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 2/24/2021 5:55 PM, Alex Williamson wrote:
> On Tue, 23 Feb 2021 18:58:18 -0500
> Steven Sistare <steven.sistare@oracle.com> wrote:
> 
>> On 2/23/2021 4:52 PM, Steven Sistare wrote:
>>> 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.  
>>
>> ... but there are no legacy bugs because size is constrained to 32-bits in do_map as
>> you pointed out, so all calls to vfio_find_dma are safe.
> 
> Right, all legacy call paths are ok afaict, but the unmap-all flag
> can't reach any mappings if there are none below an iova of SIZE_MAX.
> We should either fix vfio_find_first_dma_node() for this scenario or
> disable unmap-all where this is a possibility.  Thanks,

Changing size to u64 and using U64_MAX as the upper bound should do the trick:

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index 6cf1dad..b1be0a6 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -181,7 +181,7 @@ static struct vfio_dma *vfio_find_dma(struct vfio_iommu *iommu,
 }

 static struct rb_node *vfio_find_dma_first_node(struct vfio_iommu *iommu,
-                                               dma_addr_t start, size_t size)
+                                               dma_addr_t start, u64 size)
 {
        struct rb_node *res = NULL;
        struct rb_node *node = iommu->dma_list.rb_node;
@@ -1184,7 +1184,7 @@ 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;
+       u64 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;
@@ -1200,14 +1200,12 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
        if (unmap_all) {
                if (iova || size)
                        goto unlock;
-               size = SIZE_MAX;
-       } else if (!size || size & (pgsize - 1)) {
+               size = U64_MAX;
+       } else if (!size || size & (pgsize - 1) ||
+                  iova + size - 1 < iova || size > SIZE_MAX) {
                goto unlock;
        }

-       if (iova + size - 1 < iova || size > SIZE_MAX)
-               goto unlock;
-
        /* When dirty tracking is enabled, allow only min supported pgsize */
        if ((unmap->flags & VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP) &&
            (!iommu->dirty_page_tracking || (bitmap->pgsize != pgsize))) {