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; }