Re: [PATCH v5] x86: make sure IDT is page aligned

[Yinghai Lu]

On Tue, Jul 16, 2013 at 11:34 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> 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.
>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Reported-by: PaX Team <pageexec@xxxxxxxxx>
> ---
> v5:
>  - add comments to all IDTs about alignment reasoning, suggested by Linus

Where is thread for that?

> v4:
>  - rework using __page_aligned_bss, suggested by Yinghai LU
>  - move all the other IDT variables as well, suggested by HPA
> v3:
>  - merge 32-bit and 64-bit idt_table definition
> v2:
>  - 32-bit was already aligned
> ---
>  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(-)
>
> 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;

in that case why not add __cacheline_aligned_bss?

Yinghai
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