CVE-2026-89996 dma-buf: dma-heap: don't publish fd before copy_to_user() succeeds
A local attacker can trigger a file descriptor and memory leak in the Linux kernel's DMA heap allocator by causing a copy_to_user failure, potentially leading to resource exhaustion.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-89996?
CVE-2026-89996 is a denial of service vulnerability in Linux, published on 16 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
dma-buf: dma-heap: don't publish fd before copy_to_user() succeeds
DMA_HEAP_IOCTL_ALLOC allocates a dma-buf and installs an fd into the caller's fd table via dma_buf_fd() -> fd_install() before dma_heap_ioctl() copies the result back to userspace. If the trailing copy_to_user() fails, userspace never learns the fd number, but the fd (and the underlying dma-buf reference) are already visible to other threads in the same process and are leaked for the lifetime of the process.
The obvious "close it on the failure path" fix is unsafe: once fd_install() has run, another thread can already dup() the fd, send it via SCM_RIGHTS, or close() it and let its number be reused, so a subsequent close_fd() from the ioctl path can operate on an unrelated file. This was pointed out by Christian König on v1 [1].
Restructure the allocation path so that fd_install() is the last, unfailable step of a successful ioctl:
1. heap->ops->allocate() creates the dma_buf. 2. get_unused_fd_flags() reserves an fd number in the caller's fd table without publishing it, so no other thread can observe it. 3. copy_to_user() delivers the fd number to userspace; on failure the fd is returned with put_unused_fd() and the dma_buf reference is dropped with dma_buf_put(), leaving no user- visible state behind. 4. dma_buf_fd_install() publishes the fd and emits the trace_dma_buf_fd tracepoint -- from here on the ioctl cannot fail.
A new dma_buf_fd_install() helper is introduced in dma-buf.c to wrap fd_install() together with the DMA_BUF_TRACE() call, preserving the export tracing that dma_buf_fd() provides. dma_heap_ioctl_allocate() is refactored to return the struct dma_buf * directly (returning ERR_PTR on failure) so the caller holds the dmabuf reference across steps 3 and 4.
The failure at step 3 is easily reachable from userspace: pass a struct dma_heap_allocation_data that lives in a page whose protection is flipped to PROT_READ between copy_from_user() and copy_to_user() (e.g. via mprotect()). Before this change each such ioctl leaks one dmabuf fd; after it, the fd table is unchanged on failure and only /dev/dma_heap/<name> remains open.
No UAPI or heap-driver interface change.
[1] https://lore.kernel.org/dri-devel/[email protected]/
Which versions are affected by CVE-2026-89996?
How do I fix CVE-2026-89996?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-89996 being exploited?
There is no known exploitation of CVE-2026-89996 in the wild and no public exploit code.
Sources
Also known as AZL-101688, BELL-CVE-2026-89996, DEBIAN-CVE-2026-89996, GHSA-cr94-3ppf-3v5m, UBUNTU-CVE-2026-89996.