Re: [PATCH] x86: Use entire page for the per-cpu GDT only if paravirt-enabled

[Ingo Molnar]

* Denys Vlasenko <dvlasenk@xxxxxxxxxx> 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
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