[PATCH 5.13 134/175] perf: Fix required permissions if sigtrap is requested

From: Greg Kroah-Hartman
Date: Tue Aug 10 2021 - 14:06:08 EST


From: Marco Elver <elver@xxxxxxxxxx>

commit 9d7a6c95f62bc335b62aaf9d50590122bd03a796 upstream.

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@xxxxxxxxxx>
Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # 5.13+
Link: https://lore.kernel.org/r/20210705084453.2151729-1-elver@xxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/events/core.c | 25 ++++++++++++++++++++++++-
1 file changed, 24 insertions(+), 1 deletion(-)

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -12159,10 +12159,33 @@ 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.
*
@@ -12172,7 +12195,7 @@ 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;
}