Affected: Linux

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

btrfs: add handling for RAID1C23/DUP to btrfs_reduce_alloc_profile

Callers of `btrfs_reduce_alloc_profile` expect it to return exactly one allocation profile flag, and failing to do so may ultimately result in a WARN_ON and remount-ro when allocating new blocks, like the below transaction abort on 6.1.

`btrfs_reduce_alloc_profile` has two ways of determining the profile, first it checks if a conversion balance is currently running and uses the profile we're converting to. If no balance is currently running, it returns the max-redundancy profile which at least one block in the selected block group has.

This works by simply checking each known allocation profile bit in redundancy order. However, `btrfs_reduce_alloc_profile` has not been updated as new flags have been added - first with the `DUP` profile and later with the RAID1C34 profiles.

Because of the way it checks, if we have blocks with different profiles and at least one is known, that profile will be selected. However, if none are known we may return a flag set with multiple allocation profiles set.

This is currently only possible when a balance from one of the three unhandled profiles to another of the unhandled profiles is canceled after allocating at least one block using the new profile.

In that case, a transaction abort like the below will occur and the filesystem will need to be mounted with -o skip_balance to get it mounted rw again (but…