tracing: Free buffers when a used dynamic event is removed

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

tracing: Free buffers when a used dynamic event is removed

After 65536 dynamic events have been added and removed, the "type" field

of the event then uses the first type number that is available (not

currently used by other events). A type number is the identifier of the

binary blobs in the tracing ring buffer (known as events) to map them to

logic that can parse the binary blob.

The issue is that if a dynamic event (like a kprobe event) is traced and

is in the ring buffer, and then that event is removed (because it is

dynamic, which means it can be created and destroyed), if another dynamic

event is created that has the same number that new event's logic on

parsing the binary blob will be used.

To show how this can be an issue, the following can crash the kernel:

# cd /sys/kernel/tracing

# for i in `seq 65536`; do

echo 'p:kprobes/foo do_sys_openat2 $arg1:u32' > kprobe_events

# done

For every iteration of the above, the writing to the kprobe_events will

remove the old event and create a new one (with the same format) and

increase the type number to the next available on until the type number

reaches over 65535 which is the max number for the 16 bit type. After it

reaches that number, the logic to allocate a new number simply looks for

the next available number. When an dynamic event is removed, that number

is then available to be reused by the next dynamic event created. That is,

once the above reaches the max number, the number assigned to the event in

that loop will remain the same.

Now that means deleting one dynamic event and created another will reuse

the previous events type number. This is where bad things can happen.

After the above loop finishes, the kprobes/foo event which reads the

do_sys_openat2 function call's first parameter as an integer.

# echo 1 > kprobes/foo/enable

# cat /etc/passwd > /dev/null

# cat trace

cat-2211 [005] .... 2007.849603: foo: (do_sys_openat2+0x0/0x130) arg1=4294967196

cat-2211 [005] .... 2007.849620: foo: (do_sys_openat2+0x0/0x130) arg1=4294967196

cat-2211 [005] .... 2007.849838: foo: (do_sys_openat2+0x0/0x130) arg1=4294967196

cat-2211 [005] .... 2007.849880: foo: (do_sys_openat2+0x0/0x130) arg1=4294967196

# echo 0 > kprobes/foo/enable

Now if we delete the kprobe and create a new one that reads a string:

# echo 'p:kprobes/foo do_sys_openat2 +0($arg2):string' > kprobe_events

And now we can the trace:

# cat trace

sendmail-1942 [002] ..... 530.136320: foo: (do_sys_openat2+0x0/0x240) arg1= cat-2046 [004] ..... 530.930817: foo: (do_sys_openat2+0x0/0x240) arg1="������������������������������������������������������������������������������������������������"

cat-2046 [004] ..... 530.930961: foo: (do_sys_openat2+0x0/0x240) arg1="������������������������������������������������������������������������������������������������"

cat-2046 [004] ..... 530.934278: foo: (do_sys_openat2+0x0/0x240) arg1="������������������������������������������������������������������������������������������������"

cat-2046 [004] ..... 530.934563: foo: (do_sys_openat2+0x0/0x240) arg1="���������������������������������������

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