Re: [PATCH] uapi: export PROCFS_ROOT_INO

[Christian Brauner <brauner@kernel.org>]

On Tue, 08 Jul 2025 23:21:51 +1000, Aleksa Sarai wrote:
> The root inode of /proc having a fixed inode number has been part of the
> core kernel ABI since its inception, and recently some userspace
> programs (mainly container runtimes) have started to explicitly depend
> on this behaviour.
> 
> The main reason this is useful to userspace is that by checking that a
> suspect /proc handle has fstype PROC_SUPER_MAGIC and is PROCFS_ROOT_INO,
> they can then use openat2(RESOLVE_{NO_{XDEV,MAGICLINK},BENEATH}) to
> ensure that there isn't a bind-mount that replaces some procfs file with
> a different one. This kind of attack has lead to security issues in
> container runtimes in the past (such as CVE-2019-19921) and libraries
> like libpathrs[1] use this feature of procfs to provide safe procfs
> handling functions.
> 
> [...]

Applied to the vfs-6.17.nsfs branch of the vfs/vfs.git tree.
Patches in the vfs-6.17.nsfs branch should appear in linux-next soon.

Please report any outstanding bugs that were missed during review in a
new review to the original patch series allowing us to drop it.

It's encouraged to provide Acked-bys and Reviewed-bys even though the
patch has now been applied. If possible patch trailers will be updated.

Note that commit hashes shown below are subject to change due to rebase,
trailer updates or similar. If in doubt, please check the listed branch.

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
branch: vfs-6.17.nsfs

[1/1] uapi: export PROCFS_ROOT_INO
      https://git.kernel.org/vfs/vfs/c/76fdb7eb4e1c