Re: [PATCH 1/3] lib: test_hmm: evict device pages on file close to avoid use-after-free

[Zenghui Yu <zenghui.yu@linux.dev>]

On 3/31/26 2:34 PM, Alistair Popple wrote:
> When dmirror_fops_release() is called it frees the dmirror struct but
> doesn't migrate device private pages back to system memory first. This
> leaves those pages with a dangling zone_device_data pointer to the freed
> dmirror.
> 
> If a subsequent fault occurs on those pages (eg. during coredump) the
> dmirror_devmem_fault() callback dereferences the stale pointer causing a
> kernel panic. This was reported [1] when running mm/ksft_hmm.sh on
> arm64, where a test failure triggered SIGABRT and the resulting coredump
> walked the VMAs faulting in the stale device private pages.
> 
> Fix this by calling dmirror_device_evict_chunk() for each devmem chunk
> in dmirror_fops_release() to migrate all device private pages back to
> system memory before freeing the dmirror struct. The function is moved
> earlier in the file to avoid a forward declaration.
> 
> Fixes: b2ef9f5a5cb3 ("mm/hmm/test: add selftest driver for HMM")
> Reported-by: Zenghui Yu <zenghui.yu@linux.dev>
> Closes: https://lore.kernel.org/linux-mm/8bd0396a-8997-4d2e-a13f-5aac033083d7@linux.dev/
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
> 
> ---
> 
> Note that I wasn't able to replicate the exact crash in [1] although I
> replicated something similar. So I haven't been able to verify this
> fixes the crash conclusively, but it should.

Yup! I've verified that it indeed fixed the crash. Thanks for the fix!

Zenghui