Messages in this thread | | | Date | Mon, 28 Sep 2015 09:58:51 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] x86: Use entire page for the per-cpu GDT only if paravirt-enabled |
| |
* Denys Vlasenko <dvlasenk@redhat.com> wrote:
> On 09/26/2015 09:50 PM, H. Peter Anvin wrote: > > NAK. We really should map the GDT read-only on all 64 bit systems, > > since we can't hide the address from SLDT. Same with the IDT. > > Sorry, I don't understand your point.
So the problem is that right now the SGDT instruction (which is unprivileged) leaks the real address of the kernel image:
fomalhaut:~> ./sgdt SGDT: ffff88303fd89000 / 007f
that 'ffff88303fd89000' is a kernel address.
fomalhaut:~> cat sgdt.c #include <stdio.h> #include <stdlib.h>
int main(void) { struct gdt_desc { unsigned short limit; unsigned long addr; } __attribute__((packed)) gdt_desc = { -1, -1 };
asm volatile("sgdt %0": "=m" (gdt_desc));
printf("SGDT: %016lx / %04x\n", gdt_desc.addr, gdt_desc.limit);
return 0; }
Your observation in the changelog and your patch:
> >> It is page-sized because of paravirt. [...]
... conflicts with the intention to mark (remap) the primary GDT address read-only on native kernels as well.
So what we should do instead is to use the page alignment properly and remap the GDT to a read-only location, and load that one.
This would have a couple of advantages:
- This would give kernel address randomization more teeth on x86.
- An additional advantage would be that rootkits overwriting the GDT would have a bit more work to do.
- A third advantage would be that for NUMA systems we could 'mirror' the GDT into node-local memory and load those. This makes GDT load cache-misses a bit less expensive.
The IDT is already remapped:
fomalhaut:~> ./sidt Sidt: ffffffffff57b000 / 0fff fomalhaut:~> cat sidt.c #include <stdio.h> #include <stdlib.h>
int main(void) { struct idt_desc { unsigned short limit; unsigned long addr; } __attribute__((packed)) idt_desc = { -1, -1 };
asm volatile("sidt %0": "=m" (idt_desc));
printf("Sidt: %016lx / %04x\n", idt_desc.addr, idt_desc.limit);
return 0; }
Thanks,
Ingo
| |