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    Subject[PATCH v2 1/4] arm64: kprobes: Recover pstate.D in single-step exception handler
    Date
    On arm64, if a nested kprobes hit, it can crash the kernel with below
    error message.

    [ 152.118921] Unexpected kernel single-step exception at EL1

    This is because commit 7419333fa15e ("arm64: kprobe: Always clear
    pstate.D in breakpoint exception handler") unmask pstate.D for
    doing single step but does not recover it after single step in
    the nested kprobes. That is correct *unless* any nested kprobes
    (single-stepping) runs inside other kprobes user handler.

    When the 1st kprobe hits, do_debug_exception() will be called. At this
    point, debug exception (= pstate.D) must be masked (=1). When the 2nd
    (nested) kprobe is hit before single-step of the first kprobe, it
    unmask debug exception (pstate.D = 0) and return.
    Then, when the 1st kprobe setting up single-step, it saves current
    DAIF, mask DAIF, enable single-step, and restore DAIF.
    However, since "D" flag in DAIF is cleared by the 2nd kprobe, the
    single-step exception happens soon after restoring DAIF.

    To solve this issue, this stores all DAIF bits and restore it
    after single stepping.

    Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
    Fixes: commit 7419333fa15e ("arm64: kprobe: Always clear pstate.D in breakpoint exception handler")
    Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
    ---
    Changes in v2:
    - Save and restore all DAIF flags.
    - Operate pstate directly and remove spsr_set_debug_flag().
    ---
    arch/arm64/kernel/probes/kprobes.c | 41 ++++++------------------------------
    1 file changed, 7 insertions(+), 34 deletions(-)

    diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
    index bd5dfffca272..348e02b799a2 100644
    --- a/arch/arm64/kernel/probes/kprobes.c
    +++ b/arch/arm64/kernel/probes/kprobes.c
    @@ -29,6 +29,8 @@

    #include "decode-insn.h"

    +#define PSR_DAIF_MASK (PSR_D_BIT | PSR_A_BIT | PSR_I_BIT | PSR_F_BIT)
    +
    DEFINE_PER_CPU(struct kprobe *, current_kprobe) = NULL;
    DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);

    @@ -167,33 +169,6 @@ static void __kprobes set_current_kprobe(struct kprobe *p)
    __this_cpu_write(current_kprobe, p);
    }

    -/*
    - * When PSTATE.D is set (masked), then software step exceptions can not be
    - * generated.
    - * SPSR's D bit shows the value of PSTATE.D immediately before the
    - * exception was taken. PSTATE.D is set while entering into any exception
    - * mode, however software clears it for any normal (none-debug-exception)
    - * mode in the exception entry. Therefore, when we are entering into kprobe
    - * breakpoint handler from any normal mode then SPSR.D bit is already
    - * cleared, however it is set when we are entering from any debug exception
    - * mode.
    - * Since we always need to generate single step exception after a kprobe
    - * breakpoint exception therefore we need to clear it unconditionally, when
    - * we become sure that the current breakpoint exception is for kprobe.
    - */
    -static void __kprobes
    -spsr_set_debug_flag(struct pt_regs *regs, int mask)
    -{
    - unsigned long spsr = regs->pstate;
    -
    - if (mask)
    - spsr |= PSR_D_BIT;
    - else
    - spsr &= ~PSR_D_BIT;
    -
    - regs->pstate = spsr;
    -}
    -
    /*
    * Interrupts need to be disabled before single-step mode is set, and not
    * reenabled until after single-step mode ends.
    @@ -205,17 +180,17 @@ spsr_set_debug_flag(struct pt_regs *regs, int mask)
    static void __kprobes kprobes_save_local_irqflag(struct kprobe_ctlblk *kcb,
    struct pt_regs *regs)
    {
    - kcb->saved_irqflag = regs->pstate;
    + kcb->saved_irqflag = regs->pstate & PSR_DAIF_MASK;
    regs->pstate |= PSR_I_BIT;
    + /* Unmask PSTATE.D for enabling software step exceptions. */
    + regs->pstate &= ~PSR_D_BIT;
    }

    static void __kprobes kprobes_restore_local_irqflag(struct kprobe_ctlblk *kcb,
    struct pt_regs *regs)
    {
    - if (kcb->saved_irqflag & PSR_I_BIT)
    - regs->pstate |= PSR_I_BIT;
    - else
    - regs->pstate &= ~PSR_I_BIT;
    + regs->pstate &= ~PSR_DAIF_MASK;
    + regs->pstate |= kcb->saved_irqflag;
    }

    static void __kprobes
    @@ -252,8 +227,6 @@ static void __kprobes setup_singlestep(struct kprobe *p,

    set_ss_context(kcb, slot); /* mark pending ss */

    - spsr_set_debug_flag(regs, 0);
    -
    /* IRQs and single stepping do not mix well. */
    kprobes_save_local_irqflag(kcb, regs);
    kernel_enable_single_step(regs);
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