kernelCTF: Add CVE-2024-41010 LTS

[0xTen]

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[th3lsh3ll]

Hello @0xTen Thanks for your submission. May I ask a few questions?

Under the section "kmalloc-cg-512 -> kmalloc-cg-1k"

"We can now overlap a msg_msg with our skbuf->data object."
So the skbuf->data that msgmsg_seg/alt_ifname(from what I understand they are overlapped?) points to is made to overlap with a different msg_msg? This is done by adding another msg_msg to the queue with the overwritten msgmsg_seg?

Is it that you add another msg_msg to the same queue(which overwrites skb->data) then read until the end of msg linked list to leak pointers from kmalloc-cg-1k?

Then after leaking you write to skb->data since the msg_msg overlaps in order to free a msg_msg in another queue with the current one to end up at a double free in kmalloc-cg-1k?

[0xTen]

Your understanding is generally correct except for:

This is done by adding another msg_msg to the queue with the overwritten msgmsg_seg?

Here is very detailed explanation that hopefully will give you a very deep understanding of what is going on:

For context, msg_msg and msg_msgseg are allocated when sending ipc messages and are deallocated by receiving ipc messages. Also, skbuf->data is allocated by sending data on one end of a unix socket pair and is deallocated by receiving the data on the other end of the socket pair.

Getting to kmalloc-cg-512 in the first place

Basically, at that stage I receive from the mqueue that contains the overwritten msg_msgseg (let's call it mqueue A) only to free the skbuf->data object (residing in kmalloc-cg-512) whose pointer I discovered in the previous step (using if_inaddr). After that, you can forget about the previous steps and the corrupted msg_msgseg, because what follows is bug agnostic:
The heap chunk containing that skbuf->data will be put on top of the cg-512 freelist because I freed it by receiving the corrupted msg_msgseg (from mqueue A), and I can now reclaim that chunk by allocating msg_msg using a different mqueue (let's call it mqueue B). At this point we will have a skbuf->data object and msg_msg object overlapping with each other on kmalloc-cg-512. Keep in mind that by receiving that skbuf->data from the socket I will be:

1. reading the msg_msg struct and leaking it's contents
2. deallocating the chunk and putting it back on top of the freelist
   After that, if I send data on the socket again I will reclaim the same chunk again and overlap the msg_msg struct, so I can effectively view and edit the msg_msg object as I like.

Getting from kmalloc-512 to kmalloc-cg-1k

Now that we have a fully controllable msg_msg object (which belongs to mqueue B) in kmalloc-cg-512, in order to go from that to kmalloc-cg-1k I do the following things:

1. send another msg on mqueue B with a size such that it gets allocated in kmalloc-cg-1k, (this will populate the controllable msg_msg with a pointer) to this second msg, which lives in kmalloc-cg-1k.
2. spray some messages on different mqueues (let's all them mqueue N) such that their msg_msg objects also get allocated in kmalloc-cg1k. This allows us to get a layout such that (in kmalloc-cg-1k) there will be the 2nd msg_msg of mqueue B followed by the 1st msg_msg of some mqueue N.
3. I read the unix socket to read the 1st msg of mqueue B, which contains the pointer of the 2nd msg of mqueue B. I know for a fact that the next object following that pointer is a msg_msg belonging to a different mqueue N, so I add sizeof(kmalloc-cg-1k object) to the pointer that I just leaked, which should let me know the address of some msg_msg belonging to some mqueue N.
4. I edit the 1st msg_msg of mqueue B by writing to the unix socket, such that it now no longer points to the 2nd msg of mqueue B and now points to the 1st msg_msg of some mqueue N. The keypoint that you should have in mind here is that some msg_msg that belongs to mqueue N is now not only linked in the list of mqueue N but ALSO in the list of mqueue B and it's accessible from both queues (let's called that msg_msg X).
5. I free msg_msg X by receiving everything from mqueue B, but msg_msg X is still reacheable from mqueue N, so mqueue N references a heap chunk that is in the freelist of cg-1k.
6. I send some skbuf->data with a size such that it gets allocated on kmalloc-cg-1k, reclaiming the slot of msg_msg X from the freelist. So now mqueue N has a reference to that skbuf->data.
7. I free the same chunk again by receiveing from mqueue N, which will free the skbuf->data while it is still reacheable from the unix socket.

Going from that to root

At this point I have a similar scenario as I had on kmalloc-cg-512, which is a dangling skbuf->data object that can be overlapped with any struct I want and I will be able to edit that struct. I overlap it with pipe_buffer to control the pipe_buffer->page pointer and get arb r/w on kernel memory.

[th3lsh3ll]

thanks