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    Subject[tip:x86/pti] x86/entry/64: Clear registers for exceptions/interrupts, to reduce speculation attack surface
    Commit-ID:  3ac6d8c787b835b997eb23e43e09aa0895ef7d58
    Gitweb: https://git.kernel.org/tip/3ac6d8c787b835b997eb23e43e09aa0895ef7d58
    Author: Dan Williams <dan.j.williams@intel.com>
    AuthorDate: Mon, 5 Feb 2018 17:18:11 -0800
    Committer: Ingo Molnar <mingo@kernel.org>
    CommitDate: Tue, 6 Feb 2018 11:47:44 +0100

    x86/entry/64: Clear registers for exceptions/interrupts, to reduce speculation attack surface

    Clear the 'extra' registers on entering the 64-bit kernel for exceptions
    and interrupts. The common registers are not cleared since they are
    likely clobbered well before they can be exploited in a speculative
    execution attack.

    Originally-From: Andi Kleen <ak@linux.intel.com>
    Signed-off-by: Dan Williams <dan.j.williams@intel.com>
    Cc: <stable@vger.kernel.org>
    Cc: Andy Lutomirski <luto@kernel.org>
    Cc: Borislav Petkov <bp@alien8.de>
    Cc: Brian Gerst <brgerst@gmail.com>
    Cc: Denys Vlasenko <dvlasenk@redhat.com>
    Cc: H. Peter Anvin <hpa@zytor.com>
    Cc: Josh Poimboeuf <jpoimboe@redhat.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Link: http://lkml.kernel.org/r/151787989146.7847.15749181712358213254.stgit@dwillia2-desk3.amr.corp.intel.com
    [ Made small improvements to the changelog and the code comments. ]
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    ---
    arch/x86/entry/calling.h | 19 +++++++++++++++++++
    arch/x86/entry/entry_64.S | 6 +++++-
    2 files changed, 24 insertions(+), 1 deletion(-)

    diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
    index 3f48f69..f4b129d 100644
    --- a/arch/x86/entry/calling.h
    +++ b/arch/x86/entry/calling.h
    @@ -147,6 +147,25 @@ For 32-bit we have the following conventions - kernel is built with
    UNWIND_HINT_REGS offset=\offset
    .endm

    + /*
    + * Sanitize registers of values that a speculation attack
    + * might otherwise want to exploit. The lower registers are
    + * likely clobbered well before they could be put to use in
    + * a speculative execution gadget:
    + */
    + .macro CLEAR_REGS_NOSPEC
    + xorl %ebp, %ebp
    + xorl %ebx, %ebx
    + xorq %r8, %r8
    + xorq %r9, %r9
    + xorq %r10, %r10
    + xorq %r11, %r11
    + xorq %r12, %r12
    + xorq %r13, %r13
    + xorq %r14, %r14
    + xorq %r15, %r15
    + .endm
    +
    .macro POP_EXTRA_REGS
    popq %r15
    popq %r14
    diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
    index 065a71b..9e48002 100644
    --- a/arch/x86/entry/entry_64.S
    +++ b/arch/x86/entry/entry_64.S
    @@ -575,6 +575,7 @@ END(irq_entries_start)
    ALLOC_PT_GPREGS_ON_STACK
    SAVE_C_REGS
    SAVE_EXTRA_REGS
    + CLEAR_REGS_NOSPEC
    ENCODE_FRAME_POINTER

    testb $3, CS(%rsp)
    @@ -1133,6 +1134,7 @@ ENTRY(xen_failsafe_callback)
    ALLOC_PT_GPREGS_ON_STACK
    SAVE_C_REGS
    SAVE_EXTRA_REGS
    + CLEAR_REGS_NOSPEC
    ENCODE_FRAME_POINTER
    jmp error_exit
    END(xen_failsafe_callback)
    @@ -1178,6 +1180,7 @@ ENTRY(paranoid_entry)
    cld
    SAVE_C_REGS 8
    SAVE_EXTRA_REGS 8
    + CLEAR_REGS_NOSPEC
    ENCODE_FRAME_POINTER 8
    movl $1, %ebx
    movl $MSR_GS_BASE, %ecx
    @@ -1230,8 +1233,8 @@ ENTRY(error_entry)
    cld
    SAVE_C_REGS 8
    SAVE_EXTRA_REGS 8
    + CLEAR_REGS_NOSPEC
    ENCODE_FRAME_POINTER 8
    - xorl %ebx, %ebx
    testb $3, CS+8(%rsp)
    jz .Lerror_kernelspace

    @@ -1428,6 +1431,7 @@ ENTRY(nmi)
    pushq %r14 /* pt_regs->r14 */
    pushq %r15 /* pt_regs->r15 */
    UNWIND_HINT_REGS
    + CLEAR_REGS_NOSPEC
    ENCODE_FRAME_POINTER

    /*
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