linux-kernel - Re: [PATCH] iommufd: Prevent the use of nil data

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Message-ID: <20251008181050.GA3833649@ziepe.ca>
Date: Wed, 8 Oct 2025 15:10:50 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Edward Adam Davis <eadavis@...com>
Cc: syzbot+093a8a8b859472e6c257@...kaller.appspotmail.com,
	iommu@...ts.linux.dev, joro@...tes.org, kevin.tian@...el.com,
	linux-kernel@...r.kernel.org, robin.murphy@....com,
	syzkaller-bugs@...glegroups.com, will@...nel.org
Subject: Re: [PATCH] iommufd: Prevent the use of nil data

On Wed, Oct 08, 2025 at 03:53:07PM +0800, Edward Adam Davis wrote:
> The division exception occurs because:
> The denominator bitmap->bitmap is 0, which is derived from the nil value
> of bitmap->data passed in by the reproducer.

No, that just causes the reproducer to bail early. The reported
problem is divide by zero. Which is caused by this:

	unsigned long pgsize = 1UL << bitmap->mapped.pgshift;

	return iova / (BITS_PER_TYPE(*bitmap->bitmap) * pgsize);


  //  ioctl$IOMMU_HWPT_GET_DIRTY_BITMAP arguments: [
  //    fd: fd_iommufd (resource)
  //    cmd: const = 0x3b8c (4 bytes)
  //    arg: ptr[in, iommu_hwpt_get_dirty_bitmap] {
  //      iommu_hwpt_get_dirty_bitmap {
  //        size: len = 0x30 (4 bytes)
  //        hwpt_id: hwpt_handle (resource)
  //        flags: iommufd_hwpt_get_dirty_bitmap_flags = 0x0 (4 bytes)
  //        __reserved: const = 0x0 (4 bytes)
  //        iova: int64 = 0x0 (8 bytes)
  //        length: int64 = 0x0 (8 bytes)
  //        page_size: int64 = 0x8000000000000000 (8 bytes)
                          ^^^^^^^^^^^^^^^^^^^^^^^^^

0x8000000000000000 * BITS_PER_TYPE(*bitmap->bitmap) == 0

It should be fixed by adjusting the logic:

+       return (iova >> bitmap->mapped.pgshift) /
+              BITS_PER_TYPE(*bitmap->bitmap);

I will send a patch.

Jason