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    Subject[PATCH v2 07/36] x86/entry/64/compat: After SYSENTER, move STI after the NT fixup
    Date
    We eventually want to make it all the way into C code before
    enabling interrupts. We need to rework our flags handling slightly
    to delay enabling interrupts.

    Signed-off-by: Andy Lutomirski <luto@kernel.org>
    ---
    arch/x86/entry/entry_64_compat.S | 30 ++++++++++++++++++++++--------
    1 file changed, 22 insertions(+), 8 deletions(-)

    diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
    index aa76864a8a6b..1432d60a1f4a 100644
    --- a/arch/x86/entry/entry_64_compat.S
    +++ b/arch/x86/entry/entry_64_compat.S
    @@ -58,14 +58,9 @@ ENDPROC(native_usergs_sysret32)
    * with the int 0x80 path.
    */
    ENTRY(entry_SYSENTER_compat)
    - /*
    - * Interrupts are off on entry.
    - * We do not frame this tiny irq-off block with TRACE_IRQS_OFF/ON,
    - * it is too small to ever cause noticeable irq latency.
    - */
    + /* Interrupts are off on entry. */
    SWAPGS_UNSAFE_STACK
    movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp
    - ENABLE_INTERRUPTS(CLBR_NONE)

    /* Zero-extending 32-bit regs, do not remove */
    movl %ebp, %ebp
    @@ -76,7 +71,16 @@ ENTRY(entry_SYSENTER_compat)
    /* Construct struct pt_regs on stack */
    pushq $__USER32_DS /* pt_regs->ss */
    pushq %rbp /* pt_regs->sp */
    - pushfq /* pt_regs->flags */
    +
    + /*
    + * Push flags. This is nasty. First, interrupts are currently
    + * off, but we need pt_regs->flags to have IF set. Second, even
    + * if TF was set when SYSENTER started, it's clear by now. We fix
    + * that later using TIF_SINGLESTEP.
    + */
    + pushfq /* pt_regs->flags (except IF = 0) */
    + orl $X86_EFLAGS_IF, (%rsp) /* Fix saved flags */
    +
    pushq $__USER32_CS /* pt_regs->cs */
    pushq %r10 /* pt_regs->ip = thread_info->sysenter_return */
    pushq %rax /* pt_regs->orig_ax */
    @@ -92,12 +96,22 @@ ENTRY(entry_SYSENTER_compat)
    * Sysenter doesn't filter flags, so we need to clear NT
    * ourselves. To save a few cycles, we can check whether
    * NT was set instead of doing an unconditional popfq.
    + * This needs to happen before enabling interrupts so that
    + * we don't get preempted with NT set.
    */
    testl $X86_EFLAGS_NT, EFLAGS(%rsp)
    jnz sysenter_fix_flags
    sysenter_flags_fixed:

    /*
    + * Re-enable interrupts. IRQ tracing already thinks that IRQs are
    + * on (since we treat user mode as having IRQs on), and the
    + * prologue above is too short for it to be worth adding a
    + * tracing round trip.
    + */
    + ENABLE_INTERRUPTS(CLBR_NONE)
    +
    + /*
    * no need to do an access_ok check here because rbp has been
    * 32-bit zero extended
    */
    @@ -244,7 +258,7 @@ sysexit_audit:
    #endif

    sysenter_fix_flags:
    - pushq $(X86_EFLAGS_IF|X86_EFLAGS_FIXED)
    + pushq $X86_EFLAGS_FIXED
    popfq
    jmp sysenter_flags_fixed

    --
    2.4.3


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