[PATCH v2] perf: Require CAP_KILL if sigtrap is requested

[PATCH v2] perf: Require CAP_KILL if sigtrap is requested

[Marco Elver]

If perf_event_open() is called with another task as target and
perf_event_attr::sigtrap is set, and the target task's user does not
match the calling user, also require the CAP_KILL capability.

Otherwise, with the CAP_PERFMON capability alone it would be possible
for a user to send SIGTRAP signals via perf events to another user's
tasks. This could potentially result in those tasks being terminated if
they cannot handle SIGTRAP signals.

Note: The check complements the existing capability check, but is not
supposed to supersede the ptrace_may_access() check. At a high level we
now have:

	capable of CAP_PERFMON and (CAP_KILL if sigtrap)
		OR
	ptrace_may_access() // also checks for same thread-group and uid

Fixes: 97ba62b27867 ("perf: Add support for SIGTRAP on perf events")
Cc: <stable@vger.kernel.org> # 5.13+
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Marco Elver <elver@google.com>
---
v2:
* Drop kill_capable() and just check CAP_KILL (reported by Ondrej Mosnacek).
* Use ns_capable(__task_cred(task)->user_ns, CAP_KILL) to check for
  capability in target task's ns (reported by Ondrej Mosnacek).
---
 kernel/events/core.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index fe88d6eea3c2..43c99695dc3f 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -12152,10 +12152,23 @@ SYSCALL_DEFINE5(perf_event_open,
 	}
 
 	if (task) {
+		bool is_capable;
+
 		err = down_read_interruptible(&task->signal->exec_update_lock);
 		if (err)
 			goto err_file;
 
+		is_capable = perfmon_capable();
+		if (attr.sigtrap) {
+			/*
+			 * perf_event_attr::sigtrap sends signals to the other
+			 * task. Require the current task to have CAP_KILL.
+			 */
+			rcu_read_lock();
+			is_capable &= ns_capable(__task_cred(task)->user_ns, CAP_KILL);
+			rcu_read_unlock();
+		}
+
 		/*
 		 * Preserve ptrace permission check for backwards compatibility.
 		 *
@@ -12165,7 +12178,7 @@ SYSCALL_DEFINE5(perf_event_open,
 		 * perf_event_exit_task() that could imply).
 		 */
 		err = -EACCES;
-		if (!perfmon_capable() && !ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS))
+		if (!is_capable && !ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS))
 			goto err_cred;
 	}
 
-- 
2.32.0.93.g670b81a890-goog

Re: [PATCH v2] perf: Require CAP_KILL if sigtrap is requested

[Eric W. Biederman]

Marco Elver <elver@google.com> writes:

> If perf_event_open() is called with another task as target and
> perf_event_attr::sigtrap is set, and the target task's user does not
> match the calling user, also require the CAP_KILL capability.
>
> Otherwise, with the CAP_PERFMON capability alone it would be possible
> for a user to send SIGTRAP signals via perf events to another user's
> tasks. This could potentially result in those tasks being terminated if
> they cannot handle SIGTRAP signals.
>
> Note: The check complements the existing capability check, but is not
> supposed to supersede the ptrace_may_access() check. At a high level we
> now have:
>
> 	capable of CAP_PERFMON and (CAP_KILL if sigtrap)
> 		OR
> 	ptrace_may_access() // also checks for same thread-group and uid

Is there anyway we could have a comment that makes the required
capability checks clear?

Basically I see an inlined version of kill_ok_by_cred being implemented
without the comments on why the various pieces make sense.

Certainly ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS) should not
be a check to allow writing/changing a task.  It needs to be
PTRACE_MODE_ATTACH_REALCREDS, like /proc/self/mem uses.

Now in practice I think your patch probably has the proper checks in
place for sending a signal but it is far from clear.

Eric


> Fixes: 97ba62b27867 ("perf: Add support for SIGTRAP on perf events")
> Cc: <stable@vger.kernel.org> # 5.13+
> Reported-by: Dmitry Vyukov <dvyukov@google.com>
> Signed-off-by: Marco Elver <elver@google.com>
> ---
> v2:
> * Drop kill_capable() and just check CAP_KILL (reported by Ondrej Mosnacek).
> * Use ns_capable(__task_cred(task)->user_ns, CAP_KILL) to check for
>   capability in target task's ns (reported by Ondrej Mosnacek).
> ---
>  kernel/events/core.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index fe88d6eea3c2..43c99695dc3f 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -12152,10 +12152,23 @@ SYSCALL_DEFINE5(perf_event_open,
>  	}
>  
>  	if (task) {
> +		bool is_capable;
> +
>  		err = down_read_interruptible(&task->signal->exec_update_lock);
>  		if (err)
>  			goto err_file;
>  
> +		is_capable = perfmon_capable();
> +		if (attr.sigtrap) {
> +			/*
> +			 * perf_event_attr::sigtrap sends signals to the other
> +			 * task. Require the current task to have CAP_KILL.
> +			 */
> +			rcu_read_lock();
> +			is_capable &= ns_capable(__task_cred(task)->user_ns, CAP_KILL);
> +			rcu_read_unlock();
> +		}
> +
>  		/*
>  		 * Preserve ptrace permission check for backwards compatibility.
>  		 *
> @@ -12165,7 +12178,7 @@ SYSCALL_DEFINE5(perf_event_open,
>  		 * perf_event_exit_task() that could imply).
>  		 */
>  		err = -EACCES;
> -		if (!perfmon_capable() && !ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS))
> +		if (!is_capable && !ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS))
>  			goto err_cred;
>  	}

Re: [PATCH v2] perf: Require CAP_KILL if sigtrap is requested

[Marco Elver]

On Thu, 1 Jul 2021 at 23:41, Eric W. Biederman <ebiederm@xmission.com> wrote:
>
> Marco Elver <elver@google.com> writes:
>
> > If perf_event_open() is called with another task as target and
> > perf_event_attr::sigtrap is set, and the target task's user does not
> > match the calling user, also require the CAP_KILL capability.
> >
> > Otherwise, with the CAP_PERFMON capability alone it would be possible
> > for a user to send SIGTRAP signals via perf events to another user's
> > tasks. This could potentially result in those tasks being terminated if
> > they cannot handle SIGTRAP signals.
> >
> > Note: The check complements the existing capability check, but is not
> > supposed to supersede the ptrace_may_access() check. At a high level we
> > now have:
> >
> >       capable of CAP_PERFMON and (CAP_KILL if sigtrap)
> >               OR
> >       ptrace_may_access() // also checks for same thread-group and uid
>
> Is there anyway we could have a comment that makes the required
> capability checks clear?
>
> Basically I see an inlined version of kill_ok_by_cred being implemented
> without the comments on why the various pieces make sense.

I'll add more comments. It probably also makes sense to factor the
code here into its own helper.

> Certainly ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS) should not
> be a check to allow writing/changing a task.  It needs to be
> PTRACE_MODE_ATTACH_REALCREDS, like /proc/self/mem uses.

So if attr.sigtrap the checked ptrace mode needs to switch to
PTRACE_MODE_ATTACH_REALCREDS. Otherwise, it is possible to send a
signal if only read-ptrace permissions are granted.

Is my assumption here correct?

> Now in practice I think your patch probably has the proper checks in
> place for sending a signal but it is far from clear.

Thanks,
-- Marco