[PATCH] perf: Break deadlock involving exec_update_mutex

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Dec 07, 2020 at 10:40:11AM -0800, Linus Torvalds wrote:
> On Mon, Dec 7, 2020 at 1:10 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > > PeterZ, is there something I'm missing?
> >
> > Like this?
> >
> >   https://lkml.kernel.org/r/20200828123720.GZ1362448@hirez.programming.kicks-ass.net
> 
> Yes, except I think you should remove the old ptrace_may_access() check.

> I don't see any point at all in checking privileges twice, and I do
> see real downsides. Not just that KCSAN issue, but also lack of
> coverage (ie the second check will then effectively never be tested,
> which is bad too).

Fair enough, find below.

I suppose I'll queue the below into tip/perf/core for next merge window,
unless you want it in a hurry?

---
Subject: perf: Break deadlock involving exec_update_mutex
From: Peter Zijlstra <peterz@infradead.org>
Date: Fri, 28 Aug 2020 14:37:20 +0200

Syzbot reported a lock inversion involving perf. The sore point being
perf holding exec_update_mutex() for a very long time, specifically
across a whole bunch of filesystem ops in pmu::event_init() (uprobes)
and anon_inode_getfile().

This then inverts against procfs code trying to take
exec_update_mutex.

Move the permission checks later, such that we need to hold the mutex
over less code.

Reported-by: syzbot+db9cdf3dd1f64252c6ef@syzkaller.appspotmail.com
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/events/core.c |   46 +++++++++++++++++++++++-----------------------
 1 file changed, 23 insertions(+), 23 deletions(-)

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -11832,24 +11832,6 @@ SYSCALL_DEFINE5(perf_event_open,
 		goto err_task;
 	}
 
-	if (task) {
-		err = mutex_lock_interruptible(&task->signal->exec_update_mutex);
-		if (err)
-			goto err_task;
-
-		/*
-		 * Preserve ptrace permission check for backwards compatibility.
-		 *
-		 * We must hold exec_update_mutex across this and any potential
-		 * perf_install_in_context() call for this new event to
-		 * serialize against exec() altering our credentials (and the
-		 * perf_event_exit_task() that could imply).
-		 */
-		err = -EACCES;
-		if (!perfmon_capable() && !ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS))
-			goto err_cred;
-	}
-
 	if (flags & PERF_FLAG_PID_CGROUP)
 		cgroup_fd = pid;
 
@@ -11857,7 +11839,7 @@ SYSCALL_DEFINE5(perf_event_open,
 				 NULL, NULL, cgroup_fd);
 	if (IS_ERR(event)) {
 		err = PTR_ERR(event);
-		goto err_cred;
+		goto err_task;
 	}
 
 	if (is_sampling_event(event)) {
@@ -11976,6 +11958,24 @@ SYSCALL_DEFINE5(perf_event_open,
 		goto err_context;
 	}
 
+	if (task) {
+		err = mutex_lock_interruptible(&task->signal->exec_update_mutex);
+		if (err)
+			goto err_file;
+
+		/*
+		 * Preserve ptrace permission check for backwards compatibility.
+		 *
+		 * We must hold exec_update_mutex across this and any potential
+		 * perf_install_in_context() call for this new event to
+		 * serialize against exec() altering our credentials (and the
+		 * perf_event_exit_task() that could imply).
+		 */
+		err = -EACCES;
+		if (!perfmon_capable() && !ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS))
+			goto err_cred;
+	}
+
 	if (move_group) {
 		gctx = __perf_event_ctx_lock_double(group_leader, ctx);
 
@@ -12151,7 +12151,10 @@ SYSCALL_DEFINE5(perf_event_open,
 	if (move_group)
 		perf_event_ctx_unlock(group_leader, gctx);
 	mutex_unlock(&ctx->mutex);
-/* err_file: */
+err_cred:
+	if (task)
+		mutex_unlock(&task->signal->exec_update_mutex);
+err_file:
 	fput(event_file);
 err_context:
 	perf_unpin_context(ctx);
@@ -12163,9 +12166,6 @@ SYSCALL_DEFINE5(perf_event_open,
 	 */
 	if (!event_file)
 		free_event(event);
-err_cred:
-	if (task)
-		mutex_unlock(&task->signal->exec_update_mutex);
 err_task:
 	if (task)
 		put_task_struct(task);