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    Subject[PATCH 07/11] x86/mm: comment _PAGE_GLOBAL mystery
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    From: Dave Hansen <dave.hansen@linux.intel.com>

    I was mystified as to where the _PAGE_GLOBAL in the kernel page tables
    for kernel text came from. I audited all the places I could find, but
    I missed one: head_64.S.

    The page tables that we create in here live for a long time, and they
    also have _PAGE_GLOBAL set, despite whether the processor supports it
    or not. It's harmless, and we got *lucky* that the pageattr code
    accidentally clears it when we wipe it out of __supported_pte_mask and
    then later try to mark kernel text read-only.

    Comment some of these properties to make it easier to find and
    understand in the future.

    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>
    ---

    b/arch/x86/kernel/head_64.S | 11 ++++++++++-
    1 file changed, 10 insertions(+), 1 deletion(-)

    diff -puN arch/x86/kernel/head_64.S~comment-global-page arch/x86/kernel/head_64.S
    --- a/arch/x86/kernel/head_64.S~comment-global-page 2018-04-02 10:26:46.113661210 -0700
    +++ b/arch/x86/kernel/head_64.S 2018-04-02 10:26:46.116661210 -0700
    @@ -399,8 +399,13 @@ NEXT_PAGE(level3_ident_pgt)
    .quad level2_ident_pgt - __START_KERNEL_map + _KERNPG_TABLE_NOENC
    .fill 511, 8, 0
    NEXT_PAGE(level2_ident_pgt)
    - /* Since I easily can, map the first 1G.
    + /*
    + * Since I easily can, map the first 1G.
    * Don't set NX because code runs from these pages.
    + *
    + * Note: This sets _PAGE_GLOBAL despite whether
    + * the CPU supports it or it is enabled. But,
    + * the CPU should ignore the bit.
    */
    PMDS(0, __PAGE_KERNEL_IDENT_LARGE_EXEC, PTRS_PER_PMD)
    #else
    @@ -431,6 +436,10 @@ NEXT_PAGE(level2_kernel_pgt)
    * (NOTE: at +512MB starts the module area, see MODULES_VADDR.
    * If you want to increase this then increase MODULES_VADDR
    * too.)
    + *
    + * This table is eventually used by the kernel during normal
    + * runtime. Care must be taken to clear out undesired bits
    + * later, like _PAGE_RW or _PAGE_GLOBAL in some cases.
    */
    PMDS(0, __PAGE_KERNEL_LARGE_EXEC,
    KERNEL_IMAGE_SIZE/PMD_SIZE)
    _
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