CVE-2025-39791: dm: dm-crypt: Do not partially accept write BIOs with zoned targets

In the Linux kernel, the following vulnerability has been resolved:

dm: dm-crypt: Do not partially accept write BIOs with zoned targets

Read and write operations issued to a dm-crypt target may be split according to the dm-crypt internal limits defined by the maxreadsize and maxwritesize module parameters (default is 128 KB). The intent is to improve processing time of large BIOs by splitting them into smaller operations that can be parallelized on different CPUs.

For zoned dm-crypt targets, this BIO splitting is still done but without the parallel execution to ensure that the issuing order of write operations to the underlying devices remains sequential. However, the splitting itself causes other problems:

1) Since dm-crypt relies on the block layer zone write plugging to handle zone append emulation using regular write operations, the reminder of a split write BIO will always be plugged into the target zone write plugged. Once the on-going write BIO finishes, this reminder BIO is unplugged and issued from the zone write plug work. If this reminder BIO itself needs to be split, the reminder will be re-issued and plugged again, but that causes a call to a blkqueueenter(), which may block if a queue freeze operation was initiated. This results in a deadlock as DM submission still holds BIOs that the queue freeze side is waiting for.

2) dm-crypt relies on the emulation done by the block layer using regular write operations for processing zone append operations. This still requires to properly return the written sector as the BIO sector of the original BIO. However, this can be done correctly only and only if there is a single clone BIO used for processing the original zone append operation issued by the user. If the size of a zone append operation is larger than dm-crypt maxwritesize, then the orginal BIO will be split and processed as a chain of regular write operations. Such chaining result in an incorrect written sector being returned to the zone append issuer using the original BIO sector. This in turn results in file system data corruptions using xfs or btrfs.

Fix this by modifying getmaxrequestsize() to always return the size of the BIO to avoid it being split with dmaccpetpartialbio() in cryptmap(). getmaxrequestsize() is renamed to getmaxrequestsectors() to clarify the unit of the value returned and its interface is changed to take a struct dmtarget pointer and a pointer to the struct bio being processed. In addition to this change, to ensure that cryptallocbuffer() works correctly, set the dm-crypt device maxhwsectors limit to be at most BIOMAXVECS << PAGESECTORSSHIFT (1 MB with a 4KB page architecture). This forces DM core to split write BIOs before passing them to cryptmap(), and thus guaranteeing that dm-crypt can always accept an entire write BIO without needing to split it.

This change does not have any effect on the read path of dm-crypt. Read operations can still be split and the BIO fragments processed in parallel. There is also no impact on the performance of the write path given that all zone write BIOs were already processed inline instead of in parallel.

This change also does not affect in any way regular dm-crypt block devices.