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

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Date:   Wed, 10 Jun 2020 11:12:29 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     guptap@...eaurora.org
Cc:     joro@...tes.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] iommu/iova: Don't BUG on invalid PFNs

On 2020-06-10 10:27, guptap@...eaurora.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@...eaurora.org>
>> Signed-off-by: Robin Murphy <robin.murphy@....com>
>> ---
>>  drivers/iommu/iova.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>
> 
> Copying stable@...r.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@...eaurora.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);
>>      }