[PATCH] selinux: Perform both commoncap and selinux xattr checks

[serge@hallyn.com (Serge E. Hallyn)]

Quoting Eric W. Biederman (ebiederm at xmission.com):
> -UID: 28009                                                  
> Status: O
> 
> 
> When selinux is loaded the relax permission checks for writing
> security.capable are not honored.  Which keeps file capabilities
> from being used in user namespaces.
> 
> Stephen Smalley <sds@tycho.nsa.gov> writes:
> > Originally SELinux called the cap functions directly since there was no
> > stacking support in the infrastructure and one had to manually stack a
> > secondary module internally.  inode_setxattr and inode_removexattr
> > however were special cases because the cap functions would check
> > CAP_SYS_ADMIN for any non-capability attributes in the security.*
> > namespace, and we don't want to impose that requirement on setting
> > security.selinux.  Thus, we inlined the capabilities logic into the
> > selinux hook functions and adapted it appropriately.
> 
> Now that the permission checks in commoncap have evolved this
> inlining of their contents has become a problem.  So restructure
> selinux_inode_removexattr, and selinux_inode_setxattr to call
> both the corresponding cap_inode_ function and dentry_has_perm
> when the attribute is not a selinux security xattr.   This ensures
> the policies of both commoncap and selinux are enforced.
> 
> This results in smack and selinux having the same basic structure
> for setxattr and removexattr.  Performing their own special permission
> checks when it is their modules xattr being written to, and deferring
> to commoncap when that is not the case.  Then finally performing their
> generic module policy on all xattr writes.
> 
> This structure is fine when you only consider stacking with the
> commoncap lsm, but it becomes a problem if two lsms that don't want
> the commoncap security checks on their own attributes need to be
> stack.  This means there will need to be updates in the future as lsm
> stacking is improved, but at least now the structure between smack and
> selinux is common making the code easier to refactor.
> 
> This change also has the effect that selinux_linux_setotherxattr becomes
> unnecessary so it is removed.
> 
> Fixes: 8db6c34f1dbc ("Introduce v3 namespaced file capabilities")
> Fixes: 7bbf0e052b76 ("[PATCH] selinux merge")
> Historical Tree: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

This is hairy, but it looks right:

Reviewed-by: Serge Hallyn <serge@hallyn.com>

thanks,
-serge
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