Re: [PATCH 6.12.y] dm-verity: disable recursive forward error correction

Re: [PATCH 6.12.y] dm-verity: disable recursive forward error correction

[Eric Biggers]

[+Cc dm-devel@lists.linux.dev]

On Thu, Feb 26, 2026 at 12:35:00PM +0800, Rahul Sharma wrote:
> From: Mikulas Patocka <mpatocka@redhat.com>
> 
> [ Upstream commit d9f3e47d3fae0c101d9094bc956ed24e7a0ee801 ]
> 
> There are two problems with the recursive correction:
> 
> 1. It may cause denial-of-service. In fec_read_bufs, there is a loop that
> has 253 iterations. For each iteration, we may call verity_hash_for_block
> recursively. There is a limit of 4 nested recursions - that means that
> there may be at most 253^4 (4 billion) iterations. Red Hat QE team
> actually created an image that pushes dm-verity to this limit - and this
> image just makes the udev-worker process get stuck in the 'D' state.
> 
> 2. It doesn't work. In fec_read_bufs we store data into the variable
> "fio->bufs", but fio bufs is shared between recursive invocations, if
> "verity_hash_for_block" invoked correction recursively, it would
> overwrite partially filled fio->bufs.
> 
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> Reported-by: Guangwu Zhang <guazhang@redhat.com>
> Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
> Reviewed-by: Eric Biggers <ebiggers@kernel.org>
> [ The context change is due to the commit bdf253d580d7
> ("dm-verity: remove support for asynchronous hashes")
> in v6.18 which is irrelevant to the logic of this patch. ]
> Signed-off-by: Rahul Sharma <black.hawk@163.com>

Looks good.  The upstream patch incremented the dm target version number
too, but skipping that part looks good.  This further shows that the dm
subsystem's version numbers don't really work.

- Eric