CVE-2025-39792

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

dm: Always split write BIOs to zoned device limits

Any zoned DM target that requires zone append emulation will use the block layer zone write plugging. In such case, DM target drivers must not split BIOs using dmacceptpartial_bio() as doing so can potentially lead to deadlocks with queue freeze operations. Regular write operations used to emulate zone append operations also cannot be split by the target driver as that would result in an invalid writen sector value return using the BIO sector.

In order for zoned DM target drivers to avoid such incorrect BIO splitting, we must ensure that large BIOs are split before being passed to the map() function of the target, thus guaranteeing that the limits for the mapped device are not exceeded.

dm-crypt and dm-flakey are the only target drivers supporting zoned devices and using dmacceptpartial_bio().

In the case of dm-crypt, this function is used to split BIOs to the internal maxwritesize limit (which will be suppressed in a different patch). However, since cryptallocbuffer() uses a bioset allowing only up to BIOMAXVECS (256) vectors in a BIO. The dm-crypt device maxsegments limit, which is not set and so default to BLKMAX_SEGMENTS (128), must thus be respected and write BIOs split accordingly.

In the case of dm-flakey, since zone append emulation is not required, the block layer zone write plugging is not used and no splitting of BIOs required.

Modify the function dmzonebioneedssplit() to use the block layer helper function bioneedszonewriteplugging() to force a call to biosplittolimits() in dmsplitandprocessbio(). This allows DM target drivers to avoid using dmacceptpartialbio() for write operations on zoned DM devices.