Message-ID: <ddd39c66-81f1-44bc-95c7-832d5aff8a85@oracle.com>
Date: Tue, 7 Oct 2025 10:41:01 -0400
From: Alejandro Jimenez <alejandro.j.jimenez@...cle.com>
To: Alex Mastro <amastro@...com>
Cc: Jason Gunthorpe <jgg@...pe.ca>,
        Alex Williamson <alex.williamson@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vfio: fix VFIO_IOMMU_UNMAP_DMA when end of range would
 overflow u64



On 10/7/25 12:24 AM, Alex Mastro wrote:
> On Mon, Oct 06, 2025 at 09:23:56PM -0400, Alejandro Jimenez wrote:
>> If going this way, we'd also have to deny the MAP requests. Right now we
> 
> Agree.
> 
>>> I have doubts that anyone actually relies on MAP_DMA-ing such
>>> end-of-u64-mappings in practice, so perhaps it's OK?
>>>
>>
>> The AMD IOMMU supports a 64-bit IOVA, so when using the AMD vIOMMU with DMA
>> remapping enabled + VF passthrough + a Linux guest with iommu.forcedac=1 we
>> hit this issue since the driver (mlx5) starts requesting mappings for IOVAs
>> right at the top of the address space.
> 
> Interesting!
> 
>> The reason why I hadn't send it to the list yet is because I noticed the
>> additional locations Jason mentioned earlier in the thread (e.g.
>> vfio_find_dma(), vfio_iommu_replay()) and wanted to put together a
>> reproducer that also triggered those paths.
> 
> I am not well equipped to test some of these paths, so if you have a recipe, I'd
> be interested.
>   

Unfortunately I didn't find a good recipe yet, which is why I delayed 
sending my patch with the fix.
I tried a few things to trigger the code paths, but the "easy way" 
methods are not enough. e.g. vfio_iommu_replay() is called when enabling 
the IOMMU model on the container i.e. when issuing the VFIO_SET_IOMMU 
ioctl but at that time there are no mappings yet for the domain so there 
is no dma_list to traverse.
I also tried attaching a second group to the same container that already 
contained a group with mappings, but in vfio_iommu_type1_attach_group() 
the new group is attached to the "existing compatible domain" that was 
already created for the first, since mappings are per-domain, so there 
is no need for replay.
Just mentioning this to give an idea, I didn't have time to try 
triggering the paths involved with dirty tracking...

The basic reproducer I linked on my original patch:

https://gist.github.com/aljimenezb/f3338c9c2eda9b0a7bf5f76b40354db8

is only exercising vfio_find_dma_first_node() and vfio_dma_do_unmap(), 
which is enough to verify the fix that was causing the AMD vIOMMU failure.


>> I mentioned in the notes for the patch above why I chose a slightly more
>> complex method than the '- 1' approach, since there is a chance that
>> iova+size could also go beyond the end of the address space and actually
>> wrap around.
> 
> I think certain invariants have broken if that is possible. The current checks
> in the unmap path should prevent that (iova + size - 1 < iova).
> 
> 1330          } else if (!size || size & (pgsize - 1) ||
> 1331                     iova + size - 1 < iova || size > SIZE_MAX) {
> 1332                  goto unlock;
> 

Yes, that check properly accounts for overflow, but later 
vfio_find_dma_first_node() doesn't, so it won't find the node in the 
dma_list. I agree with the approach of sanitizing the inputs at the uapi 
boundary as Jason mentions, but the code should be robust enough to 
handle overflow anyways I'd think.

>> My goal was not to trust the inputs at all if possible. We could also use
>> check_add_overflow() if we want to add explicit error reporting in
>> vfio_iommu_type1 when an overflow is detected. i.e. catch bad callers that
> 
> I do like the explicitness of the check_* functions over the older style wrap
> checks.
> 
> Below is my partially validated attempt at a more comprehensive fix if we were
> to try and make end of address space mappings work, rather than blanking out
> the last page.
> 

In my opinion, the latter that is not optional. Ranges ending at the 
64-bit boundary are valid, and not accepting them will break drivers 
that request them.

