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    SubjectRe: [RFC][PATCH 08/10] x86/mm: do not forbid _PAGE_RW before init for __ro_after_init
    On Thu, Feb 22, 2018 at 12:37 PM, Dave Hansen
    <dave.hansen@linux.intel.com> wrote:
    >
    > From: Dave Hansen <dave.hansen@linux.intel.com>
    >
    > __ro_after_init data gets stuck in the .rodata section. That's normally
    > fine because the kernel itself manages the R/W properties.
    >
    > But, if we run __change_page_attr() on an area which is __ro_after_init,
    > the .rodata checks will trigger and force the area to be immediately
    > read-only, even if it is early-ish in boot. This caused problems when
    > trying to clear the _PAGE_GLOBAL bit for these area in the PTI code:
    > it cleared _PAGE_GLOBAL like I asked, but also took it up on itself
    > to clear _PAGE_RW. The kernel then oopses the next time it wrote to
    > a __ro_after_init data structure.
    >
    > To fix this, add the kernel_set_to_readonly check, just like we have
    > for kernel text, just a few lines below in this function.

    Yup, looks sensible. Thanks!

    >
    > Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
    > Cc: Andrea Arcangeli <aarcange@redhat.com>
    > Cc: Andy Lutomirski <luto@kernel.org>
    > Cc: Linus Torvalds <torvalds@linux-foundation.org>
    > Cc: Kees Cook <keescook@google.com>
    > Cc: Hugh Dickins <hughd@google.com>
    > Cc: Juergen Gross <jgross@suse.com>
    > Cc: x86@kernel.org
    > Cc: Nadav Amit <namit@vmware.com>

    Acked-by: Kees Cook <keescook@chromium.org>

    -Kees

    > ---
    >
    > b/arch/x86/mm/pageattr.c | 6 ++++--
    > 1 file changed, 4 insertions(+), 2 deletions(-)
    >
    > diff -puN arch/x86/mm/pageattr.c~check-kernel_set_to_readonly arch/x86/mm/pageattr.c
    > --- a/arch/x86/mm/pageattr.c~check-kernel_set_to_readonly 2018-02-22 12:36:21.531036546 -0800
    > +++ b/arch/x86/mm/pageattr.c 2018-02-22 12:36:21.535036546 -0800
    > @@ -298,9 +298,11 @@ static inline pgprot_t static_protection
    >
    > /*
    > * The .rodata section needs to be read-only. Using the pfn
    > - * catches all aliases.
    > + * catches all aliases. This also includes __ro_after_init,
    > + * so do not enforce until kernel_set_to_readonly is true.
    > */
    > - if (within(pfn, __pa_symbol(__start_rodata) >> PAGE_SHIFT,
    > + if (kernel_set_to_readonly &&
    > + within(pfn, __pa_symbol(__start_rodata) >> PAGE_SHIFT,
    > __pa_symbol(__end_rodata) >> PAGE_SHIFT))
    > pgprot_val(forbidden) |= _PAGE_RW;
    >
    > _



    --
    Kees Cook
    Pixel Security

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