The patch below does not apply to the 5.10-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 78af4dc949daaa37b3fcd5f348f373085b4e858f Mon Sep 17 00:00:00 2001

From: "peterz@infradead.org" peterz@infradead.org Date: Fri, 28 Aug 2020 14:37:20 +0200 Subject: [PATCH] perf: Break deadlock involving exec_update_mutex

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

diff --git a/kernel/events/core.c b/kernel/events/core.c index a21b0be2f22c..19ae6c931c52 100644 --- 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);