Re: [PATCH] vt: Fix potential read overflow of kernel memory

[Kees Cook]

On Wed, Aug 30, 2023 at 05:17:12PM -0600, Dan Raymond wrote:
> In my opinion strlcpy() is being used correctly here as a defensive
> precaution.  If the source string is larger than the destination buffer
> it will truncate rather than corrupt kernel memory.  However the
> return value of strlcpy() is being misused.  If truncation occurred
> the copy_to_user() call will corrupt user memory instead.
> 
> I also agree that this is not currently a bug.  It is fragile and it
> could break if someone added a very large string to the table.
> 
> Why not fix this by avoiding the redundant string copy?  How about
> something like this:
> 
> ptr = func_table[kb_func] ? : "";
> len = strlen(ptr);
> 
> if (len >= sizeof(user_kdgkb->kb_string))
> 	return -ENOSPC;
> 
> if (copy_to_user(user_kdgkb->kb_string, ptr, len + 1))
> 	return -EFAULT;

This would work if not for func_buf_lock. The bounce buffer is used to
avoid needing to hold the spin lock across copy_to_user.

-- 
Kees Cook