[PATCH 5.4 09/13] perf: Break deadlock involving exec_update_mutex

Greg Kroah-Hartman
2021-01-07T14:33:28Z

From: peterz@infradead.org <peterz@infradead.org>

[ Upstream commit 78af4dc949daaa37b3fcd5f348f373085b4e858f ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/events/core.c | 46 ++++++++++++++++++++++----------------------
1 file changed, 23 insertions(+), 23 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 9f7c2da992991..18dbdf248ed81 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -11001,24 +11001,6 @@ SYSCALL_DEFINE5(perf_event_open,
goto err_task;
}

- if (task) {
- err = mutex_lock_interruptible(&task->signal->exec_update_mutex);
- if (err)
- goto err_task;
-
- /*
- * Reuse ptrace permission checks for now.
- *
- * 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 (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS))
- goto err_cred;
- }
-
if (flags & PERF_FLAG_PID_CGROUP)
cgroup_fd = pid;

@@ -11026,7 +11008,7 @@ 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)) {
@@ -11145,6 +11127,24 @@ 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 (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS))
+ goto err_cred;
+ }
+
if (move_group) {
gctx = __perf_event_ctx_lock_double(group_leader, ctx);

@@ -11320,7 +11320,10 @@ err_locked:
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);
@@ -11332,9 +11335,6 @@ err_alloc:
*/
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);
--
2.27.0