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 dm_accept_partial_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 dm_accept_partial_bio().

In the case of dm-crypt, this function is used to split BIOs to the

internal max_write_size limit (which will be suppressed in a different

patch). However, since crypt_alloc_buffer() uses a bioset allowing only

up to BIO_MAX_VECS (256) vectors in a BIO. The dm-crypt device

max_segments limit, which is not set and so default to BLK_MAX_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 dm_zone_bio_needs_split() to use the block layer

helper function bio_needs_zone_write_plugging() to force a call to

bio_split_to_limits() in dm_split_and_process_bio(). This allows DM

target drivers to avoid using dm_accept_partial_bio() for write

operations on zoned DM devices.