Re: [PATCH v2] include/uapi/linux/xfrm.h: Fix XFRM_MSG_MAPPING ABI breakage

[Ondrej Mosnacek]

Hi,

On Sun, Sep 12, 2021 at 2:23 PM Eugene Syromiatnikov <esyr@xxxxxxxxxx> wrote:
> Commit 2d151d39073a ("xfrm: Add possibility to set the default to block
> if we have no policy") broke ABI by changing the value of the XFRM_MSG_MAPPING
> enum item, thus also evading the build-time check
> in security/selinux/nlmsgtab.c:selinux_nlmsg_lookup for presence of proper
> security permission checks in nlmsg_xfrm_perms.  Fix it by placing
> XFRM_MSG_SETDEFAULT/XFRM_MSG_GETDEFAULT to the end of the enum, right before
> __XFRM_MSG_MAX, and updating the nlmsg_xfrm_perms accordingly.
>
> Fixes: 2d151d39073a ("xfrm: Add possibility to set the default to block if we have no policy")
> References: https://lore.kernel.org/netdev/20210901151402.GA2557@xxxxxxxxxxxx/
> Signed-off-by: Eugene Syromiatnikov <esyr@xxxxxxxxxx>
> ---
> v2:
>  - Updated SELinux nlmsg_xfrm_perms permissions table and selinux_nlmsg_lookup
>    build-time check accordingly.
>
> v1: https://lore.kernel.org/lkml/20210901153407.GA20446@xxxxxxxxxxxxxxxxx/
> ---
>  include/uapi/linux/xfrm.h   | 6 +++---
>  security/selinux/nlmsgtab.c | 4 +++-
>  2 files changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/include/uapi/linux/xfrm.h b/include/uapi/linux/xfrm.h
> index b96c1ea..26f456b1 100644
> --- a/include/uapi/linux/xfrm.h
> +++ b/include/uapi/linux/xfrm.h
> @@ -213,13 +213,13 @@ enum {
>         XFRM_MSG_GETSPDINFO,
>  #define XFRM_MSG_GETSPDINFO XFRM_MSG_GETSPDINFO
>
> +       XFRM_MSG_MAPPING,
> +#define XFRM_MSG_MAPPING XFRM_MSG_MAPPING
> +
>         XFRM_MSG_SETDEFAULT,
>  #define XFRM_MSG_SETDEFAULT XFRM_MSG_SETDEFAULT
>         XFRM_MSG_GETDEFAULT,
>  #define XFRM_MSG_GETDEFAULT XFRM_MSG_GETDEFAULT
> -
> -       XFRM_MSG_MAPPING,
> -#define XFRM_MSG_MAPPING XFRM_MSG_MAPPING

Perhaps it would be a good idea to put a comment here to make it less
likely that this repeats in the future. Something like:

/* IMPORTANT: Only insert new entries right above this line, otherwise
you break ABI! */

>         __XFRM_MSG_MAX
>  };
>  #define XFRM_MSG_MAX (__XFRM_MSG_MAX - 1)
> diff --git a/security/selinux/nlmsgtab.c b/security/selinux/nlmsgtab.c
> index d59276f..94ea2a8 100644
> --- a/security/selinux/nlmsgtab.c
> +++ b/security/selinux/nlmsgtab.c
> @@ -126,6 +126,8 @@ static const struct nlmsg_perm nlmsg_xfrm_perms[] =
>         { XFRM_MSG_NEWSPDINFO,  NETLINK_XFRM_SOCKET__NLMSG_WRITE },
>         { XFRM_MSG_GETSPDINFO,  NETLINK_XFRM_SOCKET__NLMSG_READ  },
>         { XFRM_MSG_MAPPING,     NETLINK_XFRM_SOCKET__NLMSG_READ  },
> +       { XFRM_MSG_SETDEFAULT,  NETLINK_XFRM_SOCKET__NLMSG_WRITE },
> +       { XFRM_MSG_GETDEFAULT,  NETLINK_XFRM_SOCKET__NLMSG_READ  },
>  };
>
>  static const struct nlmsg_perm nlmsg_audit_perms[] =
> @@ -189,7 +191,7 @@ int selinux_nlmsg_lookup(u16 sclass, u16 nlmsg_type, u32 *perm)
>                  * structures at the top of this file with the new mappings
>                  * before updating the BUILD_BUG_ON() macro!
>                  */
> -               BUILD_BUG_ON(XFRM_MSG_MAX != XFRM_MSG_MAPPING);
> +               BUILD_BUG_ON(XFRM_MSG_MAX != XFRM_MSG_GETDEFAULT);
>                 err = nlmsg_perm(nlmsg_type, perm, nlmsg_xfrm_perms,
>                                  sizeof(nlmsg_xfrm_perms));
>                 break;
> --
> 2.1.4
>


-- 
Ondrej Mosnacek
Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.