Re: [kbuild] [linux-next:master 6931/12022] drivers/vfio/vfio_iommu_type1.c:1093 vfio_dma_do_unmap() warn: impossible condition '(size > (~0)) => (0-u32max > u32max)'

[Steven Sistare <steven.sistare@oracle.com>]

On 2/23/2021 4:10 PM, Alex Williamson wrote:
> On Tue, 23 Feb 2021 15:37:31 -0500
> Steven Sistare <steven.sistare@oracle.com> wrote:
> 
>> On 2/23/2021 12:45 PM, Alex Williamson wrote:
>>> On Tue, 23 Feb 2021 08:56:36 -0500
>>> Steven Sistare <steven.sistare@oracle.com> wrote:
>>>   
>>>> On 2/22/2021 6:17 PM, Alex Williamson wrote:  
>>>>> On Mon, 22 Feb 2021 15:51:45 -0700
>>>>> Alex Williamson <alex.williamson@redhat.com> wrote:
>>>>>     
>>>>>> On Mon, 22 Feb 2021 17:10:43 +0300
>>>>>> Dan Carpenter <dan.carpenter@oracle.com> wrote:
>>>>>>    
>>>>>>> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git  master
>>>>>>> head:   37dfbfbdca66834bc0f64ec9b35e09ac6c8898da
>>>>>>> commit: 0f53afa12baec8c00f5d1d6afb49325ada105253 [6931/12022] vfio/type1: unmap cleanup      
>>>>>>
>>>>>> It's always the patches that claim no functional change... ;)
>>>>>>    
>>>>>>> config: i386-randconfig-m021-20210222 (attached as .config)
>>>>>>> compiler: gcc-9 (Debian 9.3.0-15) 9.3.0
>>>>>>>
>>>>>>> If you fix the issue, kindly add following tag as appropriate
>>>>>>> Reported-by: kernel test robot <lkp@intel.com>
>>>>>>> Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
>>>>>>>
>>>>>>> New smatch warnings:
>>>>>>> drivers/vfio/vfio_iommu_type1.c:1093 vfio_dma_do_unmap() warn: impossible condition '(size > (~0)) => (0-u32max > u32max)'
>>>>>>>
>>>>>>> vim +1093 drivers/vfio/vfio_iommu_type1.c
>>>>>>>
>>>>>>> 73fa0d10d077d9 Alex Williamson 2012-07-31  1071  static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
>>>>>>> 331e33d2960c82 Kirti Wankhede  2020-05-29  1072  			     struct vfio_iommu_type1_dma_unmap *unmap,
>>>>>>> 331e33d2960c82 Kirti Wankhede  2020-05-29  1073  			     struct vfio_bitmap *bitmap)
>>>>>>> 73fa0d10d077d9 Alex Williamson 2012-07-31  1074  {
>>>>>>> c086de818dd81c Kirti Wankhede  2016-11-17  1075  	struct vfio_dma *dma, *dma_last = NULL;
>>>>>>> 331e33d2960c82 Kirti Wankhede  2020-05-29  1076  	size_t unmapped = 0, pgsize;
>>>>>>> 0f53afa12baec8 Steve Sistare   2021-01-29  1077  	int ret = -EINVAL, retries = 0;
>>>>>>> 331e33d2960c82 Kirti Wankhede  2020-05-29  1078  	unsigned long pgshift;
>>>>>>> 0f53afa12baec8 Steve Sistare   2021-01-29  1079  	dma_addr_t iova = unmap->iova;
>>>>>>> 0f53afa12baec8 Steve Sistare   2021-01-29  1080  	unsigned long size = unmap->size;
>>>>>>>                                                         ^^^^^^^^^^^^^^^^^^
>>>>>>>
>>>>>>> 73fa0d10d077d9 Alex Williamson 2012-07-31  1081  
>>>>>>> cade075f265b25 Kirti Wankhede  2020-05-29  1082  	mutex_lock(&iommu->lock);
