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 2/22/2021 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.
In the previous vfio_dma_do_unmap code, the u64 unmap->size would be
truncated when passed to vfio_find_dma.

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.

- Steve

> 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   
>>
>