CVE-2023-52879

In the Linux kernel, the following vulnerability has been resolved:

tracing: Have trace_event_file have ref counters

The following can crash the kernel:

# cd /sys/kernel/tracing

# echo 'p:sched schedule' > kprobe_events

# exec 5>>events/kprobes/sched/enable

# > kprobe_events

# exec 5>&-

The above commands:

1. Change directory to the tracefs directory

2. Create a kprobe event (doesn't matter what one)

3. Open bash file descriptor 5 on the enable file of the kprobe event

4. Delete the kprobe event (removes the files too)

5. Close the bash file descriptor 5

The above causes a crash!

BUG: kernel NULL pointer dereference, address: 0000000000000028

#PF: supervisor read access in kernel mode

#PF: error_code(0x0000) - not-present page

PGD 0 P4D 0

Oops: 0000 [#1] PREEMPT SMP PTI

CPU: 6 PID: 877 Comm: bash Not tainted 6.5.0-rc4-test-00008-g2c6b6b1029d4-dirty #186

Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014

RIP: 0010:tracing_release_file_tr+0xc/0x50

What happens here is that the kprobe event creates a trace_event_file

"file" descriptor that represents the file in tracefs to the event. It

maintains state of the event (is it enabled for the given instance?).

Opening the "enable" file gets a reference to the event "file" descriptor

via the open file descriptor. When the kprobe event is deleted, the file is

also deleted from the tracefs system which also frees the event "file"

descriptor.

But as the tracefs file is still opened by user space, it will not be

totally removed until the final dput() is called on it. But this is not

true with the event "file" descriptor that is already freed. If the user

does a write to or simply closes the file descriptor it will reference the

event "file" descriptor that was just freed, causing a use-after-free bug.

To solve this, add a ref count to the event "file" descriptor as well as a

new flag called "FREED". The "file" will not be freed until the last

reference is released. But the FREE flag will be set when the event is

removed to prevent any more modifications to that event from happening,

even if there's still a reference to the event "file" descriptor.