On May 22, 2012, at 6:41 AM, Jeff Layton wrote:
> statd drops all capabilities except for CAP_NET_BIND when it starts. It's
> possible though that if it ever had a compromise that an attacker would be
> able to invoke a setuid process (or something with file capabilities) in
> order to reinstate some caps.
>
> This could happen as a result of the daemon becoming compromised, or
> possibly as a result of the ha-callout program becoming compromised.
>
> In order to prevent that, have statd also prune the capability bounding
> set to nothing prior to dropping capabilities. That ensures that the
> process won't be able to reacquire capabilities via any means --
> including exec'ing a setuid program.
>
> We do however need to be cognizant of the fact that PR_CAPBSET_DROP was
> only added in 2.6.25, so check to make sure that #define exists via
> autoconf before we rely on it. In order to do that, we must add
> ax_check_define.m4 from the GNU autoconf macro archive.
>
> Furthermore, do a runtime check to see if /proc/sys/kernel/cap-bound
> exists before attempting to clear the bounding set. If it does, then
> don't bother trying since it won't work. In that event though, do
> throw a warning however since the presence of that file indicates that
> there is a disconnect between the build and runtime environments.
>
> Cc: Chuck Lever <chuck.lever@xxxxxxxxxx>
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
Acked-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
> ---
> support/nsm/file.c | 57 ++++++++++++++++++++++++++++++++++++++++++++++++++-
> 1 files changed, 55 insertions(+), 2 deletions(-)
>
> diff --git a/support/nsm/file.c b/support/nsm/file.c
> index 5dd52c1..5476446 100644
> --- a/support/nsm/file.c
> +++ b/support/nsm/file.c
> @@ -338,10 +338,10 @@ nsm_is_default_parentdir(void)
> *
> * Returns true if successful, or false if some error occurred.
> */
> +#ifdef HAVE_SYS_CAPABILITY_H
> static _Bool
> nsm_clear_capabilities(void)
> {
> -#ifdef HAVE_SYS_CAPABILITY_H
> cap_t caps;
>
> caps = cap_from_text("cap_net_bind_service=ep");
> @@ -357,10 +357,60 @@ nsm_clear_capabilities(void)
> }
>
> (void)cap_free(caps);
> -#endif
> return true;
> }
>
> +#define CAP_BOUND_PROCFILE "/proc/sys/kernel/cap-bound"
> +static _Bool
> +prune_bounding_set(void)
> +{
> +#ifdef PR_CAPBSET_DROP
> + int ret;
> + unsigned long i;
> + struct stat st;
> +
> + /*
> + * Prior to kernel 2.6.25, the capabilities bounding set was a global
> + * value. Check to see if /proc/sys/kernel/cap-bound exists and don't
> + * bother to clear the bounding set if it does.
> + */
> + ret = stat(CAP_BOUND_PROCFILE, &st);
> + if (!ret) {
> + xlog(L_WARNING, "%s exists. Not attempting to clear "
> + "capabilities bounding set.",
> + CAP_BOUND_PROCFILE);
> + return true;
> + } else if (errno != ENOENT) {
> + /* Warn, but attempt to clear the bounding set anyway. */
> + xlog(L_WARNING, "Unable to stat %s: %m", CAP_BOUND_PROCFILE);
> + }
> +
> + /* prune the bounding set to nothing */
> + for (i = 0; i <= CAP_LAST_CAP; ++i) {
> + ret = prctl(PR_CAPBSET_DROP, i, 0, 0, 0);
> + if (ret) {
> + xlog(L_ERROR, "Unable to prune capability %lu from "
> + "bounding set: %m", i);
> + return false;
> + }
> + }
> +#endif /* PR_CAPBSET_DROP */
> + return true;
> +}
> +#else /* !HAVE_SYS_CAPABILITY_H */
> +static _Bool
> +nsm_clear_capabilities(void)
> +{
> + return true;
> +}
> +
> +static _Bool
> +prune_bounding_set(void)
> +{
> + return true;
> +}
> +#endif /* HAVE_SYS_CAPABILITY_H */
> +
> /**
> * nsm_drop_privileges - drop root privileges
> * @pidfd: file descriptor of a pid file
> @@ -393,6 +443,9 @@ nsm_drop_privileges(const int pidfd)
> return false;
> }
>
> + if (!prune_bounding_set())
> + return false;
> +
> if (st.st_uid == 0) {
> xlog_warn("Running as root. "
> "chown %s to choose different user", nsm_base_dirname);
> --
> 1.7.7.6
>
> --
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--
Chuck Lever
chuck[dot]lever[at]oracle[dot]com
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