Re: [PATCH 03/14] x86/entry/64: Fix unwind hints in register clearing code

[Andy Lutomirski]

> On Mar 12, 2020, at 10:31 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> 
> ïThe PUSH_AND_CLEAR_REGS macro zeroes each register immediately after
> pushing it.  If an NMI or exception hits after a register is cleared,
> but before the UNWIND_HINT_REGS annotation, the ORC unwinder will
> wrongly think the previous value of the register was zero.  This can
> confuse the unwinding process and cause it to exit early.
> 
> Because ORC is simpler than DWARF, there are a limited number of unwind
> annotation states, so it's not possible to add an individual unwind hint
> after each push/clear combination.  Instead, the register clearing
> instructions need to be consolidated and moved to after the
> UNWIND_HINT_REGS annotation.

I donât suppose you know how bad t he performance hit is on a non-PTI machine?

> 
> Fixes: 3f01daecd545 ("x86/entry/64: Introduce the PUSH_AND_CLEAN_REGS macro")
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> ---
> arch/x86/entry/calling.h | 40 +++++++++++++++++++++-------------------
> 1 file changed, 21 insertions(+), 19 deletions(-)
> 
> diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
> index 0789e13ece90..1c7f13bb6728 100644
> --- a/arch/x86/entry/calling.h
> +++ b/arch/x86/entry/calling.h
> @@ -98,13 +98,6 @@ For 32-bit we have the following conventions - kernel is built with
> #define SIZEOF_PTREGS    21*8
> 
> .macro PUSH_AND_CLEAR_REGS rdx=%rdx rax=%rax save_ret=0
> -    /*
> -     * Push registers and 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.
> -     * Interleave XOR with PUSH for better uop scheduling:
> -     */
>    .if \save_ret
>    pushq    %rsi        /* pt_regs->si */
>    movq    8(%rsp), %rsi    /* temporarily store the return address in %rsi */
> @@ -114,34 +107,43 @@ For 32-bit we have the following conventions - kernel is built with
>    pushq   %rsi        /* pt_regs->si */
>    .endif
>    pushq    \rdx        /* pt_regs->dx */
> -    xorl    %edx, %edx    /* nospec   dx */
>    pushq   %rcx        /* pt_regs->cx */
> -    xorl    %ecx, %ecx    /* nospec   cx */
>    pushq   \rax        /* pt_regs->ax */
>    pushq   %r8        /* pt_regs->r8 */
> -    xorl    %r8d, %r8d    /* nospec   r8 */
>    pushq   %r9        /* pt_regs->r9 */
> -    xorl    %r9d, %r9d    /* nospec   r9 */
>    pushq   %r10        /* pt_regs->r10 */
> -    xorl    %r10d, %r10d    /* nospec   r10 */
>    pushq   %r11        /* pt_regs->r11 */
> -    xorl    %r11d, %r11d    /* 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 */
> -    xorl    %r12d, %r12d    /* nospec   r12*/
>    pushq    %r13        /* pt_regs->r13 */
> -    xorl    %r13d, %r13d    /* nospec   r13*/
>    pushq    %r14        /* pt_regs->r14 */
> -    xorl    %r14d, %r14d    /* nospec   r14*/
>    pushq    %r15        /* pt_regs->r15 */
> -    xorl    %r15d, %r15d    /* nospec   r15*/
>    UNWIND_HINT_REGS
> +
>    .if \save_ret
>    pushq    %rsi        /* return address on top of stack */
>    .endif
> +
> +    /*
> +     * 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.
> +     */
> +    xorl    %edx,  %edx    /* nospec dx  */
> +    xorl    %ecx,  %ecx    /* nospec cx  */
> +    xorl    %r8d,  %r8d    /* nospec r8  */
> +    xorl    %r9d,  %r9d    /* nospec r9  */
> +    xorl    %r10d, %r10d    /* nospec r10 */
> +    xorl    %r11d, %r11d    /* nospec r11 */
> +    xorl    %ebx,  %ebx    /* nospec rbx */
> +    xorl    %ebp,  %ebp    /* nospec rbp */
> +    xorl    %r12d, %r12d    /* nospec r12 */
> +    xorl    %r13d, %r13d    /* nospec r13 */
> +    xorl    %r14d, %r14d    /* nospec r14 */
> +    xorl    %r15d, %r15d    /* nospec r15 */
> +
> .endm
> 
> .macro POP_REGS pop_rdi=1 skip_r11rcx=0
> -- 
> 2.21.1
>