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    Subject[PATCH 4.14 040/167] x86/entry/64: Clear extra registers beyond syscall arguments, to reduce speculation attack surface
    Date
    4.14-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Dan Williams <dan.j.williams@intel.com>

    commit 8e1eb3fa009aa7c0b944b3c8b26b07de0efb3200 upstream.

    At entry userspace may have (maliciously) populated the extra registers
    outside the syscall calling convention with arbitrary values that could
    be useful in a speculative execution (Spectre style) attack.

    Clear these registers to minimize the kernel's attack surface.

    Note, this only clears the extra registers and not the unused
    registers for syscalls less than 6 arguments, since those registers are
    likely to be clobbered well before their values could be put to use
    under speculation.

    Note, Linus found that the XOR instructions can be executed with
    minimized cost if interleaved with the PUSH instructions, and Ingo's
    analysis found that R10 and R11 should be included in the register
    clearing beyond the typical 'extra' syscall calling convention
    registers.

    Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
    Reported-by: 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: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Link: http://lkml.kernel.org/r/151787988577.7847.16733592218894189003.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>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/x86/entry/entry_64.S | 13 +++++++++++++
    1 file changed, 13 insertions(+)

    --- a/arch/x86/entry/entry_64.S
    +++ b/arch/x86/entry/entry_64.S
    @@ -231,13 +231,26 @@ GLOBAL(entry_SYSCALL_64_after_hwframe)
    pushq %r8 /* pt_regs->r8 */
    pushq %r9 /* pt_regs->r9 */
    pushq %r10 /* pt_regs->r10 */
    + /*
    + * Clear extra registers that a speculation attack might
    + * otherwise want to exploit. Interleave XOR with PUSH
    + * for better uop scheduling:
    + */
    + xorq %r10, %r10 /* nospec r10 */
    pushq %r11 /* pt_regs->r11 */
    + xorq %r11, %r11 /* nospec r11 */
    pushq %rbx /* pt_regs->rbx */
    + xorl %ebx, %ebx /* nospec rbx */
    pushq %rbp /* pt_regs->rbp */
    + xorl %ebp, %ebp /* nospec rbp */
    pushq %r12 /* pt_regs->r12 */
    + xorq %r12, %r12 /* nospec r12 */
    pushq %r13 /* pt_regs->r13 */
    + xorq %r13, %r13 /* nospec r13 */
    pushq %r14 /* pt_regs->r14 */
    + xorq %r14, %r14 /* nospec r14 */
    pushq %r15 /* pt_regs->r15 */
    + xorq %r15, %r15 /* nospec r15 */
    UNWIND_HINT_REGS

    TRACE_IRQS_OFF

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