What happens when I printk a char * that was initialized in userspace?

Question

I implemented a new system call as an intro exercise. All it does is take in a buffer and printk that buffer. I later learned that the correct practice would be to use copy_from_user.

Is this just a precautionary measure to validate the address, or is my system call causing some error (page fault?) that I cannot see?

If it is just a precautionary measure, what is it protecting against?

Thanks!

Answer 1

There are several reasons.

1. Some architectures employ segmented memory, where there is a separate segment for the user memory. In that case, copy_from_user is essential to actually get the right memory address.
2. The kernel has access to everything, including (almost by definition) a lot of privileged information. Not using copy_from_user could allow information disclosure if a user passes in a kernel address. Worse, if you are writing to a user-supplied buffer without copy_to_user, the user could overwrite kernel memory.
3. You'd like to prevent the user from crashing the kernel module just by passing in a bad pointer; using copy_from_user protects against faults so e.g. a system call handler can return EFAULT in response to a bad user pointer.