iommufd: Don't overflow during division for dirty tracking

JIRA: https://issues.redhat.com/browse/RHEL-131315

Upstream Status: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git

commit cb30dfa
Author: Jason Gunthorpe <jgg@ziepe.ca>
Date:   Wed Oct 8 15:17:18 2025 -0300

    iommufd: Don't overflow during division for dirty tracking

    If pgshift is 63 then BITS_PER_TYPE(*bitmap->bitmap) * pgsize will overflow
    to 0 and this triggers divide by 0.

    In this case the index should just be 0, so reorganize things to divide
    by shift and avoid hitting any overflows.

    Link: https://patch.msgid.link/r/0-v1-663679b57226+172-iommufd_dirty_div0_jgg@nvidia.com
    Cc: stable@vger.kernel.org
    Fixes: 58ccf01 ("vfio: Add an IOVA bitmap support")
    Reviewed-by: Joao Martins <joao.m.martins@oracle.com>
    Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
    Reviewed-by: Kevin Tian <kevin.tian@intel.com>
    Reported-by: syzbot+093a8a8b859472e6c257@syzkaller.appspotmail.com
    Closes: https://syzkaller.appspot.com/bug?extid=093a8a8b859472e6c257
    Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

Signed-off-by: Eder Zulian <ezulian@redhat.com>

Author: efzrh

drivers/iommu/iommufd/iova_bitmap.c

@@ -130,9 +130,8 @@ struct iova_bitmap {
 static unsigned long iova_bitmap_offset_to_index(struct iova_bitmap *bitmap,
 						 unsigned long iova)
 {
-	unsigned long pgsize = 1UL << bitmap->mapped.pgshift;
-
-	return iova / (BITS_PER_TYPE(*bitmap->bitmap) * pgsize);
+	return (iova >> bitmap->mapped.pgshift) /
+	       BITS_PER_TYPE(*bitmap->bitmap);
 }
 
 /*