Re: [RFC PATCH] security: Allow dropping bounding set process-wide

[Serge E. Hallyn]

Jinjie, could you reproduce your timing experiments against the
SetProc code?

On Tue, Sep 22, 2026 at 07:10:11PM -0700, Andrew G. Morgan wrote:
> The https://pkg.go.dev/kernel.org/pub/linux/libs/security/libcap/cap#IAB.SetProc
> already handles this for the whole process. It can be run from main()
> if you need it to happen early, and it will track down all of the
> threads in the runtime.
> 
> To Serge's point, I am also curious what benefit there is from doing
> it more quickly. After all, this is pretty much a one-time function
> request for any executable.
> 
> FYI The "sendmail capabilities bug" reference is written up here:
> https://sites.google.com/site/fullycapable/thesendmailcapabilitiesissue
> 
> Cheers
> 
> Andrew
> 
> On Tue, Sep 22, 2026 at 10:01 AM Serge E. Hallyn <serge@xxxxxxxxxx> wrote:
> >
> > On Tue, Sep 22, 2026 at 05:58:16PM +0800, Jinjie Ruan wrote:
> > > The capability bounding set is per-thread: PR_CAPBSET_DROP only affects
> > > the calling thread, since its bounding set lives in the per-task struct
> > > cred.  User space that wants to drop capabilities for a whole process
> > > must therefore invoke PR_CAPBSET_DROP once per capability per thread,
> > > which on a many-threaded process is expensive, and from a Go runtime
> > > requires stopping the world and signalling every thread.
> > >
> > > Add PR_CAPBSET_DROP_MASK, an opt-in prctl that removes a set of
> > > capabilities, given as a 64-bit mask in arg2 (low 32 bits) and arg3
> > > (high 32 bits), from the bounding set of every thread of the calling
> > > thread group in a single call.
> > >
> > > The calling thread drops the capabilities synchronously, last; every
> > > sibling that still holds any of them is asked to drop them through a
> > > task_work item, so that it applies the drop in its own context.  This
> > > avoids racing with a sibling's concurrent credential updates such as
> > > setuid() or capset().  The call does not wait for the siblings: a
> > > sibling may be parked in a wait that is not woken by TIF_NOTIFY_SIGNAL
> > > (e.g. futex), so waiting could block for an unbounded time.  This is
> > > still safe, because TIF_NOTIFY_SIGNAL is handled on the way out to user
> > > mode, so a sibling applies the drop before executing any further
> > > userspace code.  It does not synchronize against a sibling that is
> > > concurrently creating threads, so callers must keep the thread group
> > > quiescent while dropping.
> > >
> > > Measured with gVisor's "bounding set trimmed" boot phase on an arm64 KVM
> > > guest (medians over repeated boots):
> > >   - 8 vCPUs:  ~1.6ms  -> ~0.10ms
> > >   - 32 vCPUs: ~3.5ms  -> ~0.11ms
> > >   - 64 vCPUs: ~10.1ms -> ~0.1-0.6ms
> > >   - cost goes from O(capabilities * threads) stop-the-world prctls to a
> > >     single thread-group walk.
> >
> > That is impressive, but please do detail the specific use case where
> > you need to drop from the bounding set after the go scheduler has started.
> > I can imagine some cases where you need to do some early setup and then
> > want to drop privileges, but you could also do that by re-exec'ing, so
> > I'd like to hear specifics.
> >
> > This makes me nervous, reminding me of the 'sendmail capabilities bug'.
> > If some program specifically locks down one thread, I could imagine the
> > locked down thread forcing wrong behavior from the privileged threads
> > by calling this.
> >
> > >
> > > Cc: Serge Hallyn <serge@xxxxxxxxxx>
> > > Cc: Paul Moore <paul@xxxxxxxxxxxxxx>
> > > Cc: James Morris <jmorris@xxxxxxxxx>
> > > Cc: Paul Walmsley <pjw@xxxxxxxxxx>
> > > Cc: Thomas Gleixner <tglx@xxxxxxxxxx>
> > > Cc: Zong Li <zong.li@xxxxxxxxxx>
> > > Cc: Deepak Gupta <debug@xxxxxxxxxxxx>
> > > Cc: "Peter Zijlstra (Intel)" <peterz@xxxxxxxxxxxxx>
> > > Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
> > > ---
> > >  include/uapi/linux/prctl.h |   1 +
> > >  security/commoncap.c       | 129 +++++++++++++++++++++++++++++++++++++
> > >  2 files changed, 130 insertions(+)
> > >
> > > diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> > > index b6ec6f693719..750a7824d3bc 100644
> > > --- a/include/uapi/linux/prctl.h
> > > +++ b/include/uapi/linux/prctl.h
> > > @@ -70,6 +70,7 @@
> > >  /* Get/set the capability bounding set (as per security/commoncap.c) */
> > >  #define PR_CAPBSET_READ 23
> > >  #define PR_CAPBSET_DROP 24
> > > +#define PR_CAPBSET_DROP_MASK 83
> > >
> > >  /* Get/set the process' ability to use the timestamp counter instruction */
> > >  #define PR_GET_TSC 25
> > > diff --git a/security/commoncap.c b/security/commoncap.c
> > > index 3399535808fe..ae7ce50a8151 100644
> > > --- a/security/commoncap.c
> > > +++ b/security/commoncap.c
> > > @@ -19,7 +19,14 @@
> > >  #include <linux/hugetlb.h>
> > >  #include <linux/mount.h>
> > >  #include <linux/sched.h>
> > > +#include <linux/cred.h>
> > > +#include <linux/rcupdate.h>
> > > +#include <linux/sched/signal.h>
> > > +#include <linux/sched/task.h>
> > > +#include <linux/slab.h>
> > > +#include <linux/task_work.h>