I'll test this change when I have a chance, though I'd really like to 
have solid test cases so I am hoping there is some advice on how to 
trigger all the paths.

Thank you,
Alejandro

> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index 08242d8ce2ca..66a25de35446 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -28,6 +28,7 @@
>   #include <linux/iommu.h>
>   #include <linux/module.h>
>   #include <linux/mm.h>
> +#include <linux/overflow.h>
>   #include <linux/kthread.h>
>   #include <linux/rbtree.h>
>   #include <linux/sched/signal.h>
> @@ -165,12 +166,14 @@ static struct vfio_dma *vfio_find_dma(struct vfio_iommu *iommu,
>   {
>   	struct rb_node *node = iommu->dma_list.rb_node;
>   
> +	BUG_ON(size == 0);
> +
>   	while (node) {
>   		struct vfio_dma *dma = rb_entry(node, struct vfio_dma, node);
>   
> -		if (start + size <= dma->iova)
> +		if (start + size - 1 < dma->iova)
>   			node = node->rb_left;
> -		else if (start >= dma->iova + dma->size)
> +		else if (start > dma->iova + dma->size - 1)
>   			node = node->rb_right;
>   		else
>   			return dma;
> @@ -186,10 +189,12 @@ static struct rb_node *vfio_find_dma_first_node(struct vfio_iommu *iommu,
>   	struct rb_node *node = iommu->dma_list.rb_node;
>   	struct vfio_dma *dma_res = NULL;
>   
> +	BUG_ON(size == 0);
> +
>   	while (node) {
>   		struct vfio_dma *dma = rb_entry(node, struct vfio_dma, node);
>   
> -		if (start < dma->iova + dma->size) {
> +		if (start <= dma->iova + dma->size - 1) {
>   			res = node;
>   			dma_res = dma;
>   			if (start >= dma->iova)
> @@ -199,7 +204,7 @@ static struct rb_node *vfio_find_dma_first_node(struct vfio_iommu *iommu,
>   			node = node->rb_right;
>   		}
>   	}
> -	if (res && size && dma_res->iova > start + size - 1)
> +	if (res && dma_res->iova > start + size - 1)
>   		res = NULL;
>   	return res;
>   }
> @@ -213,7 +218,7 @@ static void vfio_link_dma(struct vfio_iommu *iommu, struct vfio_dma *new)
>   		parent = *link;
>   		dma = rb_entry(parent, struct vfio_dma, node);
>   
> -		if (new->iova + new->size <= dma->iova)
> +		if (new->iova + new->size - 1 < dma->iova)
>   			link = &(*link)->rb_left;
>   		else
>   			link = &(*link)->rb_right;
> @@ -825,14 +830,24 @@ static int vfio_iommu_type1_pin_pages(void *iommu_data,
>   	unsigned long remote_vaddr;
>   	struct vfio_dma *dma;
>   	bool do_accounting;
> +	u64 end, to_pin;
>   
> -	if (!iommu || !pages)
> +	if (!iommu || !pages || npage < 0)
>   		return -EINVAL;
>   
>   	/* Supported for v2 version only */
>   	if (!iommu->v2)
>   		return -EACCES;
>   
> +	if (npage == 0)
> +		return 0;
> +
> +	if (check_mul_overflow(npage, PAGE_SIZE, &to_pin))
> +		return -EINVAL;
> +
> +	if (check_add_overflow(user_iova, to_pin - 1, &end))
> +		return -EINVAL;
> +
>   	mutex_lock(&iommu->lock);
>   
>   	if (WARN_ONCE(iommu->vaddr_invalid_count,
> @@ -997,7 +1012,7 @@ static long vfio_sync_unpin(struct vfio_dma *dma, struct vfio_domain *domain,
>   #define VFIO_IOMMU_TLB_SYNC_MAX		512
>   
>   static size_t unmap_unpin_fast(struct vfio_domain *domain,
> -			       struct vfio_dma *dma, dma_addr_t *iova,
> +			       struct vfio_dma *dma, dma_addr_t iova,
