Linux Kernel: copy_from_user - struct with pointers

Question

Hello,

I've implemented some kind of character device and I need help with copy_ from_user function.

I've a structure:

struct  my_struct{

int a;

int *b;
};

I initialize it in user space and pass pointer to my_struct to my char device using 'write' function. In Kernel's Space character device 'write' function I cast it from a *char to this kind of structure. I alloc some memory for a struct using kmalloc and do copy_from_user into it.

It's fine for simple 'int a', but it copies only pointer (address) of b value, not value pointed by b, so I'm now in Kernel Space and I'm working with a pointer that points to a user space memory. Is this incorrect and I shouldn't access to user space pointer directly and I have to copy_from_user every single pointer in my struct and then copy back every pointer in "read" function using copy_to_user function?

Answer 1

You are correct in your surmising. If you need to access the value *b, you will need to use copy_from_user (and copy_to_user to update it back in the user process).

Answer 2

You must always use copy_from_user and similar to access user space memory from kernel space, regardless of how you got the pointer. Since b is a pointer to user space memory, you must use copy_from_user to access it.

These functions do two important additional tasks:

1. They make sure the pointer points into user space and not kernel space. Without this check, user space programs might be able to read or write to kernel memory, bypassing normal security.
2. They handle page faults correctly. Normally a page fault in kernel mode will result in an OOPS or panic - the copy_*_user family of functions have a special override that tells the PF handler that all is well, and the fault should be handled normally; and in the event that the fault cannot be satisfied by IO (ie, what would normally cause a SIGSEGV or SIGBUS), return an error code instead so their caller can do any necessary cleanup before returning to userspace with -EFAULT.