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>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
[PM: minor comment and variable tweaks, approved by RS]
Signed-off-by: Paul Moore <paul@paul-moore.com>