>   			       size_t len, phys_addr_t phys, long *unlocked,
>   			       struct list_head *unmapped_list,
>   			       int *unmapped_cnt,
> @@ -1007,18 +1022,16 @@ static size_t unmap_unpin_fast(struct vfio_domain *domain,
>   	struct vfio_regions *entry = kzalloc(sizeof(*entry), GFP_KERNEL);
>   
>   	if (entry) {
> -		unmapped = iommu_unmap_fast(domain->domain, *iova, len,
> +		unmapped = iommu_unmap_fast(domain->domain, iova, len,
>   					    iotlb_gather);
>   
>   		if (!unmapped) {
>   			kfree(entry);
>   		} else {
> -			entry->iova = *iova;
> +			entry->iova = iova;
>   			entry->phys = phys;
>   			entry->len  = unmapped;
>   			list_add_tail(&entry->list, unmapped_list);
> -
> -			*iova += unmapped;
>   			(*unmapped_cnt)++;
>   		}
>   	}
> @@ -1037,18 +1050,17 @@ static size_t unmap_unpin_fast(struct vfio_domain *domain,
>   }
>   
>   static size_t unmap_unpin_slow(struct vfio_domain *domain,
> -			       struct vfio_dma *dma, dma_addr_t *iova,
> +			       struct vfio_dma *dma, dma_addr_t iova,
>   			       size_t len, phys_addr_t phys,
>   			       long *unlocked)
>   {
> -	size_t unmapped = iommu_unmap(domain->domain, *iova, len);
> +	size_t unmapped = iommu_unmap(domain->domain, iova, len);
>   
>   	if (unmapped) {
> -		*unlocked += vfio_unpin_pages_remote(dma, *iova,
> +		*unlocked += vfio_unpin_pages_remote(dma, iova,
>   						     phys >> PAGE_SHIFT,
>   						     unmapped >> PAGE_SHIFT,
>   						     false);
> -		*iova += unmapped;
>   		cond_resched();
>   	}
>   	return unmapped;
> @@ -1057,12 +1069,12 @@ static size_t unmap_unpin_slow(struct vfio_domain *domain,
>   static long vfio_unmap_unpin(struct vfio_iommu *iommu, struct vfio_dma *dma,
>   			     bool do_accounting)
>   {
> -	dma_addr_t iova = dma->iova, end = dma->iova + dma->size;
>   	struct vfio_domain *domain, *d;
>   	LIST_HEAD(unmapped_region_list);
>   	struct iommu_iotlb_gather iotlb_gather;
>   	int unmapped_region_cnt = 0;
>   	long unlocked = 0;
> +	size_t pos = 0;
>   
>   	if (!dma->size)
>   		return 0;
> @@ -1086,13 +1098,14 @@ static long vfio_unmap_unpin(struct vfio_iommu *iommu, struct vfio_dma *dma,
>   	}
>   
>   	iommu_iotlb_gather_init(&iotlb_gather);
> -	while (iova < end) {
> +	while (pos < dma->size) {
>   		size_t unmapped, len;
>   		phys_addr_t phys, next;
> +		dma_addr_t iova = dma->iova + pos;
>   
>   		phys = iommu_iova_to_phys(domain->domain, iova);
>   		if (WARN_ON(!phys)) {
> -			iova += PAGE_SIZE;
> +			pos += PAGE_SIZE;
>   			continue;
>   		}
>   
> @@ -1101,7 +1114,7 @@ static long vfio_unmap_unpin(struct vfio_iommu *iommu, struct vfio_dma *dma,
>   		 * may require hardware cache flushing, try to find the
>   		 * largest contiguous physical memory chunk to unmap.
>   		 */
> -		for (len = PAGE_SIZE; iova + len < end; len += PAGE_SIZE) {
> +		for (len = PAGE_SIZE; len + pos < dma->size; len += PAGE_SIZE) {
>   			next = iommu_iova_to_phys(domain->domain, iova + len);
>   			if (next != phys + len)
>   				break;
> @@ -1111,16 +1124,18 @@ static long vfio_unmap_unpin(struct vfio_iommu *iommu, struct vfio_dma *dma,
>   		 * First, try to use fast unmap/unpin. In case of failure,