>>>>>>> cade075f265b25 Kirti Wankhede  2020-05-29  1083  
>>>>>>> 331e33d2960c82 Kirti Wankhede  2020-05-29  1084  	pgshift = __ffs(iommu->pgsize_bitmap);
>>>>>>> 331e33d2960c82 Kirti Wankhede  2020-05-29  1085  	pgsize = (size_t)1 << pgshift;
>>>>>>> cade075f265b25 Kirti Wankhede  2020-05-29  1086  
>>>>>>> 0f53afa12baec8 Steve Sistare   2021-01-29  1087  	if (iova & (pgsize - 1))
>>>>>>> cade075f265b25 Kirti Wankhede  2020-05-29  1088  		goto unlock;
>>>>>>> cade075f265b25 Kirti Wankhede  2020-05-29  1089  
>>>>>>> 0f53afa12baec8 Steve Sistare   2021-01-29  1090  	if (!size || size & (pgsize - 1))
>>>>>>> cade075f265b25 Kirti Wankhede  2020-05-29  1091  		goto unlock;
>>>>>>> 73fa0d10d077d9 Alex Williamson 2012-07-31  1092  
>>>>>>> 0f53afa12baec8 Steve Sistare   2021-01-29 @1093  	if (iova + size - 1 < iova || size > SIZE_MAX)
>>>>>>>
>>>>>>> size is unsigned long and SIZE_MAX is ULONG_MAX so "size > SIZE_MAX"
>>>>>>> does not make sense.      
>>>>>>
>>>>>> I think it made sense before the above commit, where unmap->size is a
>>>>>> __u64 and a user could provide a value that exceeds SIZE_MAX on ILP32.
>>>>>> Seems like the fix is probably to use a size_t for the local variable
>>>>>> and restore this test to compare (unmap->size > SIZE_MAX).  Steve?    
>>>>>
>>>>> Actually it seems like VFIO_DMA_UNMAP_FLAG_ALL doesn't work when
>>>>> PHYS_ADDR_MAX != SIZE_MAX (ex. x86 PAE - I think).      
>>>>
>>>> It seems like PAE causes problems even before VFIO_DMA_UNMAP_FLAG_ALL.  
>>>
>>> This wouldn't surprise me, I don't know of any actual non-64bit users
>>> and pure 32bit support was only lightly validated ages ago.
>>>   
>>>> In the previous vfio_dma_do_unmap code, the u64 unmap->size would be
>>>> truncated when passed to vfio_find_dma.  
>>>
>>> We would have failed with -EINVAL before we get there due to this
>>> SIZE_MAX test.  I think the existing (previous) PAE interface is at
>>> least self consistent; I see the mapping path also attempts to check
>>> that casting map->size as size_t still matches the original value.  
>>
>> Good point, and it also checks for vaddr and iova overflow and wrap:
>>
>> vfio_dma_do_map()
>>         if (map->size != size || map->vaddr != vaddr || map->iova != iova)
>>                 return -EINVAL;
>>         if (iova + size - 1 < iova || vaddr + size - 1 < vaddr) {
>>                 ret = -EINVAL;
>>
>> With that, I don't see a problem with PAE, for unmap-all or otherwise.
>> We just need "u64 size" in vfio_dma_do_unmap to avoid the smatch warning.
> 
> I'm not convinced.  My understanding is that on PAE phys_addr_t is
> 64-bit while size_t is 32-bit.  dma_addr_t (iova above) seems to follow
> phys_addr_t.  That suggests to me that our {un}map.iova lives in a
> 64-bit address space, but each mapping is limited to 32-bits.  The

