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    Subject[tip:x86/pti] x86/mm: Do not forbid _PAGE_RW before init for __ro_after_init
    Commit-ID:  efad2b4151521c944e405272035a673c74125c65
    Gitweb: https://git.kernel.org/tip/efad2b4151521c944e405272035a673c74125c65
    Author: Dave Hansen <dave.hansen@linux.intel.com>
    AuthorDate: Fri, 6 Apr 2018 13:55:14 -0700
    Committer: Ingo Molnar <mingo@kernel.org>
    CommitDate: Mon, 9 Apr 2018 18:27:34 +0200

    x86/mm: Do not forbid _PAGE_RW before init for __ro_after_init

    __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.

    Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
    Acked-by: Kees Cook <keescook@chromium.org>
    Cc: Andrea Arcangeli <aarcange@redhat.com>
    Cc: Andy Lutomirski <luto@kernel.org>
    Cc: Arjan van de Ven <arjan@linux.intel.com>
    Cc: Borislav Petkov <bp@alien8.de>
    Cc: Dan Williams <dan.j.williams@intel.com>
    Cc: David Woodhouse <dwmw2@infradead.org>
    Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: Josh Poimboeuf <jpoimboe@redhat.com>
    Cc: Juergen Gross <jgross@suse.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Nadav Amit <namit@vmware.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: linux-mm@kvack.org
    Link: http://lkml.kernel.org/r/20180406205514.8D898241@viggo.jf.intel.com
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    ---
    arch/x86/mm/pageattr.c | 6 ++++--
    1 file changed, 4 insertions(+), 2 deletions(-)

    diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
    index 968f51a2e39b..a7324045d87d 100644
    --- a/arch/x86/mm/pageattr.c
    +++ b/arch/x86/mm/pageattr.c
    @@ -298,9 +298,11 @@ static inline pgprot_t static_protections(pgprot_t prot, unsigned long address,

    /*
    * 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;

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