CVE ID:	CVE-2023-52879
Description:	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.
Test IDs:	None available
Cross References:	Common Vulnerability Exposure (CVE) ID: CVE-2023-52879 https://git.kernel.org/stable/c/2c9de867ca285c397cd71af703763fe416265706 https://git.kernel.org/stable/c/2c9de867ca285c397cd71af703763fe416265706 https://git.kernel.org/stable/c/2fa74d29fc1899c237d51bf9a6e132ea5c488976 https://git.kernel.org/stable/c/2fa74d29fc1899c237d51bf9a6e132ea5c488976 https://git.kernel.org/stable/c/9034c87d61be8cff989017740a91701ac8195a1d https://git.kernel.org/stable/c/9034c87d61be8cff989017740a91701ac8195a1d https://git.kernel.org/stable/c/961c4511c7578d6b8f39118be919016ec3db1c1e https://git.kernel.org/stable/c/961c4511c7578d6b8f39118be919016ec3db1c1e https://git.kernel.org/stable/c/a98172e36e5f1b3d29ad71fade2d611cfcc2fe6f https://git.kernel.org/stable/c/a98172e36e5f1b3d29ad71fade2d611cfcc2fe6f https://git.kernel.org/stable/c/bb32500fb9b78215e4ef6ee8b4345c5f5d7eafb4 https://git.kernel.org/stable/c/bb32500fb9b78215e4ef6ee8b4345c5f5d7eafb4 https://git.kernel.org/stable/c/cbc7c29dff0fa18162f2a3889d82eeefd67305e0 https://git.kernel.org/stable/c/cbc7c29dff0fa18162f2a3889d82eeefd67305e0