Re: [PATCH] x86, traps: maps all IDTs to fixmap area.

[Andy Lutomirski]

On Thu, Feb 26, 2015 at 8:45 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Thu, Feb 26, 2015 at 7:17 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> On Wed, Feb 25, 2015 at 11:06 PM, Wang Nan <wangnan0@xxxxxxxxxx> wrote:
>>> The reason why mapping idt_table to fixmap area should also be applied
>>> to debug_idt_table and trace_idt_table. This patch does same thing for
>>> all IDTs.
>>>
>>> Signed-off-by: Wang Nan <wangnan0@xxxxxxxxxx>
>>> ---
>>>
>>> I believe trace_idt_table and debug_idt_table should be symmetrical with
>>> idt_table. However, Like my previous patch 'x86, traps: install gates
>>> using IST after cpu_init()', I'm not sure whether this is a practical
>>> fix.
>>
>> It shouldn't matter, since we should never enter userspace with these
>> IDTs loaded.
>>
>> --Andy
>>
>> [patch kept below for Kees' benefit]
>
> Is there a reason to use fixmap entries for these IDTs? Or rather, is
> there a situation where these IDTs are ever visible to userspace? (The
> reason to use the fixmap is to hide their "true" location from
> userspace.)

There's also the F00F workaround, which IIRC we get for free by using
the fixmap, but that also shouldn't matter here.

>
> -Kees
>
>>
>>>
>>> ---
>>>  arch/x86/include/asm/fixmap.h |  6 ++++++
>>>  arch/x86/kernel/tracepoint.c  |  2 +-
>>>  arch/x86/kernel/traps.c       | 13 +++++++++++--
>>>  arch/x86/xen/mmu.c            |  6 ++++++
>>>  4 files changed, 24 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/arch/x86/include/asm/fixmap.h b/arch/x86/include/asm/fixmap.h
>>> index f80d700..79550f4 100644
>>> --- a/arch/x86/include/asm/fixmap.h
>>> +++ b/arch/x86/include/asm/fixmap.h
>>> @@ -90,6 +90,12 @@ enum fixed_addresses {
>>>         FIX_IO_APIC_BASE_END = FIX_IO_APIC_BASE_0 + MAX_IO_APICS - 1,
>>>  #endif
>>>         FIX_RO_IDT,     /* Virtual mapping for read-only IDT */
>>> +#ifdef CONFIG_X86_64
>>> +       FIX_RO_DEBUG_IDT,       /* Virtual mapping for read-only debug_idt_table */
>>> +#endif
>>> +#ifdef CONFIG_TRACING
>>> +       FIX_RO_TRACE_IDT,       /* Virtual mapping for read-only trace_idt_table */
>>> +#endif
>>>  #ifdef CONFIG_X86_32
>>>         FIX_KMAP_BEGIN, /* reserved pte's for temporary kernel mappings */
>>>         FIX_KMAP_END = FIX_KMAP_BEGIN+(KM_TYPE_NR*NR_CPUS)-1,
>>> diff --git a/arch/x86/kernel/tracepoint.c b/arch/x86/kernel/tracepoint.c
>>> index 1c113db..296e130 100644
>>> --- a/arch/x86/kernel/tracepoint.c
>>> +++ b/arch/x86/kernel/tracepoint.c
>>> @@ -12,7 +12,7 @@ atomic_t trace_idt_ctr = ATOMIC_INIT(0);
>>>  struct desc_ptr trace_idt_descr = { NR_VECTORS * 16 - 1,
>>>                                 (unsigned long) trace_idt_table };
>>>
>>> -/* No need to be aligned, but done to keep all IDTs defined the same way. */
>>> +/* Must be page-aligned because the real IDT is used in a fixmap. */
>>>  gate_desc trace_idt_table[NR_VECTORS] __page_aligned_bss;
>>>
>>>  static int trace_irq_vector_refcount;
>>> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
>>> index cf7898e..6d88c37 100644
>>> --- a/arch/x86/kernel/traps.c
>>> +++ b/arch/x86/kernel/traps.c
>>> @@ -67,7 +67,7 @@
>>>  #include <asm/pgalloc.h>
>>>  #include <asm/proto.h>
>>>
>>> -/* No need to be aligned, but done to keep all IDTs defined the same way. */
>>> +/* Must be page-aligned because the real IDT is used in a fixmap. */
>>>  gate_desc debug_idt_table[NR_VECTORS] __page_aligned_bss;
>>>  #else
>>>  #include <asm/processor-flags.h>
>>> @@ -998,9 +998,18 @@ void __init trap_init(void)
>>>          * Set the IDT descriptor to a fixed read-only location, so that the
>>>          * "sidt" instruction will not leak the location of the kernel, and
>>>          * to defend the IDT against arbitrary memory write vulnerabilities.
>>> -        * It will be reloaded in cpu_init() */
>>> +        * It will be reloaded in cpu_init()
>>> +        */
>>>         __set_fixmap(FIX_RO_IDT, __pa_symbol(idt_table), PAGE_KERNEL_RO);
>>>         idt_descr.address = fix_to_virt(FIX_RO_IDT);
>>> +#ifdef CONFIG_X86_64
>>> +       __set_fixmap(FIX_RO_DEBUG_IDT, __pa_symbol(debug_idt_table), PAGE_KERNEL_RO);
>>> +       debug_idt_descr.address = fix_to_virt(FIX_RO_DEBUG_IDT);
>>> +#endif
>>> +#ifdef CONFIG_TRACING
>>> +       __set_fixmap(FIX_RO_TRACE_IDT, __pa_symbol(trace_idt_table), PAGE_KERNEL_RO);
>>> +       trace_idt_descr.address = fix_to_virt(FIX_RO_TRACE_IDT);
>>> +#endif
>>>
>>>         /*
>>>          * Should be a barrier for any external CPU state:
>>> diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
>>> index adca9e2..1fd4a4c 100644
>>> --- a/arch/x86/xen/mmu.c
>>> +++ b/arch/x86/xen/mmu.c
>>> @@ -1984,6 +1984,12 @@ static void xen_set_fixmap(unsigned idx, phys_addr_t phys, pgprot_t prot)
>>>         switch (idx) {
>>>         case FIX_BTMAP_END ... FIX_BTMAP_BEGIN:
>>>         case FIX_RO_IDT:
>>> +#ifdef CONFIG_X86_64
>>> +       case FIX_RO_DEBUG_IDT:
>>> +#endif
>>> +#ifdef CONFIG_TRACING
>>> +       case FIX_RO_TRACE_IDT:
>>> +#endif
>>>  #ifdef CONFIG_X86_32
>>>         case FIX_WP_TEST:
>>>  # ifdef CONFIG_HIGHMEM
>>> --
>>> 1.8.4
>>>
>>
>>
>>
>> --
>> Andy Lutomirski
>> AMA Capital Management, LLC
>
>
>
> --
> Kees Cook
> Chrome OS Security



-- 
Andy Lutomirski
AMA Capital Management, LLC
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