kern: add a security knob to disable unprivileged access to kenv

We sometimes store sensitive things in the kenv that get zapped, but we
really shouldn't rely on that zapping to actually happen.  Most
unprivileged processes don't really need to read from the kernel
environment in the first place, so add a knob that allows it to be
disabled.

Note that we consider jailed root to be unprivileged from this
perspective; they have their own meta/env concepts and we should
encourage users to take advantage of those for passing information to
jails.

"Hey we should do something about that":	dch
Reviewed by:	imp, ziaee, zlei (all slightly previous version)
Differential Revision:	https://reviews.freebsd.org/D57697

1 files changed, 3 insertions, 1 deletions

diff --git a/share/man/man7/security.7 b/share/man/man7/security.7
index 395cf082c2fc..bec5c4f0b001 100644
--- a/share/man/man7/security.7
+++ b/share/man/man7/security.7
@@ -26,7 +26,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd March 22, 2026
+.Dd June 22, 2026
 .Dt SECURITY 7
 .Os
 .Sh NAME
@@ -987,6 +987,8 @@ and usual termination signals like
 and
 .Dv SIGTERM ,
 to the processes executing programs with changed uids.
+.It Va security.bsd.unprivileged_kenv_read
+Controls availability of kernel environment variables to non-root users.
 .It Va security.bsd.unprivileged_proc_debug
 Controls availability of the process debugging facilities to non-root users.
 See also