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perf: Fix required permissions if sigtrap is requested

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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: 97ba62b278 ("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
This commit is contained in:
Marco Elver 2021-07-05 10:44:52 +02:00 • committed by Peter Zijlstra
parent 7fef2edf7c
commit 9d7a6c95f6
1 changed files with 24 additions and 1 deletions
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25
kernel/events/core.c
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@ -12158,10 +12158,33 @@ SYSCALL_DEFINE5(perf_event_open,
} }
if (task) { if (task) {
unsigned int ptrace_mode = PTRACE_MODE_READ_REALCREDS;
bool is_capable;
err = down_read_interruptible(&task->signal->exec_update_lock); err = down_read_interruptible(&task->signal->exec_update_lock);
if (err) if (err)
goto err_file; 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. * Preserve ptrace permission check for backwards compatibility.
* *
@ -12171,7 +12194,7 @@ SYSCALL_DEFINE5(perf_event_open,
* perf_event_exit_task() that could imply). * perf_event_exit_task() that could imply).
*/ */
err = -EACCES; 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; goto err_cred;
} }