lkml.org 
[lkml]   [2015]   [Sep]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [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


    \
     
     \ /
      Last update: 2015-09-28 10:21    [W:4.263 / U:0.012 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site