On 10/10/2014 08:14 PM, Baoquan He wrote: > On 10/08/14 at 03:27pm, Vivek Goyal wrote: >> On Wed, Oct 08, 2014 at 08:09:59AM -0700, H. Peter Anvin wrote: > >>> Sorry... this makes no sense. >>> >>> For x86-64, there is no direct connection between the physical and >>> virtual address spaces that the kernel runs in... >> >> I am sorry I did not understand this one. I thought that initial >> relocatable kernel implementaion did not have any direct connection >> between virtual and physical address. One could load kernel anywhere >> and kernel virtual address will not change and we will just adjust >> page tables to map virtual address to right physical address. >> >> Now handle_relocation() stuff seems to introduce a close coupling >> between physical and virtual address. So if kernel shifts by 16MB >> in physical address space, then it will shift by equal amount >> in virtual address space. So there seems to be a direct connection >> between virtual and physical address space in this case. > > Yeah, it's exactly as Vivek said. > > Before kaslr was introduced, x86_64 kernel can be put anywhere, and > always _text is 0xffffffff81000000. Meanwhile phys_base contains the > offset between the compiled addr (namely 0x1000000) and kernel loaded > addr. After kaslr implementation was added, as long as kernel loaded > addr is different 0x1000000, it will call handle_relocations(). The > offset now is added onto each symbols including _text and phys_base > becomes 0. > > It's clearly showing that by checking /proc/kallsyms and value of > phys_base. > This really shouldn't have happened this way on x86-64. It has to happen this way on i386, but I worry that this may be a serious misdesign in kaslr on x86-64. I'm also wondering if there is any other fallout of this? -hpa
