0CTF2018 zerofs Writeup
zerofs.ko is a driver module of a custom filesystem.
The kernel and the module is compiled by randstruct plugin, which I found in the magic string –
vermagic=4.13.0 SMP mod_unload modversionsRANDSTRUCT_PLUGIN_3c73df5cc8285309b74c8a4caaf831205da45096402d3b1a80caab1d7fa1b03a`.
run.sh and /init show that the kernel is protected by SMEP, SMAP, KASLR, kptr_restrict and dmesg_restrict.
I found the module may be modified from simplefs after the game.
By reversing zerofs.ko, I knew the blocksize is 4096 bits. The first block of the image is the superblock. It consists of magic, block_size, inode_count and free_blocks bitmap.
The second block records all of the inodes in an array. ino is inode number, and dno is the block number of the image.
There is a root inode which ino is 1. It indicates the root dictionary. There is a block corresponding to root dictionary to indicates files in the dictionary. It is an array of zerofs_dir_record structure.
There isn’t any bound or size check in read and write function.
If the filesize we set in image is bigger than blocksize(0x1000), there will be an out-of-bound read/write when invoking copy_to_user/copy_from_user.
To exploit the vulnerabilities, I need to construct an malicious image. Here is the script. I put a file named 666 with size of 0xffffffffffffffff.
After mounting the image, I could trigger out-of-bound read by read the file 666.
I tried to find CRED struct in leaked memory. Fortunately, I found some by searching the uid. It took me some time to locate CRED struct because of the radomization of structures.
I still didn’t know which CRED is valid and which process the CRED belongs to although I could find some CRED structures. The exploit is not stable, so I run the exploit serval times. After leaking the memory, the exploit will check if it gets root privilege in a loop. If so, it invokes
The last step is to write the CRED. I invoked llseek to set offset to the CRED, and invoked write to modify the CRED, setting uid to 0.
Here is the expliot