> > >  #include <linux/prctl.h>
> > > +#include <linux/printk.h>
> > >  #include <linux/securebits.h>
> > >  #include <linux/user_namespace.h>
> > >  #include <linux/binfmts.h>
> > > @@ -1283,6 +1290,123 @@ static int cap_prctl_drop(unsigned long cap)
> > >       return commit_creds(new);
> > >  }
> > >
> > > +/*
> > > + * Structure used to queue process-wide bounding set drops via task_work.
> > > + */
> > > +struct cap_bset_drop_work {
> > > +     struct callback_head work;
> > > +     struct task_struct *task;
> > > +     kernel_cap_t mask;
> > > +     struct cap_bset_drop_work *next;
> > > +};
> > > +
> > > +static void cap_bset_drop_work_fn(struct callback_head *work)
> > > +{
> > > +     struct cap_bset_drop_work *w = container_of(work, struct cap_bset_drop_work, work);
> > > +     struct cred *new = prepare_creds();
> > > +
> > > +     if (!new) {
> > > +             /* Out of memory: bounding set drop failed silently for this thread. */
> > > +             pr_warn_ratelimited("capability bounding set drop failed for pid %d (%s)\n",
> > > +                                 task_pid_nr(current), current->comm);
> > > +             goto out;
> > > +     }
> > > +
> > > +     new->cap_bset = cap_drop(new->cap_bset, w->mask);
> > > +     commit_creds(new);
> > > +
> > > +out:
> > > +     put_task_struct(w->task);
> > > +     kfree(w);
> > > +}
> > > +
> > > +/*
> > > + * cap_bset_drop_process - Drop capabilities from all threads in the group.
> > > + * @mask: Mask of capabilities to drop from the bounding set.
> > > + *
> > > + * Drops @mask from the calling thread synchronously, and queues a task_work
> > > + * item for each sibling thread to safely apply the drop in its own context.
> > > + *
> > > + * The caller must hold CAP_SETPCAP. Thread group must be quiescent to avoid
> > > + * racing with concurrent thread creation.
> > > + *
> > > + * Returns 0 on success, or -ENOMEM if allocations fail (all-or-nothing).
> > > + */
> > > +static int cap_bset_drop_process(kernel_cap_t mask)
> > > +{
> > > +     struct cap_bset_drop_work *list = NULL, *w, *next;
> > > +     struct task_struct *thread;
> > > +     struct cred *new = NULL;
> > > +     int ret = 0;
> > > +
> > > +     rcu_read_lock();
> > > +     for_each_thread(current, thread) {
> > > +             const struct cred *cred;
> > > +
> > > +             if (thread == current || (thread->flags & PF_EXITING))
> > > +                     continue;
> > > +
> > > +             cred = __task_cred(thread);
> > > +             if (cap_isclear(cap_intersect(cred->cap_bset, mask)))
> > > +                     continue;
> > > +
> > > +             w = kmalloc_obj(*w, GFP_ATOMIC);
> > > +             if (!w) {
> > > +                     ret = -ENOMEM;
> > > +                     break;
> > > +             }
> > > +
> > > +             w->task = get_task_struct(thread);
> > > +             w->mask = mask;
> > > +             w->next = list;
> > > +             list = w;
> > > +     }
> > > +     rcu_read_unlock();
> > > +
> > > +     if (!ret) {
> > > +             new = prepare_creds();
> > > +             if (!new)
> > > +                     ret = -ENOMEM;
> > > +     }
> > > +
> > > +     if (ret) {
> > > +             while (list) {
> > > +                     next = list->next;
> > > +                     put_task_struct(list->task);
> > > +                     kfree(list);
> > > +                     list = next;
> > > +             }
> > > +             return ret;
> > > +     }
> > > +
> > > +     for (w = list; w; w = next) {
> > > +             next = w->next;
> > > +             init_task_work(&w->work, cap_bset_drop_work_fn);
> > > +             if (task_work_add(w->task, &w->work, TWA_SIGNAL)) {
> > > +                     put_task_struct(w->task);
> > > +                     kfree(w);
> > > +             }
> > > +     }
> > > +
> > > +     new->cap_bset = cap_drop(new->cap_bset, mask);
> > > +     commit_creds(new);
> > > +
> > > +     return 0;
> > > +}
> > > +
> > > +static int cap_prctl_drop_mask(unsigned long low, unsigned long high)
> > > +{
> > > +     kernel_cap_t mask = mk_kernel_cap((u32)low, (u32)high);
> > > +
> > > +     if (cap_isclear(mask))
> > > +             return 0;
> > > +
> > > +     if (!ns_capable(current_user_ns(), CAP_SETPCAP))
> > > +             return -EPERM;
> > > +
> > > +     return cap_bset_drop_process(mask);
> > > +}
> > > +
> > >  /**
> > >   * cap_task_prctl - Implement process control functions for this security module
> > >   * @option: The process control function requested
> > > @@ -1313,6 +1437,11 @@ int cap_task_prctl(int option, unsigned long arg2, unsigned long arg3,
> > >       case PR_CAPBSET_DROP:
> > >               return cap_prctl_drop(arg2);
> > >
> > > +     case PR_CAPBSET_DROP_MASK:
> > > +             if (arg4 || arg5)
> > > +                     return -EINVAL;
> > > +             return cap_prctl_drop_mask(arg2, arg3);
> > > +
> > >       /*
> > >        * The next four prctl's remain to assist with transitioning a
> > >        * system from legacy UID=0 based privilege (when filesystem
> > > --
> > > 2.34.1