Re: [PATCH 5/5] iommu/iova: Avoid double-negatives in magazine helpers

[Will Deacon]

On Fri, Sep 24, 2021 at 06:01:57PM +0800, John Garry wrote:
> A similar crash to the following could be observed if initial CPU rcache
> magazine allocations fail in init_iova_rcaches():
> 
> Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
> Mem abort info:
> 
>   free_iova_fast+0xfc/0x280
>   iommu_dma_free_iova+0x64/0x70
>   __iommu_dma_unmap+0x9c/0xf8
>   iommu_dma_unmap_sg+0xa8/0xc8
>   dma_unmap_sg_attrs+0x28/0x50
>   cq_thread_v3_hw+0x2dc/0x528
>   irq_thread_fn+0x2c/0xa0
>   irq_thread+0x130/0x1e0
>   kthread+0x154/0x158
>   ret_from_fork+0x10/0x34
> 
> The issue is that expression !iova_magazine_full(NULL) evaluates true; this
> falls over in __iova_rcache_insert() when we attempt to cache a mag and
> cpu_rcache->loaded == NULL:
> 
> if (!iova_magazine_full(cpu_rcache->loaded)) {
> 	can_insert = true;
> ...
> 
> if (can_insert)
> 	iova_magazine_push(cpu_rcache->loaded, iova_pfn);
> 
> As above, can_insert is evaluated true, which it shouldn't be, and we try
> to insert pfns in a NULL mag, which is not safe.
> 
> To avoid this, stop using double-negatives, like !iova_magazine_full() and
> !iova_magazine_empty(), and use positive tests, like
> iova_magazine_has_space() and iova_magazine_has_pfns(), respectively; these
> can safely deal with cpu_rcache->{loaded, prev} = NULL.

I don't understand why you're saying that things like !iova_magazine_empty()
are double-negatives. What about e.g. !list_empty() elsewhre in the kernel?

The crux of the fix seems to be:

> @@ -783,8 +787,9 @@ static bool __iova_rcache_insert(struct iova_caching_domain *rcached,
>  		if (new_mag) {
>  			spin_lock(&rcache->lock);
>  			if (rcache->depot_size < MAX_GLOBAL_MAGS) {
> -				rcache->depot[rcache->depot_size++] =
> -						cpu_rcache->loaded;
> +				if (cpu_rcache->loaded)
> +					rcache->depot[rcache->depot_size++] =
> +							cpu_rcache->loaded;

Which could be independent of the renaming?

Will