Re: x86: Make sure IDT is page aligned

[Kees Cook]

On Thu, Jul 18, 2013 at 6:30 PM, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> On Fri, Jul 19, 2013 at 01:15:26AM +0000, Linux Kernel Mailing List wrote:
>> Gitweb:     http://git.kernel.org/linus/;a=commit;h=4df05f361937ee86e5a8c9ead8aeb6a19ea9b7d7
>> Commit:     4df05f361937ee86e5a8c9ead8aeb6a19ea9b7d7
>> Parent:     5ff560fd48d5b3d82fa0c3aff625c9da1a301911
>> Author:     Kees Cook <keescook@xxxxxxxxxxxx>
>> AuthorDate: Tue Jul 16 11:34:41 2013 -0700
>> Committer:  H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
>> CommitDate: Tue Jul 16 15:14:48 2013 -0700
>>
>>     x86: Make sure IDT is page aligned
>>
>>     Since the IDT is referenced from a fixmap, make sure it is page aligned.
>>     Merge with 32-bit one, since it was already aligned to deal with F00F
>>     bug. Since bss is cleared before IDT setup, it can live there. This also
>>     moves the other *_idt_table variables into common locations.
>>
>>     This avoids the risk of the IDT ever being moved in the bss and having
>>     the mapping be offset, resulting in calling incorrect handlers. In the
>>     current upstream kernel this is not a manifested bug, but heavily patched
>>     kernels (such as those using the PaX patch series) did encounter this bug.
>>
>>     The tables other than idt_table technically do not need to be page
>>     aligned, at least not at the current time, but using a common
>>     declaration avoids mistakes.  On 64 bits the table is exactly one page
>>     long, anyway.
>>
>>     Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>>     Link: http://lkml.kernel.org/r/20130716183441.GA14232@xxxxxxxxxxxxxxx
>>     Reported-by: PaX Team <pageexec@xxxxxxxxx>
>>     Signed-off-by: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
>> ---
>>  arch/x86/kernel/head_64.S    |   15 ---------------
>>  arch/x86/kernel/tracepoint.c |    6 ++----
>>  arch/x86/kernel/traps.c      |   12 ++++++------
>>  3 files changed, 8 insertions(+), 25 deletions(-)
>
> This patch is now in Linus's tree.  Kees, did you also want this in the
> -stable tree(s)?

The potential problem was introduced with
4eefbe792baedb474e256d35370849992fcf1c79, so 3.10 needs it, yes. I had
also sent a much smaller version here:
http://git.kernel.org/cgit/linux/kernel/git/kees/linux.git/commit/?h=idt-stable&id=794c1e0df641e13050cfc4af340fc3c85bed4ea3

Either will address the problem. If there is no problem with taking
the larger clean-up for stable, then that's probably easiest.

Thanks!

-Kees

>
> thanks,
>
> greg k-h
>
>
>>
>> diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
>> index 5e4d8a8..e1aabdb 100644
>> --- a/arch/x86/kernel/head_64.S
>> +++ b/arch/x86/kernel/head_64.S
>> @@ -512,21 +512,6 @@ ENTRY(phys_base)
>>
>>  #include "../../x86/xen/xen-head.S"
>>
>> -     .section .bss, "aw", @nobits
>> -     .align L1_CACHE_BYTES
>> -ENTRY(idt_table)
>> -     .skip IDT_ENTRIES * 16
>> -
>> -     .align L1_CACHE_BYTES
>> -ENTRY(debug_idt_table)
>> -     .skip IDT_ENTRIES * 16
>> -
>> -#ifdef CONFIG_TRACING
>> -     .align L1_CACHE_BYTES
>> -ENTRY(trace_idt_table)
>> -     .skip IDT_ENTRIES * 16
>> -#endif
>> -
>>       __PAGE_ALIGNED_BSS
>>  NEXT_PAGE(empty_zero_page)
>>       .skip PAGE_SIZE
>> diff --git a/arch/x86/kernel/tracepoint.c b/arch/x86/kernel/tracepoint.c
>> index 4e584a8..1c113db 100644
>> --- a/arch/x86/kernel/tracepoint.c
>> +++ b/arch/x86/kernel/tracepoint.c
>> @@ -12,10 +12,8 @@ atomic_t trace_idt_ctr = ATOMIC_INIT(0);
>>  struct desc_ptr trace_idt_descr = { NR_VECTORS * 16 - 1,
>>                               (unsigned long) trace_idt_table };
>>
>> -#ifndef CONFIG_X86_64
>> -gate_desc trace_idt_table[NR_VECTORS] __page_aligned_data
>> -                                     = { { { { 0, 0 } } }, };
>> -#endif
>> +/* No need to be aligned, but done to keep all IDTs defined the same way. */
>> +gate_desc trace_idt_table[NR_VECTORS] __page_aligned_bss;
>>
>>  static int trace_irq_vector_refcount;
>>  static DEFINE_MUTEX(irq_vector_mutex);
>> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
>> index b0865e8..1b23a1c 100644
>> --- a/arch/x86/kernel/traps.c
>> +++ b/arch/x86/kernel/traps.c
>> @@ -63,19 +63,19 @@
>>  #include <asm/x86_init.h>
>>  #include <asm/pgalloc.h>
>>  #include <asm/proto.h>
>> +
>> +/* No need to be aligned, but done to keep all IDTs defined the same way. */
>> +gate_desc debug_idt_table[NR_VECTORS] __page_aligned_bss;
>>  #else
>>  #include <asm/processor-flags.h>
>>  #include <asm/setup.h>
>>
>>  asmlinkage int system_call(void);
>> -
>> -/*
>> - * The IDT has to be page-aligned to simplify the Pentium
>> - * F0 0F bug workaround.
>> - */
>> -gate_desc idt_table[NR_VECTORS] __page_aligned_data = { { { { 0, 0 } } }, };
>>  #endif
>>
>> +/* Must be page-aligned because the real IDT is used in a fixmap. */
>> +gate_desc idt_table[NR_VECTORS] __page_aligned_bss;
>> +
>>  DECLARE_BITMAP(used_vectors, NR_VECTORS);
>>  EXPORT_SYMBOL_GPL(used_vectors);
>>
>> --
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--
Kees Cook
Chrome OS Security
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