On Wed, Mar 3, 2021 at 3:57 AM Tyler Hicks <tyhicks@xxxxxxxxxxxxxxxxxxx> wrote:
> On 2021-02-12 19:59:30, Ondrej Mosnacek wrote:
> > Commit 02a52c5c8c3b ("selinux: move policy commit after updating
> > selinuxfs") moved the selinux_policy_commit() call out of
> > security_load_policy() into sel_write_load(), which caused a subtle yet
> > rather serious bug.
> >
> > The problem is that security_load_policy() passes a reference to the
> > convert_params local variable to sidtab_convert(), which stores it in
> > the sidtab, where it may be accessed until the policy is swapped over
> > and RCU synchronized. Before 02a52c5c8c3b, selinux_policy_commit() was
> > called directly from security_load_policy(), so the convert_params
> > pointer remained valid all the way until the old sidtab was destroyed,
> > but now that's no longer the case and calls to sidtab_context_to_sid()
> > on the old sidtab after security_load_policy() returns may cause invalid
> > memory accesses.
> >
> > This can be easily triggered using the stress test from commit
> > ee1a84fdfeed ("selinux: overhaul sidtab to fix bug and improve
> > performance"):
> > ```
> > function rand_cat() {
> > echo $(( $RANDOM % 1024 ))
> > }
> >
> > function do_work() {
> > while true; do
> > echo -n "system_u:system_r:kernel_t:s0:c$(rand_cat),c$(rand_cat)" \
> > >/sys/fs/selinux/context 2>/dev/null || true
> > done
> > }
> >
> > do_work >/dev/null &
> > do_work >/dev/null &
> > do_work >/dev/null &
> >
> > while load_policy; do echo -n .; sleep 0.1; done
> >
> > kill %1
> > kill %2
> > kill %3
> > ```
> >
> > Fix this by allocating the temporary sidtab convert structures
> > dynamically and passing them among the
> > selinux_policy_{load,cancel,commit} functions.
> >
> > Note that this commit also fixes the minor issue of logging a
> > MAC_POLICY_LOAD audit record in case sel_make_policy_nodes() fails (in
> > which case the new policy isn't actually loaded).
> >
> > Fixes: 02a52c5c8c3b ("selinux: move policy commit after updating selinuxfs")
> > Signed-off-by: Ondrej Mosnacek <omosnace@xxxxxxxxxx>
>
> Tested-by: Tyler Hicks <tyhicks@xxxxxxxxxxxxxxxxxxx>
> Reviewed-by: Tyler Hicks <tyhicks@xxxxxxxxxxxxxxxxxxx>
>
> Feel free to leave those tags on your v3 submission after making the two
> small changes requested by Paul.
Thanks!
--
Ondrej Mosnacek
Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.
