Re: [PATCH] iommu/iova: Fix alloc iova overflows issue

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

On 2023-01-09 08:34, yf.wang@mediatek.com wrote:
> From: Yunfei Wang <yf.wang@mediatek.com>
> 
> In __alloc_and_insert_iova_range, there is an issue that retry_pfn
> overflows. The value of iovad->anchor.pfn_hi is ~0UL, then when
> iovad->cached_node is iovad->anchor, curr_iova->pfn_hi + 1 will
> overflow. As a result, if the retry logic is executed, low_pfn is
> updated to 0, and then new_pfn < low_pfn returns false to make the
> allocation successful.
> 
> This issue occurs in the following two situations:
> 1. The first iova size exceeds the domain size. When initializing
> iova domain, iovad->cached_node is assigned as iovad->anchor. For
> example, the iova domain size is 10M, start_pfn is 0x1_F000_0000,
> and the iova size allocated for the first time is 11M. The
> following is the log information, new->pfn_lo is smaller than
> iovad->cached_node.
> 
> Example log:
> [  223.798112][T1705487] sh: [name:iova&]__alloc_and_insert_iova_range
> start_pfn:0x1f0000,retry_pfn:0x0,size:0xb00,limit_pfn:0x1f0a00
> [  223.799590][T1705487] sh: [name:iova&]__alloc_and_insert_iova_range
> success start_pfn:0x1f0000,new->pfn_lo:0x1efe00,new->pfn_hi:0x1f08ff
> 
> 2. The node with the largest iova->pfn_lo value in the iova domain
> is deleted, iovad->cached_node will be updated to iovad->anchor,
> and then the alloc iova size exceeds the maximum iova size that can
> be allocated in the domain.
> 
> Adding judgment that retry_pfn must be greater than iovad->start_pfn
> can fix this issue.

Hmm, indeed that's a sneaky little bug - thanks for the thorough 
analysis - but couldn't we avoid the overflow entirely? I don't have the 
complete logic paged in, but superficially it seems like:

	retry_pfn = curr_iova->pfn_hi;
	...
		retry_pfn <= limit_pfn
	...
		low_pfn = retry_pfn + 1;

should still work, shouldn't it?

Thanks,
Robin.

> Signed-off-by: jianjiao zeng <jianjiao.zeng@mediatek.com>
> Signed-off-by: Yunfei Wang <yf.wang@mediatek.com>
> ---
>   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 a44ad92fc5eb..0073206c2a95 100644
> --- a/drivers/iommu/iova.c
> +++ b/drivers/iommu/iova.c
> @@ -209,7 +209,8 @@ static int __alloc_and_insert_iova_range(struct iova_domain *iovad,
>   	} while (curr && new_pfn <= curr_iova->pfn_hi && new_pfn >= low_pfn);
>   
>   	if (high_pfn < size || new_pfn < low_pfn) {
> -		if (low_pfn == iovad->start_pfn && retry_pfn < limit_pfn) {
> +		if (low_pfn == iovad->start_pfn &&
> +		    retry_pfn >= iovad->start_pfn && retry_pfn < limit_pfn) {
>   			high_pfn = limit_pfn;
>   			low_pfn = retry_pfn;
>   			curr = iova_find_limit(iovad, limit_pfn);