the original goal (then destroyed by a following change) or intended
to be a future enhancement...