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

[Yi Liu <yi.l.liu@intel.com>]

On 10/29/26 15:25, Tian, Kevin wrote:
>> From: Nicolin Chen <nicolinc@nvidia.com>
>> 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@vger.kernel.org
>> Assisted-by: LLM
>> Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
> 
> Reviewed-by: Kevin Tian <kevin.tian@intel.com>
> 
> 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...

There was a related discussion[1]. With this patch, a page request in
the replace window gets IOMMU_PAGE_RESP_INVALID instead of being
serviced - spec only says re-issuing the same PRG won't succeed unless
the mapping changes, it doesn't forbid retry. So I think this is just a
more conservative instance of the failure case Jason already called OK
in [1], not a new hazard.

[1] https://lore.kernel.org/linux-iommu/20241105151042.GC458827@nvidia.com/

Regards,
Yi Liu