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

[Jinjie Ruan <ruanjinjie@huawei.com>]

在 2026/9/25 22:42, Serge E. Hallyn 写道:
> Also (not sure whether this is what Andrew is referring to here), in your
> code below, what keeps one thread from doing a clone in between the
> task harvesting loop and the task_work_add() loop?  It seems like you'd
> need to do another loop after to make sure you didn't miss any new
> tasks?

Yes, We need to propagate the dropped cap to child threads created via
clone.

> 
> And of course we'd need a more robust solution to
> 
>>>>>> +          /* Out of memory: bounding set drop failed silently for this thread. */

I agree. I'm thinking about a more complete approach that won't miss
newly cloned child threads, while also avoiding the inability to roll
back if a child thread's prepare commit fails.

> 
> -serge
> 
> On Fri, Sep 25, 2026 at 07:28:08AM -0700, Andrew G. Morgan wrote:
>> I don't see a race condition in the (*cap.IAB).Set() code. The
>> semantics of this sort of thing can't be allowed to include a race;
>> otherwise, back-to-back calls to this "system call" might generate
>> divergent thread state. How you do that without some serialization
>> isn't clear to me.
>>
>> Cheers
>>
>> Andrew
>>
>> On Fri, Sep 25, 2026 at 6:17 AM Serge E. Hallyn <serge@hallyn.com> wrote:
>>>
>>> On Thu, Sep 24, 2026 at 04:05:59PM +0800, Jinjie Ruan wrote:
>>>>
>>>>
>>>> 在 2026/9/23 1:01, Serge E. Hallyn 写道:
>>>>> 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.
>>>>
>>>> Hi Serge,
>>>>
>>>> Thanks for the review.
>>>>
>>>> The use case is gVisor's sentry: a long-lived, pure-Go process that
>>>> starts a sandbox per container/pod. The final capability set depends on
>>>> the spec, the platform (ptrace requires CAP_SYS_PTRACE),
>>>> directfs/networking, and the capabilities granted to the runtime's user
>>>> namespace.
>>>>
>>>> It is therefore only known after the runtime and gVisor's own threads
>>>> are already up — the trim happens multi-threaded. We do have a re-exec
>>>> path, but it re-loads a ~100 MB binary and re-runs Go init, so we only
>>>> take it when forced.
>>>>
>>>>>
>>>>> 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.
>>>>
>>>> Fair concern. This new prcoess-wide drop is also a drop — it only
>>>> removes capabilities from all threads, and requires CAP_SETPCAP. But it
>>>
>>> Good point, CAP_SETPCAP requirement might mitigate my concern.  I'll look
>>> at this a bit more, thanks.
>>>
>>>> must actually reach every thread, so the thread-creation race still
>>>> needs to be addressed.
>>>>
>>>>>
>>>>>>
>>>>>> Cc: Serge Hallyn <serge@hallyn.com>
>>>>>> Cc: Paul Moore <paul@paul-moore.com>
>>>>>> Cc: James Morris <jmorris@namei.org>
>>>>>> Cc: Paul Walmsley <pjw@kernel.org>
>>>>>> Cc: Thomas Gleixner <tglx@kernel.org>
>>>>>> Cc: Zong Li <zong.li@sifive.com>
>>>>>> Cc: Deepak Gupta <debug@rivosinc.com>
>>>>>> Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
>>>>>> Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
>>>>>> ---
>>>>>>  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
>>>>
>>>> --
>>>> Best regards,
>>>> Jinjie

-- 
Best regards,
Jinjie