dm-verity: disable recursive forward error correction

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: dm-verity: disable recursive forward error correction 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.

References

• https://git.kernel.org/stable/c/8b821ca892cfeeaf0bedc9fc72717294f67144d5
• https://git.kernel.org/stable/c/e227d2b229c7529bd98d348efc55262ccf24ab35
• https://git.kernel.org/stable/c/897d9006e75f46f8bd7df78faa424327ae6a4bcf
• https://git.kernel.org/stable/c/4220cb37406915c926c0e4a3dbab77cd9cceeb1e
• https://git.kernel.org/stable/c/232948cf600fba69aff36b25d85ef91a73a35756
• https://git.kernel.org/stable/c/d9f3e47d3fae0c101d9094bc956ed24e7a0ee801