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    Subject[PATCH 4.19 035/103] x86/efistub: Disable paging at mixed mode entry
    Date
    From: Ard Biesheuvel <ardb@kernel.org>

    commit 4911ee401b7ceff8f38e0ac597cbf503d71e690c upstream.

    The EFI mixed mode entry code goes through the ordinary startup_32()
    routine before jumping into the kernel's EFI boot code in 64-bit
    mode. The 32-bit startup code must be entered with paging disabled,
    but this is not documented as a requirement for the EFI handover
    protocol, and so we should disable paging explicitly when entering
    the kernel from 32-bit EFI firmware.

    Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
    Cc: <stable@vger.kernel.org>
    Cc: Arvind Sankar <nivedita@alum.mit.edu>
    Cc: Hans de Goede <hdegoede@redhat.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: linux-efi@vger.kernel.org
    Link: https://lkml.kernel.org/r/20191224132909.102540-4-ardb@kernel.org
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/x86/boot/compressed/head_64.S | 5 +++++
    1 file changed, 5 insertions(+)

    --- a/arch/x86/boot/compressed/head_64.S
    +++ b/arch/x86/boot/compressed/head_64.S
    @@ -244,6 +244,11 @@ ENTRY(efi32_stub_entry)
    leal efi32_config(%ebp), %eax
    movl %eax, efi_config(%ebp)

    + /* Disable paging */
    + movl %cr0, %eax
    + btrl $X86_CR0_PG_BIT, %eax
    + movl %eax, %cr0
    +
    jmp startup_32
    ENDPROC(efi32_stub_entry)
    #endif

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