[PATCH][v2] vfio/type1: Sanitize user-supplied inputs to prevent undefined __ffs() behavior

[lirongqing]

From: Li RongQing <lirongqing@xxxxxxxxx>

The __ffs() function expects a non-zero input. When passed 0, its return
value is undefined (garbage) but does not trigger a hardware fault.
Although downstream logic may eventually catch invalid derived values,
passing unchecked user inputs into __ffs() is a robust-ness and code
quality issue.

Fix this by validating user-supplied inputs early in the UNMAP_DMA and
DIRTY_PAGES ioctl paths before they reach any bit scan operations:

1. Reject an empty dirty.flags in vfio_iommu_type1_dirty_pages() to
   ensure the subsequent __ffs() / __fls() single-bit check is safe.
2. Ensure bitmap.pgsize and range.bitmap.pgsize are valid non-zero
   powers of two before calculating pgshift via __ffs().

This change improves the overall robustness of the VFIO type1 IOMMU
driver against erratic or malicious user-space inputs.

Signed-off-by: Li RongQing <lirongqing@xxxxxxxxx>
---
Diff with v1: Add the two check in vfio_iommu_type1_dirty_pages
              and rewrite the commit message

 drivers/vfio/vfio_iommu_type1.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index c8151ba..b74f56c 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -38,6 +38,7 @@
 #include <linux/workqueue.h>
 #include <linux/mm_inline.h>
 #include <linux/overflow.h>
+#include <linux/log2.h>
 #include "vfio.h"
 
 #define DRIVER_VERSION  "0.2"
@@ -2949,6 +2950,9 @@ static int vfio_iommu_type1_unmap_dma(struct vfio_iommu *iommu,
 		if (!access_ok((void __user *)bitmap.data, bitmap.size))
 			return -EINVAL;
 
+		if (unlikely(!is_power_of_2(bitmap.pgsize)))
+			return -EINVAL;
+
 		pgshift = __ffs(bitmap.pgsize);
 		ret = verify_bitmap_size(unmap.size >> pgshift,
 					 bitmap.size);
@@ -2985,6 +2989,9 @@ static int vfio_iommu_type1_dirty_pages(struct vfio_iommu *iommu,
 	if (dirty.argsz < minsz || dirty.flags & ~mask)
 		return -EINVAL;
 
+	if (!dirty.flags)
+		return -EINVAL;
+
 	/* only one flag should be set at a time */
 	if (__ffs(dirty.flags) != __fls(dirty.flags))
 		return -EINVAL;
@@ -3039,6 +3046,9 @@ static int vfio_iommu_type1_dirty_pages(struct vfio_iommu *iommu,
 			       range.bitmap.size))
 			return -EINVAL;
 
+		if (unlikely(!is_power_of_2(range.bitmap.pgsize)))
+			return -EINVAL;
+
 		pgshift = __ffs(range.bitmap.pgsize);
 		ret = verify_bitmap_size(size >> pgshift,
 					 range.bitmap.size);
-- 
2.9.4