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

[Andy Lutomirski]

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]

>
> ---
>  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
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