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

Amir Goldstein
2024-11-29T14:34:53Z

On Fri, Nov 29, 2024 at 3:27 PM Jeff Layton <jlayton@kernel.org> wrote:
>
> 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>

Yep. look good

Reviewed-by: Amir Goldstein <amir73il@gmail.com>