Re: [PATCH v2] perf: Require CAP_KILL if sigtrap is requested
From: Eric W. Biederman <hidden>
Date: 2021-07-01 21:41:30
Marco Elver [off-list ref] 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
quoted hunk
Fixes: 97ba62b27867 ("perf: Add support for SIGTRAP on perf events") Cc: <redacted> # 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; }