Re: [PATCH v12 1/4] security: Allow all LSMs to provide xattrs for inode_init_security hook

Paul Moore
2023-07-07T14:35:14Z

On Fri, Jul 7, 2023 at 2:49 AM Roberto Sassu <roberto.sassu@huawei.com> wrote:
> > From: Paul Moore [mailto:paul@paul-moore.com]
> > Sent: Friday, July 7, 2023 3:44 AM
> > On Jun 10, 2023 Roberto Sassu <roberto.sassu@huaweicloud.com> wrote:
> > >
> > > Currently, the LSM infrastructure supports only one LSM providing an xattr
> > > and EVM calculating the HMAC on that xattr, plus other inode metadata.
> > >
> > > Allow all LSMs to provide one or multiple xattrs, by extending the security
> > > blob reservation mechanism. Introduce the new lbs_xattr_count field of the
> > > lsm_blob_sizes structure, so that each LSM can specify how many xattrs it
> > > needs, and the LSM infrastructure knows how many xattr slots it should
> > > allocate.
> > >
> > > Modify the inode_init_security hook definition, by passing the full
> > > xattr array allocated in security_inode_init_security(), and the current
> > > number of xattr slots in that array filled by LSMs. The first parameter
> > > would allow EVM to access and calculate the HMAC on xattrs supplied by
> > > other LSMs, the second to not leave gaps in the xattr array, when an LSM
> > > requested but did not provide xattrs (e.g. if it is not initialized).
> > >
> > > Introduce lsm_get_xattr_slot(), which LSMs can call as many times as the
> > > number specified in the lbs_xattr_count field of the lsm_blob_sizes
> > > structure. During each call, lsm_get_xattr_slot() increments the number of
> > > filled xattrs, so that at the next invocation it returns the next xattr
> > > slot to fill.
> > >
> > > Cleanup security_inode_init_security(). Unify the !initxattrs and
> > > initxattrs case by simply not allocating the new_xattrs array in the
> > > former. Update the documentation to reflect the changes, and fix the
> > > description of the xattr name, as it is not allocated anymore.
> > >
> > > Adapt both SELinux and Smack to use the new definition of the
> > > inode_init_security hook, and to call lsm_get_xattr_slot() to obtain and
> > > fill the reserved slots in the xattr array.
> > >
> > > Move the xattr->name assignment after the xattr->value one, so that it is
> > > done only in case of successful memory allocation.
> > >
> > > Finally, change the default return value of the inode_init_security hook
> > > from zero to -EOPNOTSUPP, so that BPF LSM correctly follows the hook
> > > conventions.
> > >
> > > Reported-by: Nicolas Bouchinet <nicolas.bouchinet@clip-os.org>
> > > Link: https://lore.kernel.org/linux-integrity/Y1FTSIo+1x+4X0LS@archlinux/
> > > Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> > > ---
> > > include/linux/lsm_hook_defs.h | 6 +--
> > > include/linux/lsm_hooks.h | 20 ++++++++++
> > > security/security.c | 71 +++++++++++++++++++++++------------
> > > security/selinux/hooks.c | 17 +++++----
> > > security/smack/smack_lsm.c | 25 ++++++------
> > > 5 files changed, 92 insertions(+), 47 deletions(-)
> >
> > Two *very* small suggestions below, but I can make those during the
> > merge if you are okay with that Roberto?
>
> Hi Paul
>
> yes, sure, I'm ok with them. Please make them during the merge.

Great, I'll queue this up for merging once the merge window closes.
I'll send confirmation once it's done but just a heads-up that things
might be a little delayed next week.

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paul-moore.com