Episode Details

36 year old UFS bug fixed, a BSD for the road, automatic upgrades with OpenBSD, DTrace ext2fs support in FreeBSD, Dedicated SSH tunnel user, upgrading VMM VMs to OpenBSD 6.5, and more.

Headlines

36+ year old bug in FFS/UFS discovered and patched

This update eliminates a kernel stack disclosure bug in UFS/FFS directory entries that is caused by uninitialized directory entry padding written to the disk.

• When the directory entry is written to disk, it is written as a full 32bit entry, and the unused bytes were not initialized, so could possibly contain sensitive data from the kernel stack It can be viewed by any user with read access to that directory. Up to 3 bytes of kernel stack are disclosed per file entry, depending on the the amount of padding the kernel needs to pad out the entry to a 32 bit boundary. The offset in the kernel stack that is disclosed is a function of the filename size. Furthermore, if the user can create files in a directory, this 3 byte window can be expanded 3 bytes at a time to a 254 byte window with 75% of the data in that window exposed. The additional exposure is done by removing the entry, creating a new entry with a 4-byte longer name, extracting 3 more bytes by reading the directory, and repeating until a 252 byte name is created. This exploit works in part because the area of the kernel stack that is being disclosed is in an area that typically doesn't change that often (perhaps a few times a second on a lightly loaded system), and these file creates and unlinks themselves don't overwrite the area of kernel stack being disclosed. It appears that this bug originated with the creation of the Fast File System in 4.1b-BSD (Circa 1982, more than 36 years ago!), and is likely present in every Unix or Unix-like system that uses UFS/FFS. Amazingly, nobody noticed until now. This update also adds the -z flag to fsck_ffs to have it scrub the leaked information in the name padding of existing directories. It only needs to be run once on each UFS/FFS filesystem after a patched kernel is installed and running. Submitted by: David G. Lawrence dg@dglawrence.com
• So a patched kernel will no longer leak this data, and running the fsck_ffs -z command will erase any leaked data that may exist on your system
• OpenBSD commit with additional detail on mitigations The impact on OpenBSD is very limited: 1 - such stack bytes can be found in raw-device reads, from group operator. If you can read the raw disks you can undertake other more powerful actions. 2 - read(2) upon directory fd was disabled July 1997 because I didn't like how grep * would display garbage and mess up the tty, and applying vis(3) for just directory reads seemed silly. read(2) was changed to return 0 (EOF). Sep 2016 this was further changed to EISDIR, so you still cannot see the bad bytes. 3 - In 2013 when guenther adapted the getdents(2) directory-reading system call to 64-bit ino_t, the userland data format changed to 8-byte-alignment, making it incompatible with the 4-byte-alignment UFS on-disk format. As a result of code refactoring the bad bytes were not copied to userland. Bad bytes will remain in old directories on old filesystems, but nothing makes those bytes user visible. There will be no errata or syspatch issued. I urge other systems which do expose the information to userland to issue errata quickly, since this is a 254 byte infoleak of the stack which is great for ROP-chain building to attack some other bug. Especially if the kernel has no layout/link-order randomization ...

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