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 events 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 calls 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=���������������������������������������

—truncated—

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/1603feac154ff38514e8354e3079a455eb4801e2
• https://git.kernel.org/stable/c/417d5ea6e735e5d88ffb6c436cf2938f3f476dd1
• https://git.kernel.org/stable/c/4313e5a613049dfc1819a6dfb5f94cf2caff9452
• https://git.kernel.org/stable/c/be111ebd8868d4b7c041cb3c6102e1ae27d6dc1d
• https://git.kernel.org/stable/c/c52d0c8c4f38f7580cff61c4dfe1034c580cedfd