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Wed, 08 Oct 2025 11:10:52 -0700 (PDT) Received: from ziepe.ca ([140.209.225.78]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-794d4b3c43asm393694b3a.1.2025.10.08.11.10.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 08 Oct 2025 11:10:51 -0700 (PDT) Received: from jgg by wakko with local (Exim 4.97) (envelope-from ) id 1v6Yck-0000000GA19-0vdm; Wed, 08 Oct 2025 15:10:50 -0300 Date: Wed, 8 Oct 2025 15:10:50 -0300 From: Jason Gunthorpe To: Edward Adam Davis Cc: syzbot+093a8a8b859472e6c257@syzkaller.appspotmail.com, iommu@lists.linux.dev, joro@8bytes.org, kevin.tian@intel.com, linux-kernel@vger.kernel.org, robin.murphy@arm.com, syzkaller-bugs@googlegroups.com, will@kernel.org Subject: Re: [PATCH] iommufd: Prevent the use of nil data Message-ID: <20251008181050.GA3833649@ziepe.ca> References: <68e5a30d.050a0220.256323.002e.GAE@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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