Decrypt device driver

AccumPlus · 03-17-2017, 04:36 AM

I got a task to make a small image, encrypt it with the simpliest method and write a decrypt driver.

Making an image:

Code:

dd if=/dev/zero of=myFS bs=1024 count=60
mkfs.ext2 myFS

Encrypting with XOR:

Code:

int main()
{
    std::fstream streamIn;
    std::fstream streamOut;

    const char key = 0b00001111;

    streamIn.open("myFS", std::ios_base::binary | std::ios_base::in);
    streamOut.open("newFS", std::ios_base::binary | std::ios_base::trunc | std::ios_base::out);

    char b;
    while (streamIn.read(&b, 1))
    {
        b ^= key;
        streamOut.write(&b, 1);
    }

    streamIn.close();
    streamOut.close();

    return 0;
}

Then writing driver. It is based on device-mapper target and the main working function has this prototype:

Code:

static int sddm_target_map(struct dm_target *ti, struct bio *bio);

You see, I have a bio structure, and I think, that I should decrypt every byte it contains. What I have now:

Code:

struct bio_vec vec;
struct bvec_iter it;
unsigned int len;
char *addr;
unsigned int i;

bio_for_each_segment(vec, bio, it)
{
    len = vec.bv_len;
    addr = (char*)(page_address(vec.bv_page) + vec.bv_offset);

    for (i = 0; i != len; ++i)
    {
        *(addr+i) ^= 0b00001111;
    }
}

Of course, it doesn't work. Attempts to mount it finished with error:

Code:

mount: wrong fs type, bad option, bad superblock on /dev/mapper/mydevice

Also saw another algorithm:

Code:

bio_for_each_segment(vec, bio, it)
{
    len = vec.bv_len;
    addr = kmap_atomic(vec.bv_page);
    pointer = (char *)(addr + vec.bv_offset);

    for (i = 0; i != len; ++i)
    {
        *(pointer+i) ^= 0b00001111;
    }

    kunmap_atomic(addr);
}

But it goes with the same error.

How can I implement my plan? Thanks in advance!