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

[Wang Nan]

On 2015/2/27 2:31, Andy Lutomirski wrote:
> 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.
> 

What about a flaw module triggering the F00F bug in kernel space? Instead of
kernel panic, the system will hang. I think tis should be a case for which
my patch can help. However, the trigger condition is critical.

>>
>> -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
> 
> 
> 


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