UBUNTU-CVE-2026-97416: Medium severity ubuntu/linux vulnerability

In the Linux kernel, the following vulnerability has been resolved: btrfs: balance: fix potential bg lookup failure in btrfsmayallocdatachunk() [BUG] Running btrfs balance can trigger a null-ptr-deref before relocating a data chunk when metadata corruption leaves a chunk in the chunk tree without a corresponding block group in the in-memory cache: KASAN: null-ptr-deref in range [0x0000000000000088-0x000000000000008f] RIP: 0010:btrfsmayallocdatachunk+0x40/0x1c0 fs/btrfs/volumes.c:3601 Call Trace: btrfsbalance fs/btrfs/volumes.c:4217 [inline] btrfsbalance+0x2516/0x42b0 fs/btrfs/volumes.c:4604 btrfsioctlbalance fs/btrfs/ioctl.c:3577 [inline] btrfsioctl+0x25cf/0x5b90 fs/btrfs/ioctl.c:5313 ... [CAUSE] btrfsbalance() iterates the on-disk chunk tree and passes the chunk logical bytenr to btrfsmayallocdatachunk() before relocating a data chunk. That helper then queries the in-memory block group cache: cache = btrfslookupblockgroup(fsinfo, chunkoffset); chunktype = cache->flags; / cache may be NULL / A corrupt image can contain a chunk item whose matching block group item is missing, so no block group is ever inserted into the cache. In that case btrfslookupblockgroup() returns NULL. The code only guards this with ASSERT(cache), which becomes a no-op when CONFIGBTRFSASSERT is disabled. The subsequent dereference of cache->flags therefore crashes the kernel. [FIX] Add a NULL check after btrfslookupblockgroup() in btrfsmayallocdatachunk() and print and error message for clarity.