CVE-2025-38073

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

block: fix race between set_blocksize and read paths

With the new large sector size support, its now the case that set_blocksize can change i_blksize and the folio order in a manner that conflicts with a concurrent reader and causes a kernel crash.

Specifically, lets say that udev-worker calls libblkid to detect the labels on a block device. The read call can create an order-0 folio to read the first 4096 bytes from the disk. But then udev is preempted.

Next, someone tries to mount an 8k-sectorsize filesystem from the same block device. The filesystem calls set_blksize, which sets i_blksize to 8192 and the minimum folio order to 1.

Now udev resumes, still holding the order-0 folio it allocated. It then tries to schedule a read bio and do_mpage_readahead tries to create bufferheads for the folio. Unfortunately, blocks_per_folio == 0 because the page size is 4096 but the blocksize is 8192 so no bufferheads are attached and the bh walk never sets bdev. We then submit the bio with a NULL block device and crash.

Therefore, truncate the page cache after flushing but before updating i_blksize. However, thats not enough – we also need to lock out file IO and page faults during the update. Take both the i_rwsem and the invalidate_lock in exclusive mode for invalidations, and in shared mode for read/write operations.

I dont know if this is the correct fix, but xfs/259 found it.

Affected Software

References

• https://git.kernel.org/stable/c/64f505b08e0cfd8163491c8c082d4f47a88e51d4
• https://git.kernel.org/stable/c/8c5cf440a378801d313eb58be996fdc81a8878a4
• https://git.kernel.org/stable/c/c0e473a0d226479e8e925d5ba93f751d8df628e9