[PATCH v3 1/2] perf: Fix required permissions if sigtrap is requested

[Marco Elver]

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 at 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 at vger.kernel.org> # 5.13+
> Reported-by: Dmitry Vyukov <dvyukov at google.com>
> Signed-off-by: Marco Elver <elver at 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
>