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

[Jason Gunthorpe]

On Thu, Oct 29, 2026 at 07:00:18PM +0800, Yi Liu wrote:
> On 10/29/26 15:25, Tian, Kevin wrote:
> > > 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...
> 
> 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@xxxxxxxxxx/

I called using either the old or new translation OK, using neither
seems suboptimal. So returning IOMMU_PAGE_RESP_INVALID doesn't sound
ideal..

Jason