Re: [PATCH RFC v2 0/3] pidfs: file handle preliminaries

Jeff Layton
2024-11-29T14:27:38Z

On Fri, 2024-11-29 at 14:02 +0100, Christian Brauner wrote:
> Hey,
>
> This reworks the inode number allocation for pidfs in order to support
> file handles properly.
>
> Recently we received a patchset that aims to enable file handle encoding
> and decoding via name_to_handle_at(2) and open_by_handle_at(2).
>
> A crucical step in the patch series is how to go from inode number to
> struct pid without leaking information into unprivileged contexts. The
> issue is that in order to find a struct pid the pid number in the
> initial pid namespace must be encoded into the file handle via
> name_to_handle_at(2). This can be used by containers using a separate
> pid namespace to learn what the pid number of a given process in the
> initial pid namespace is. While this is a weak information leak it could
> be used in various exploits and in general is an ugly wart in the
> design.
>
> To solve this problem a new way is needed to lookup a struct pid based
> on the inode number allocated for that struct pid. The other part is to
> remove the custom inode number allocation on 32bit systems that is also
> an ugly wart that should go away.
>
> So, a new scheme is used that I was discusssing with Tejun some time
> back. A cyclic ida is used for the lower 32 bits and a the high 32 bits
> are used for the generation number. This gives a 64 bit inode number
> that is unique on both 32 bit and 64 bit. The lower 32 bit number is
> recycled slowly and can be used to lookup struct pids.
>
> Thanks!
> Christian
>
> ---
> Changes in v2:
> - Remove __maybe_unused pidfd_ino_get_pid() function that was only there
> for initial illustration purposes.
> - Link to v1: https://lore.kernel.org/r/20241128-work-pidfs-v1-0-80f267639d98@kernel.org
>
> ---
> Christian Brauner (3):
> pidfs: rework inode number allocation
> pidfs: remove 32bit inode number handling
> pidfs: support FS_IOC_GETVERSION
>
> fs/pidfs.c | 118 ++++++++++++++++++++++++++++++++------------------
> include/linux/pidfs.h | 2 +
> kernel/pid.c | 14 +++---
> 3 files changed, 86 insertions(+), 48 deletions(-)
> ---
> base-commit: b86545e02e8c22fb89218f29d381fa8e8b91d815
> change-id: 20241128-work-pidfs-2bd42c7ea772
>
>

This seems like a good stopgap fix until we can sort out how to get to
64-bit inode numbers internally everywhere.

Reviewed-by: Jeff Layton <jlayton@kernel.org>