>   		 * switch to slow unmap/unpin path.
>   		 */
> -		unmapped = unmap_unpin_fast(domain, dma, &iova, len, phys,
> +		unmapped = unmap_unpin_fast(domain, dma, iova, len, phys,
>   					    &unlocked, &unmapped_region_list,
>   					    &unmapped_region_cnt,
>   					    &iotlb_gather);
>   		if (!unmapped) {
> -			unmapped = unmap_unpin_slow(domain, dma, &iova, len,
> +			unmapped = unmap_unpin_slow(domain, dma, iova, len,
>   						    phys, &unlocked);
>   			if (WARN_ON(!unmapped))
>   				break;
>   		}
> +
> +		pos += unmapped;
>   	}
>   
>   	dma->iommu_mapped = false;
> @@ -1212,7 +1227,7 @@ static int update_user_bitmap(u64 __user *bitmap, struct vfio_iommu *iommu,
>   }
>   
>   static int vfio_iova_dirty_bitmap(u64 __user *bitmap, struct vfio_iommu *iommu,
> -				  dma_addr_t iova, size_t size, size_t pgsize)
> +				  dma_addr_t iova, u64 end, size_t pgsize)
>   {
>   	struct vfio_dma *dma;
>   	struct rb_node *n;
> @@ -1229,8 +1244,8 @@ static int vfio_iova_dirty_bitmap(u64 __user *bitmap, struct vfio_iommu *iommu,
>   	if (dma && dma->iova != iova)
>   		return -EINVAL;
>   
> -	dma = vfio_find_dma(iommu, iova + size - 1, 0);
> -	if (dma && dma->iova + dma->size != iova + size)
> +	dma = vfio_find_dma(iommu, end, 1);
> +	if (dma && dma->iova + dma->size - 1 != end)
>   		return -EINVAL;
>   
>   	for (n = rb_first(&iommu->dma_list); n; n = rb_next(n)) {
> @@ -1239,7 +1254,7 @@ static int vfio_iova_dirty_bitmap(u64 __user *bitmap, struct vfio_iommu *iommu,
>   		if (dma->iova < iova)
>   			continue;
>   
> -		if (dma->iova > iova + size - 1)
> +		if (dma->iova > end)
>   			break;
>   
>   		ret = update_user_bitmap(bitmap, iommu, dma, iova, pgsize);
> @@ -1305,6 +1320,7 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
>   	unsigned long pgshift;
>   	dma_addr_t iova = unmap->iova;
>   	u64 size = unmap->size;
> +	u64 unmap_end;
>   	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;
> @@ -1327,11 +1343,13 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
>   		if (iova || size)
>   			goto unlock;
>   		size = U64_MAX;
> -	} else if (!size || size & (pgsize - 1) ||
> -		   iova + size - 1 < iova || size > SIZE_MAX) {
> +	} else if (!size || size & (pgsize - 1) || size > SIZE_MAX) {
>   		goto unlock;
>   	}
>   
> +	if (check_add_overflow(iova, size - 1, &unmap_end))
> +		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))) {
> @@ -1376,8 +1394,8 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
>   		if (dma && dma->iova != iova)
>   			goto unlock;
>   
> -		dma = vfio_find_dma(iommu, iova + size - 1, 0);
> -		if (dma && dma->iova + dma->size != iova + size)
> +		dma = vfio_find_dma(iommu, unmap_end, 1);
> +		if (dma && dma->iova + dma->size - 1 != unmap_end)
>   			goto unlock;
>   	}
>   
> @@ -1386,7 +1404,7 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
>   
>   	while (n) {
>   		dma = rb_entry(n, struct vfio_dma, node);
> -		if (dma->iova > iova + size - 1)
> +		if (dma->iova > unmap_end)
>   			break;
>   
>   		if (!iommu->v2 && iova > dma->iova)
> @@ -1713,12 +1731,12 @@ static int vfio_iommu_replay(struct vfio_iommu *iommu,
>   
