Re: [PATCH] iommu/iova: Don't BUG on invalid PFNs

Re: [PATCH] iommu/iova: Don't BUG on invalid PFNs

[guptap]

On 2020-06-02 18:38, Robin Murphy wrote:
> Unlike the other instances which represent a complete loss of
> consistency within the rcache mechanism itself, or a fundamental
> and obvious misconfiguration by an IOMMU driver, the BUG_ON() in
> iova_magazine_free_pfns() can be provoked at more or less any time
> in a "spooky action-at-a-distance" manner by any old device driver
> passing nonsense to dma_unmap_*() which then propagates through to
> queue_iova().
> 
> Not only is this well outside the IOVA layer's control, it's also
> nowhere near fatal enough to justify panicking anyway - all that
> really achieves is to make debugging the offending driver more
> difficult. Let's simply WARN and otherwise ignore bogus PFNs.
> 
> Reported-by: Prakash Gupta <guptap@codeaurora.org>
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> ---
>  drivers/iommu/iova.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 

Copying stable@vger.kernel.org

You can add
Reviewed-by: Prakash Gupta <guptap@codeaurora.org>

> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
> index 0e6a9536eca6..612cbf668adf 100644
> --- a/drivers/iommu/iova.c
> +++ b/drivers/iommu/iova.c
> @@ -811,7 +811,9 @@ iova_magazine_free_pfns(struct iova_magazine *mag,
> struct iova_domain *iovad)
>  	for (i = 0 ; i < mag->size; ++i) {
>  		struct iova *iova = private_find_iova(iovad, mag->pfns[i]);
> 
> -		BUG_ON(!iova);
> +		if (WARN_ON(!iova))
> +			continue;
> +
>  		private_free_iova(iovad, iova);
>  	}

Re: [PATCH] iommu/iova: Don't BUG on invalid PFNs

[Robin Murphy]

On 2020-06-10 10:27, guptap@codeaurora.org wrote:
> On 2020-06-02 18:38, Robin Murphy wrote:
>> Unlike the other instances which represent a complete loss of
>> consistency within the rcache mechanism itself, or a fundamental
>> and obvious misconfiguration by an IOMMU driver, the BUG_ON() in
>> iova_magazine_free_pfns() can be provoked at more or less any time
>> in a "spooky action-at-a-distance" manner by any old device driver
>> passing nonsense to dma_unmap_*() which then propagates through to
>> queue_iova().
>>
>> Not only is this well outside the IOVA layer's control, it's also
>> nowhere near fatal enough to justify panicking anyway - all that
>> really achieves is to make debugging the offending driver more
>> difficult. Let's simply WARN and otherwise ignore bogus PFNs.
>>
>> Reported-by: Prakash Gupta <guptap@codeaurora.org>
>> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
>> ---
>>  drivers/iommu/iova.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>
> 
> Copying stable@vger.kernel.org

Per Documentation/process/stable-kernel-rules.rst, I'm not convinced 
this meets the criteria for stable, which is why I deliberately left 
that out. This change isn't fixing any bug in itself, it is merely 
relaxing a behaviour that only comes into play if a serious and 
effectively unrecoverable bug has already occurred elsewhere.

If Joerg feels like sending it to stable anyway then fair enough, but 
FYI there is no relevant "Fixes" tag.

> You can add
> Reviewed-by: Prakash Gupta <guptap@codeaurora.org>

Thanks,
Robin.

>> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
>> index 0e6a9536eca6..612cbf668adf 100644
>> --- a/drivers/iommu/iova.c
>> +++ b/drivers/iommu/iova.c
>> @@ -811,7 +811,9 @@ iova_magazine_free_pfns(struct iova_magazine *mag,
>> struct iova_domain *iovad)
>>      for (i = 0 ; i < mag->size; ++i) {
>>          struct iova *iova = private_find_iova(iovad, mag->pfns[i]);
>>
>> -        BUG_ON(!iova);
>> +        if (WARN_ON(!iova))
>> +            continue;
>> +
>>          private_free_iova(iovad, iova);
>>      }