A flaw in the Linux Kernel found. If unprivileged users can mount FUSE filesystems, then can trigger use after free (UAF) that reads of write() buffers, allowing theft of (partial) /etc/shadow hashes or any other data from filesystem.

FUSE allows the userspace filesystem to specify on FUSEOPEN whether the file should use the normal kernel pagecache for handling read()/write() or just send FUSEREAD/FUSEWRITE requests directly to the userspace filesystem (using the flag FOPENDIRECTIO in fuseopenout::openflags).

In FOPENDIRECTIO mode, fusefilewriteiter() calls fusedirectwriteiter(), which normally calls fusedirectio(), which then imports the write buffer with fusegetuserpages(), which uses iovitergetpages() to grab references to userspace pages instead of actually copying memory.

On the filesystem device side, these pages can then either be read to userspace (via fusedevread()), or splice()d over into a pipe using fusedevspliceread() as pipe buffers with &nostealpipebufops.

This is wrong because after fusedevdoread() unlocks the FUSE request, the userspace filesystem can mark the request as completed, causing write() to return. At that point, the write buffer may be reused for other purposes, and the userspace filesystem should no longer have access to it.

A bug was found in Moby (Docker Engine) where containers were incorrectly started with non-empty inheritable Linux process capabilities, creating an atypical Linux environment and enabling programs with inheritable file capabilities to elevate those capabilities to the permitted set during execve(2). Normally, when executable programs have specified permitted file capabilities, otherwise unprivileged users and processes can execute those programs and gain the specified file capabilities up to the bounding set. Due to this bug, containers which included executable programs with inheritable file capabilities allowed otherwise unprivileged users and processes to additionally gain these inheritable file capabilities up to the container's bounding set. Containers which use Linux users and groups to perform privilege separation inside the container are most directly impacted.