It'd be good to get this sorted -- please take another look.

Many thanks, -- Marco

On Mon, 5 Jul 2021 at 10:45, Marco Elver elver@google.com wrote:

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 or PTRACE_MODE_ATTACH permissions.

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

---

v3:

• Upgrade ptrace mode check to ATTACH if attr.sigtrap, otherwise it's possible to change the target task (send signal) even if only read ptrace permissions were granted (reported by Eric W. Biederman).

v2: https://lkml.kernel.org/r/20210701083842.580466-1-elver@google.com

• 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).

v1: https://lkml.kernel.org/r/20210630093709.3612997-1-elver@google.com

kernel/events/core.c | 25 ++++++++++++++++++++++++- 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c index fe88d6eea3c2..f79ee82e644a 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -12152,10 +12152,33 @@ SYSCALL_DEFINE5(perf_event_open, }

    if (task) {

•           unsigned int ptrace_mode = PTRACE_MODE_READ_REALCREDS;
  

•           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 also have
  

•                    * CAP_KILL.
  

•                    */
  

•                   rcu_read_lock();
  

•                   is_capable &= ns_capable(__task_cred(task)->user_ns, CAP_KILL);
  

•                   rcu_read_unlock();
  

•                   /*
  

•                    * If the required capabilities aren't available, checks
  

•                    * for ptrace permissions: upgrade to ATTACH, since
  

•                    * sending signals can effectively change the target
  

•                    * task.
  

•                    */
  

•                   ptrace_mode = PTRACE_MODE_ATTACH_REALCREDS;
  

•           }
  

•           /*
             * Preserve ptrace permission check for backwards compatibility.
             *
  

@@ -12165,7 +12188,7 @@ 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))
                    goto err_cred;
    }
  
  

-- 2.32.0.93.g670b81a890-goog

The following commit has been merged into the perf/urgent branch of tip:

Commit-ID: 9d7a6c95f62bc335b62aaf9d50590122bd03a796 Gitweb: https://git.kernel.org/tip/9d7a6c95f62bc335b62aaf9d50590122bd03a796 Author: Marco Elver elver@google.com AuthorDate: Mon, 05 Jul 2021 10:44:52 +02:00 Committer: Peter Zijlstra peterz@infradead.org CommitterDate: Fri, 16 Jul 2021 18:46:38 +02:00

perf: Fix required permissions if sigtrap is requested

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 or PTRACE_MODE_ATTACH permissions.

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") Reported-by: Dmitry Vyukov dvyukov@google.com Signed-off-by: Marco Elver elver@google.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Acked-by: Dmitry Vyukov dvyukov@google.com Cc: stable@vger.kernel.org # 5.13+ Link: https://lore.kernel.org/r/20210705084453.2151729-1-elver@google.com --- kernel/events/core.c | 25 ++++++++++++++++++++++++- 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c index 4649170..c13730b 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -12158,10 +12158,33 @@ SYSCALL_DEFINE5(perf_event_open, }

if (task) { + unsigned int ptrace_mode = PTRACE_MODE_READ_REALCREDS; + 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 also have + * CAP_KILL. + */ + rcu_read_lock(); + is_capable &= ns_capable(__task_cred(task)->user_ns, CAP_KILL); + rcu_read_unlock(); + + /* + * If the required capabilities aren't available, checks + * for ptrace permissions: upgrade to ATTACH, since + * sending signals can effectively change the target + * task. + */ + ptrace_mode = PTRACE_MODE_ATTACH_REALCREDS; + } + /* * Preserve ptrace permission check for backwards compatibility. * @@ -12171,7 +12194,7 @@ 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)) goto err_cred; }