Re: [RESEND PATCH v3 2/4] iommu/iova: Avoid double-negatives in magazine helpers

["Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>]

On 2020/12/9 19:39, John Garry wrote:
> On 09/12/2020 09:03, Leizhen (ThunderTown) wrote:
>>
>>
>> On 2020/11/17 18:25, 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:
>>>     ESR = 0x96000004
>>>     EC = 0x25: DABT (current EL), IL = 32 bits
>>>     SET = 0, FnV = 0
>>>     EA = 0, S1PTW = 0
>>> Data abort info:
>>>     ISV = 0, ISS = 0x00000004
>>>     CM = 0, WnR = 0
>>> [0000000000000000] user address but active_mm is swapper
>>> Internal error: Oops: 96000004 [#1] PREEMPT SMP
>>> Modules linked in:
>>> CPU: 11 PID: 696 Comm: irq/40-hisi_sas Not tainted 5.9.0-rc7-dirty #109
>>> Hardware name: Huawei D06 /D06, BIOS Hisilicon D06 UEFI RC0 - V1.16.01 03/15/2019
>>> Call trace:
>>>    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
>>>
>>> Code: f9400060 f102001f 54000981 d4210000 (f9400043)
>>>
>>>   ---[ end trace 4afcbdfc61b60467 ]---
>>>
>>> The issue is that expression !iova_magazine_full(NULL) evaluates true; this
>>> falls over in 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.
>>>
>>> Signed-off-by: John Garry <john.garry@huawei.com>
> 
> Thanks for checking here...
> 
>>> ---
>>>   drivers/iommu/iova.c | 29 +++++++++++++++++------------
>>>   1 file changed, 17 insertions(+), 12 deletions(-)
>>>
>>> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
>>> index 81b7399dd5e8..1f3f0f8b12e0 100644
>>> --- a/drivers/iommu/iova.c
>>> +++ b/drivers/iommu/iova.c
>>> @@ -827,14 +827,18 @@ iova_magazine_free_pfns(struct iova_magazine *mag, struct iova_domain *iovad)
>>>       mag->size = 0;
>>>   }
>>>   -static bool iova_magazine_full(struct iova_magazine *mag)
>>> +static bool iova_magazine_has_space(struct iova_magazine *mag)
>>>   {
>>> -    return (mag && mag->size == IOVA_MAG_SIZE);
>>> +    if (!mag)
>>> +        return false;
>>> +    return mag->size < IOVA_MAG_SIZE;
>>>   }
>>>   -static bool iova_magazine_empty(struct iova_magazine *mag)
>>> +static bool iova_magazine_has_pfns(struct iova_magazine *mag)
>>>   {
>>> -    return (!mag || mag->size == 0);
>>> +    if (!mag)
>>> +        return false;
>>> +    return mag->size;
>>>   }
>>>     static unsigned long iova_magazine_pop(struct iova_magazine *mag,
>>> @@ -843,7 +847,7 @@ static unsigned long iova_magazine_pop(struct iova_magazine *mag,
>>>       int i;
>>>       unsigned long pfn;
>>>   -    BUG_ON(iova_magazine_empty(mag));
>>> +    BUG_ON(!iova_magazine_has_pfns(mag));
>>>         /* Only fall back to the rbtree if we have no suitable pfns at all */
>>>       for (i = mag->size - 1; mag->pfns[i] > limit_pfn; i--)
>>> @@ -859,7 +863,7 @@ static unsigned long iova_magazine_pop(struct iova_magazine *mag,
>>>     static void iova_magazine_push(struct iova_magazine *mag, unsigned long pfn)
>>>   {
>>> -    BUG_ON(iova_magazine_full(mag));
>>> +    BUG_ON(!iova_magazine_has_space(mag));
>>>         mag->pfns[mag->size++] = pfn;
>>>   }
>>> @@ -905,9 +909,9 @@ static bool __iova_rcache_insert(struct iova_domain *iovad,
>>>       cpu_rcache = raw_cpu_ptr(rcache->cpu_rcaches);
>>>       spin_lock_irqsave(&cpu_rcache->lock, flags);
>>>   -    if (!iova_magazine_full(cpu_rcache->loaded)) {
> 
> *
> 
>>> +    if (iova_magazine_has_space(cpu_rcache->loaded)) {
>>>           can_insert = true;
>>> -    } else if (!iova_magazine_full(cpu_rcache->prev)) {
>>> +    } else if (iova_magazine_has_space(cpu_rcache->prev)) {
>>>           swap(cpu_rcache->prev, cpu_rcache->loaded);
>>>           can_insert = true;
>>>       } else {
>>> @@ -916,8 +920,9 @@ static bool __iova_rcache_insert(struct iova_domain *iovad,
>>>           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)
>>
>> Looks like it just needs to change this place. Compiler ensures that mag->size
>> will not be accessed when mag is NULL.
> 
> Not sure about that. We would get a crash prior to touching this codepath:
> 
> So if cpu_rcache->loaded == NULL at entry, then !iova_magazine_full(cpu_rcache->loaded (==NULL) ) evaluates true, * above, so can_insert is set true, and we then attempt iova_magazine_push(cpu_rcache->loaded (==NULL), iova_pfn), which is not safe.
> 
> ok?

OK, I got it.

Reviewed-by: Zhen Lei <thunder.leizhen@huawei.com>

> 
> Cheers,
> John
> 
>>
>> static bool iova_magazine_full(struct iova_magazine *mag)
>> {
>>          return (mag && mag->size == IOVA_MAG_SIZE);
>> }
>>
>> static bool iova_magazine_empty(struct iova_magazine *mag)
>> {
>>          return (!mag || mag->size == 0);
>> }
>>
> 
> 
> 
> 
>>
>>> +                    rcache->depot[rcache->depot_size++] =
>>> +                            cpu_rcache->loaded;
>>>               } else {
>>>                   mag_to_free = cpu_rcache->loaded;
>>>               }
>>> @@ -968,9 +973,9 @@ static unsigned long __iova_rcache_get(struct iova_rcache *rcache,
>>>       cpu_rcache = raw_cpu_ptr(rcache->cpu_rcaches);
>>>       spin_lock_irqsave(&cpu_rcache->lock, flags);
>>>   -    if (!iova_magazine_empty(cpu_rcache->loaded)) {
>>> +    if (iova_magazine_has_pfns(cpu_rcache->loaded)) {
>>>           has_pfn = true;
>>> -    } else if (!iova_magazine_empty(cpu_rcache->prev)) {
>>> +    } else if (iova_magazine_has_pfns(cpu_rcache->prev)) {
>>>           swap(cpu_rcache->prev, cpu_rcache->loaded);
>>>           has_pfn = true;
>>>       } else {
>>>
>>
> 
> 
> .
>