[PATCH] fix namespaced fscaps when !CONFIG_SECURITY

[James Morris]

[Adding LSM list]

On Tue, 17 Nov 2020, Serge E. Hallyn wrote:

> Namespaced file capabilities were introduced in 8db6c34f1dbc .
> When userspace reads an xattr for a namespaced capability, a
> virtualized representation of it is returned if the caller is
> in a user namespace owned by the capability's owning rootid.
> The function which performs this virtualization was not hooked
> up if CONFIG_SECURITY=n.  Therefore in that case the original
> xattr was shown instead of the virtualized one.
> 
> To test this using libcap-bin (*1),
> 
> $ v=$(mktemp)
> $ unshare -Ur setcap cap_sys_admin-eip $v
> $ unshare -Ur setcap -v cap_sys_admin-eip $v
> /tmp/tmp.lSiIFRvt8Y: OK
> 
> "setcap -v" verifies the values instead of setting them, and
> will check whether the rootid value is set.  Therefore, with
> this bug un-fixed, and with CONFIG_SECURITY=n, setcap -v will
> fail:
> 
> $ v=$(mktemp)
> $ unshare -Ur setcap cap_sys_admin=eip $v
> $ unshare -Ur setcap -v cap_sys_admin=eip $v
> nsowner[got=1000, want=0],/tmp/tmp.HHDiOOl9fY differs in []
> 
> Fix this bug by calling cap_inode_getsecurity() in
> security_inode_getsecurity() instead of returning
> -EOPNOTSUPP, when CONFIG_SECURITY=n.
> 
> *1 - note, if libcap is too old for getcap to have the '-n'
> option, then use verify-caps instead.
> 
> Signed-off-by: Serge Hallyn <serge at hallyn.com>
> Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1593431
> Cc: Hervé Guillemet <herve at guillemet.org>
> Cc: Andrew G. Morgan <morgan at kernel.org>
> Cc: Casey Schaufler <casey at schaufler-ca.com>
> ---
>  include/linux/security.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/security.h b/include/linux/security.h
> index bc2725491560..39642626a707 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -869,7 +869,7 @@ static inline int security_inode_killpriv(struct dentry *dentry)
>  
>  static inline int security_inode_getsecurity(struct inode *inode, const char *name, void **buffer, bool alloc)
>  {
> -	return -EOPNOTSUPP;
> +	return cap_inode_getsecurity(inode, name, buffer, alloc);
>  }
>  
>  static inline int security_inode_setsecurity(struct inode *inode, const char *name, const void *value, size_t size, int flags)
> 

-- 
James Morris
<jmorris at namei.org>