>   	for (; n; n = rb_next(n)) {
>   		struct vfio_dma *dma;
> -		dma_addr_t iova;
> +		size_t pos = 0;
>   
>   		dma = rb_entry(n, struct vfio_dma, node);
> -		iova = dma->iova;
>   
> -		while (iova < dma->iova + dma->size) {
> +		while (pos < dma->size) {
> +			dma_addr_t iova = dma->iova + pos;
>   			phys_addr_t phys;
>   			size_t size;
>   
> @@ -1734,14 +1752,15 @@ static int vfio_iommu_replay(struct vfio_iommu *iommu,
>   				phys = iommu_iova_to_phys(d->domain, iova);
>   
>   				if (WARN_ON(!phys)) {
> -					iova += PAGE_SIZE;
> +					pos += PAGE_SIZE;
>   					continue;
>   				}
>   
> +
>   				size = PAGE_SIZE;
>   				p = phys + size;
>   				i = iova + size;
> -				while (i < dma->iova + dma->size &&
> +				while (size + pos < dma->size &&
>   				       p == iommu_iova_to_phys(d->domain, i)) {
>   					size += PAGE_SIZE;
>   					p += PAGE_SIZE;
> @@ -1782,7 +1801,7 @@ static int vfio_iommu_replay(struct vfio_iommu *iommu,
>   				goto unwind;
>   			}
>   
> -			iova += size;
> +			pos += size;
>   		}
>   	}
>   
> @@ -1799,29 +1818,29 @@ static int vfio_iommu_replay(struct vfio_iommu *iommu,
>   unwind:
>   	for (; n; n = rb_prev(n)) {
>   		struct vfio_dma *dma = rb_entry(n, struct vfio_dma, node);
> -		dma_addr_t iova;
> +		size_t pos = 0;
>   
>   		if (dma->iommu_mapped) {
>   			iommu_unmap(domain->domain, dma->iova, dma->size);
>   			continue;
>   		}
>   
> -		iova = dma->iova;
> -		while (iova < dma->iova + dma->size) {
> +		while (pos < dma->size) {
> +			dma_addr_t iova = dma->iova + pos;
>   			phys_addr_t phys, p;
>   			size_t size;
>   			dma_addr_t i;
>   
>   			phys = iommu_iova_to_phys(domain->domain, iova);
>   			if (!phys) {
> -				iova += PAGE_SIZE;
> +				pos += PAGE_SIZE;
>   				continue;
>   			}
>   
>   			size = PAGE_SIZE;
>   			p = phys + size;
>   			i = iova + size;
> -			while (i < dma->iova + dma->size &&
> +			while (pos + size < dma->size &&
>   			       p == iommu_iova_to_phys(domain->domain, i)) {
>   				size += PAGE_SIZE;
>   				p += PAGE_SIZE;
> @@ -2908,6 +2927,7 @@ static int vfio_iommu_type1_dirty_pages(struct vfio_iommu *iommu,
>   		unsigned long pgshift;
>   		size_t data_size = dirty.argsz - minsz;
>   		size_t iommu_pgsize;
> +		u64 end;
>   
>   		if (!data_size || data_size < sizeof(range))
>   			return -EINVAL;
> @@ -2916,8 +2936,12 @@ static int vfio_iommu_type1_dirty_pages(struct vfio_iommu *iommu,
>   				   sizeof(range)))
>   			return -EFAULT;
>   
> -		if (range.iova + range.size < range.iova)
> +		if (range.size == 0)
> +			return 0;
> +
> +		if (check_add_overflow(range.iova, range.size - 1, &end))
>   			return -EINVAL;
> +
>   		if (!access_ok((void __user *)range.bitmap.data,
>   			       range.bitmap.size))
>   			return -EINVAL;
> @@ -2949,7 +2973,7 @@ static int vfio_iommu_type1_dirty_pages(struct vfio_iommu *iommu,
>   		if (iommu->dirty_page_tracking)
>   			ret = vfio_iova_dirty_bitmap(range.bitmap.data,
>   						     iommu, range.iova,
> -						     range.size,
> +						     end,
>   						     range.bitmap.pgsize);
>   		else
>   			ret = -EINVAL;

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