Re: [PATCH] iommu/iova: Clear the slab cache pointers when destroying them

[Robin Murphy <robin.murphy@arm.com>]

On 16/08/2026 7:43 pm, Davidlohr Bueso wrote:
> Both iova_cache_get() failure path and iova_cache_put() destroy the two
> slab caches without clearing the pointers, leaving them dangling with
> iova_cache_users at zero. The next iova_cache_get() then re-enters the
> respective block, and if it fails early enough to reach 'out_err' before
> re-creating both caches, it calls kmem_cache_destroy() a second time on
> whichever cache is still stale:
> 
>    BUG: KASAN: slab-use-after-free in iova_cache_get+0x216/0x280
>    Read of size 1 at addr ffff888001b9fdc0 by task swapper/0/1
>    CPU: 1 UID: 0 PID: 1 Comm: swapper/0 Not tainted 7.2.0-rc1 #2
>    Call Trace:
>     <TASK>
>     dump_stack_lvl+0x53/0x70
>     print_report+0xce/0x620
>     kasan_report+0xce/0x100
>     __kasan_check_byte+0x36/0x50
>     kmem_cache_destroy+0x1b/0x1c0
>     iova_cache_get+0x216/0x280
>     ...
>    Freed by task 1:
>     kasan_save_stack+0x33/0x60
>     kasan_save_track+0x14/0x30
>     kasan_save_free_info+0x3b/0x60
>     __kasan_slab_free+0x43/0x70
>     kmem_cache_free+0xbe/0x3c0
>     kobject_put+0x14d/0x280
>     iova_cache_put+0x8e/0xd0
> 
> Clear both pointers after destroying them, in both places.

Out of curiosity, what architecture/kernel config have you found this 
with? I see the logic, but off the top of my head I'm somewhat 
struggling to imagine the scenario in which iova_cache_get() succeeds, 
the last user (so no iommu-dma) cleanly calls iova_cache_put() to be 
able to free the state, then another iova_cache_get() fails. Is 
everything else also falling apart in flames anyway at this point?

Thanks,
Robin.

> Fixes: 84e6f56be9c6 ("iommu/iova: use named kmem_cache for iova magazines")
> Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
> ---
>   drivers/iommu/iova.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
> index b710e5ad37e2..0e8ea04b824f 100644
> --- a/drivers/iommu/iova.c
> +++ b/drivers/iommu/iova.c
> @@ -984,6 +984,8 @@ int iova_cache_get(void)
>   out_err:
>   	kmem_cache_destroy(iova_cache);
>   	kmem_cache_destroy(iova_magazine_cache);
> +	iova_cache = NULL;
> +	iova_magazine_cache = NULL;
>   	mutex_unlock(&iova_cache_mutex);
>   	return err;
>   }
> @@ -1001,6 +1003,8 @@ void iova_cache_put(void)
>   		cpuhp_remove_multi_state(CPUHP_IOMMU_IOVA_DEAD);
>   		kmem_cache_destroy(iova_cache);
>   		kmem_cache_destroy(iova_magazine_cache);
> +		iova_cache = NULL;
> +		iova_magazine_cache = NULL;
>   	}
>   	mutex_unlock(&iova_cache_mutex);
>   }