OK, the "map->iova != iova" test does not help because dma_addr_t is 64-bit. My bad.
So, I re-propose my fix for unmap-all from previous email.

I am not keen on proposing a fix for the potential legacy bugs, vfio_find_dma() and
its callers, if no one is reporting bugs and no one uses it with vfio.  It has the 
potential for regression with no upside.

- Steve 

> unmap-all logic only looks for a first entry to unmap in the
> [0..SIZE_MAX] range.  If an entry happens to exist there, we'll unmap
> everything, but the user would have no requirement to have a mapping
> within that range, their first mapping could exist at iova = (SIZE_MAX
> + 1).  So unmap-all would effectively need a special case to use
> rb_first(), which mostly negates the reason we added
> vfio_find_dma_first_node().  Right?  Thanks,
> 
> Alex
> 
> 
>>>> For unmap, these fixes should suffice, and I would rather do this than
>>>> disable the unmap-all flag for a corner case:
>>>>
>>>>   vfio_dma_do_unmap()
>>>>     size_t unmapped = 0;
>>>>     unsigned long size = unmap->size;  
>>>>     ==>    
>>>>     u64 unmapped = 0;
>>>>     u64 size = unmap->size;
>>>>
>>>>   static struct rb_node *vfio_find_dma_first_node(
>>>>       struct vfio_iommu *iommu, dma_addr_t start, size_t size)  
>>>>   ==>    
>>>>   static struct rb_node *vfio_find_dma_first_node(
>>>>       struct vfio_iommu *iommu, dma_addr_t start, u64 size)
>>>>
>>>> And maybe use dma_addr_t instead of u64 in the above (which is 64 bits for
>>>> CONFIG_X86_PAE).
>>>>
>>>> However, there are other places in the existing code that need tweaking
>>>> to be safe for PAE, the vfio_find_dma() size arg for one.  
>>>
>>> Yes, it looks like the IOMMU aperture checking using vfio_find_dma()
>>> could have issues where dma_addr_t > size_t.  Do you want to propose a
>>> patch?  Thanks,
>>>
>>> Alex
>>>   
>>>>> I can't say I'm
>>>>> really interested in adding complexity to make it work in such a case
>>>>> either.  Maybe we can just not expose it, ex:
>>>>>
>>>>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>>>>> index ed03f3fcb07e..6b69a74b3db0 100644
>>>>> --- a/drivers/vfio/vfio_iommu_type1.c
>>>>> +++ b/drivers/vfio/vfio_iommu_type1.c
>>>>> @@ -1207,7 +1207,7 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
>>>>>  	int ret = -EINVAL, retries = 0;
>>>>>  	unsigned long pgshift;
>>>>>  	dma_addr_t iova = unmap->iova;
>>>>> -	unsigned long size = unmap->size;
>>>>> +	size_t size = unmap->size;
>>>>>  	bool unmap_all = unmap->flags & VFIO_DMA_UNMAP_FLAG_ALL;
>>>>>  	bool invalidate_vaddr = unmap->flags & VFIO_DMA_UNMAP_FLAG_VADDR;
>>>>>  	struct rb_node *n, *first_n;
>>>>> @@ -1228,7 +1228,7 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
>>>>>  		goto unlock;
>>>>>  	}
>>>>>  
>>>>> -	if (iova + size - 1 < iova || size > SIZE_MAX)
>>>>> +	if (iova + size - 1 < iova || unmap->size > SIZE_MAX)
>>>>>  		goto unlock;
>>>>>  
>>>>>  	/* When dirty tracking is enabled, allow only min supported pgsize */
>>>>> @@ -2657,9 +2657,10 @@ static int vfio_iommu_type1_check_extension(struct vfio_iommu *iommu,
>>>>>  	case VFIO_TYPE1_IOMMU:
>>>>>  	case VFIO_TYPE1v2_IOMMU:
>>>>>  	case VFIO_TYPE1_NESTING_IOMMU:
>>>>> -	case VFIO_UNMAP_ALL:
>>>>>  	case VFIO_UPDATE_VADDR:
>>>>>  		return 1;
>>>>> +	case VFIO_UNMAP_ALL:
>>>>> +		return PHYS_ADDR_MAX == SIZE_MAX ? 1 : 0;
>>>>>  	case VFIO_DMA_CC_IOMMU:
>>>>>  		if (!iommu)
>>>>>  			return 0;
>>>>> @@ -2868,6 +2869,10 @@ static int vfio_iommu_type1_unmap_dma(struct vfio_iommu *iommu,
>>>>>  			    VFIO_DMA_UNMAP_FLAG_VADDR)))
>>>>>  		return -EINVAL;
>>>>>  
>>>>> +	if ((PHYS_ADDR_MAX != SIZE_MAX) &&
>>>>> +	    (unmap.flags & VFIO_DMA_UNMAP_FLAG_ALL))
>>>>> +		return -EINVAL;
>>>>> +
>>>>>  	if (unmap.flags & VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP) {
>>>>>  		unsigned long pgshift;
>>>>>  
>>>>>
>>>>>  
>>>>>
>>>>>     
>>>>>>> Is the " - 1" intentional on the other overflow check?  As in it's okay
>>>>>>> to wrap around to zero but not further than that?  Sometimes this is
>>>>>>> intentional but it requires more subsystem expertise than I possess.      
>>>>>>
>>>>>> Yes, since we're dealing with a start + length we need to account for
>>>>>> the -1 in the end value, otherwise the user could never unmap the last
>>>>>> page of the address space.  Thanks for the report!
>>>>>>
>>>>>> Alex
>>>>>>    
>>>>>>> cade075f265b25 Kirti Wankhede  2020-05-29  1094  		goto unlock;
>>>>>>> 73fa0d10d077d9 Alex Williamson 2012-07-31  1095  
>>>>>>> 331e33d2960c82 Kirti Wankhede  2020-05-29  1096  	/* When dirty tracking is enabled, allow only min supported pgsize */
>>>>>>> 331e33d2960c82 Kirti Wankhede  2020-05-29  1097  	if ((unmap->flags & VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP) &&
>>>>>>> 331e33d2960c82 Kirti Wankhede  2020-05-29  1098  	    (!iommu->dirty_page_tracking || (bitmap->pgsize != pgsize))) {
>>>>>>> 331e33d2960c82 Kirti Wankhede  2020-05-29  1099  		goto unlock;
>>>>>>> 331e33d2960c82 Kirti Wankhede  2020-05-29  1100  	}
>>>>>>> 73fa0d10d077d9 Alex Williamson 2012-07-31  1101  
>>>>>>> 331e33d2960c82 Kirti Wankhede  2020-05-29  1102  	WARN_ON((pgsize - 1) & PAGE_MASK);
>>>>>>> 331e33d2960c82 Kirti Wankhede  2020-05-29  1103  again:
>>>>>>> 1ef3e2bc04223f Alex Williamson 2014-02-26  1104  	/*
>>>>>>>
>>>>>>> ---
>>>>>>> 0-DAY CI Kernel Test Service, Intel Corporation
>>>>>>> https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org       
>>>>>>    
>>>>>     
>>>>  
>>>   
>>
>