Re: [PATCH v2] iommu/iova: Add NULL check in iova_magazine_free()

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

On 2026-03-04 8:06 am, Lynn Liu wrote:
> When iova_domain_init_rcaches() fails to allocate an iova_magazine
> during the initialization of per-cpu rcaches, it jumps to out_err and
> calls free_iova_rcaches() for cleanup.
> 
> In free_iova_rcaches(), the code iterates through all possible CPUs to
> free both cpu_rcache->loaded and cpu_rcache->prev. However, if the
> original allocation failed mid-way through the CPU loop, the pointers
> for the remaining CPUs remain NULL.
> 
> Since kmem_cache_free() does not explicitly handle NULL pointers like
> kfree() does, passing these NULL pointers leads to a kernel paging
> request fault.
> 
> The crash can be observed when fault injection triggers a failure in
> iova_magazine_alloc():
> 
> pc : _compound_head include/linux/page-flags.h:251 [inline]
> pc : virt_to_folio include/linux/mm.h:1395 [inline]
> pc : virt_to_slab mm/slab.h:215 [inline]
> pc : kmem_cache_free+0x90/0x5c4 mm/slub.c:4769
> ...
> Call trace:
>   trace_kmem_cache_free include/trace/events/kmem.h:114 [inline]
>   kmem_cache_free+0x90/0x5c4 mm/slub.c:4768
>   iova_magazine_free drivers/iommu/iova.c:623 [inline]
>   free_iova_rcaches+0x114/0x344 drivers/iommu/iova.c:894
>   iova_domain_init_rcaches drivers/iommu/iova.c:762 [inline]
>   iova_domain_init_rcaches+0x3d0/0x528 drivers/iommu/iova.c:717
> 
> Add a NULL check in iova_magazine_free() to safely handle partially
> initialized rcaches in error paths.

Wouldn't it be even better to just break the for_each_possible_cpu() 
loop in free_iova_rcaches() on the first NULL, much like the outer 
rcache->cpu_rcaches condition? Since the cleanup is already trying to be 
symmetric to the allocation, I'd rather fix that than add a blanket 
check that's redundant for every other caller.

Thanks,
Robin.

> Fixes: 84e6f56be9c6 ("iommu/iova: use named kmem_cache for iova magazines")
> Signed-off-by: Lynn Liu <liulynn@google.com>
> ---
> v1: https://lore.kernel.org/linux-iommu/20260214080919.390846-1-liulynn@google.com/T/#u
> Changelog in v1 -> v2:
>   - Update author name to full name.
>   - Add Fixes tag.
>   - Include the Call trace from the kernel crash.
> 
>   drivers/iommu/iova.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
> index 18f839721813..e026be5e068b 100644
> --- a/drivers/iommu/iova.c
> +++ b/drivers/iommu/iova.c
> @@ -611,7 +611,8 @@ static struct iova_magazine *iova_magazine_alloc(gfp_t flags)
>   
>   static void iova_magazine_free(struct iova_magazine *mag)
>   {
> -	kmem_cache_free(iova_magazine_cache, mag);
> +	if (mag)
> +		kmem_cache_free(iova_magazine_cache, mag);
>   }
>   
>   static void