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Message-ID: <CAGXu5jJs=aj4yGOk-sCmzt1Ti6FpHs3PiG1iO=OFhwSq8Qtv7w@mail.gmail.com>
Date: Thu, 26 Feb 2015 08:45:55 -0800
From: Kees Cook <keescook@...omium.org>
To: Andy Lutomirski <luto@...capital.net>
Cc: Wang Nan <wangnan0@...wei.com>,
Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
Steven Rostedt <rostedt@...dmis.org>,
Ingo Molnar <mingo@...hat.com>,
"H. Peter Anvin" <hpa@...or.com>,
Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>,
Oleg Nesterov <oleg@...hat.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Li Zefan <lizefan@...wei.com>
Subject: Re: [PATCH] x86, traps: maps all IDTs to fixmap area.
On Thu, Feb 26, 2015 at 7:17 AM, Andy Lutomirski <luto@...capital.net> wrote:
> On Wed, Feb 25, 2015 at 11:06 PM, Wang Nan <wangnan0@...wei.com> 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@...wei.com>
>> ---
>>
>> 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.)
-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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