In the Linux kernel, the following vulnerability has been resolved:
binfmt_misc: don't let an 'F' entry pin its own instance
An entry registered with 'F' opens its interpreter at registration time and holds that file until the entry is freed. Any entry nobody removes by hand only gets closed once the binfmt_misc superblock is shut down. If the interpreter lives on a mount that keeps that superblock alive the two pin each other:
binfmt_misc sb -> inode -> entry -> interp_file -> vfsmount -> binfmt_misc sb
TL;DR the file is never closed. Once the mount namespace is gone there is nothing left to unregister through either.
There are two ways to trigger this bug:
- Point the interpreter at the instance itself. Its files are regular
- Use the instance as an overlayfs lower layer. The overlay superblock
That's a DoS. And it isn't only the superblock that leaks. It pins the user namespace it was mounted in, so every iteration permanently eats one of the caller's user namespace charges.
So let's just do the sane thing. SB_I_NOEXEC makes open_exec() fail on the instance's own files and s_stack_depth makes overlayfs reject the layer before it ever takes a clone. That also covers the ecryptfs and fuse passthrough variants. What 'F' promises is unchanged.
The stable tag is narrower than the Fixes tags on purpose. Before sandboxed mounts this needed global root against the single instance everyone shares, and the change doesn't apply to those trees anyway.
Note that SB_I_NODEV is implicitly raised for userns mounts but raise it explicitly here as well.