linux-kernel - Re: [PATCH v3 1/4] arm64: kprobes: Recover pstate.D in single-step exception handler

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Date:   Thu, 1 Aug 2019 13:16:22 +0100
From:   Will Deacon <will@...nel.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Dan Rue <dan.rue@...aro.org>,
        Daniel Diaz <daniel.diaz@...aro.org>,
        Anders Roxell <anders.roxell@...aro.org>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        linux-kernel@...r.kernel.org, James Morse <james.morse@....com>,
        Matt Hart <matthew.hart@...aro.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 1/4] arm64: kprobes: Recover pstate.D in single-step
 exception handler

On Thu, Jul 25, 2019 at 05:15:54PM +0900, Masami Hiramatsu wrote:
> kprobes manipulates the interrupted PSTATE for single step, and
> doesn't restore it. Thus, if we put a kprobe where the pstate.D
> (debug) masked, the mask will be cleared after the kprobe hits.
> 
> Moreover, in the most complicated case, this can lead a kernel
> crash with below message when a nested kprobe hits.
> 
> [  152.118921] Unexpected kernel single-step exception at EL1
> 
> When the 1st kprobe hits, do_debug_exception() will be called.
> At this point, debug exception (= pstate.D) must be masked (=1).
> But if another kprobes hits before single-step of the first kprobe
> (e.g. inside user pre_handler), it unmask the 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.
> 
> This has been introduced by commit 7419333fa15e ("arm64: kprobe:
> Always clear pstate.D in breakpoint exception handler")
> 
> To solve this issue, this stores all DAIF bits and restore it
> after single stepping.
> 
> Reported-by: Naresh Kamboju <naresh.kamboju@...aro.org>
> Fixes: commit 7419333fa15e ("arm64: kprobe: Always clear pstate.D in breakpoint exception handler")
> Reviewed-by: James Morse <james.morse@....com>
> Tested-by: James Morse <james.morse@....com>
> Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
> ---
>   Changes in v3:
>    - Update patch description
>    - move PSR_DAIF_MASK in daifflags.h
>   Changes in v2:
>    - Save and restore all DAIF flags.
>    - Operate pstate directly and remove spsr_set_debug_flag().
> ---
>  arch/arm64/include/asm/daifflags.h |    2 ++
>  arch/arm64/kernel/probes/kprobes.c |   39 +++++-------------------------------
>  2 files changed, 7 insertions(+), 34 deletions(-)

I'm seeing an allmodconfig build failure with this:

arch/arm64/kernel/probes/kprobes.c: In function ‘kprobes_save_local_irqflag’:
arch/arm64/kernel/probes/kprobes.c:181:38: error: ‘DAIF_MASK’ undeclared (first use in this function); did you mean ‘BIT_MASK’?
  kcb->saved_irqflag = regs->pstate & DAIF_MASK;
                                      ^~~~~~~~~
                                      BIT_MASK
arch/arm64/kernel/probes/kprobes.c:181:38: note: each undeclared identifier is reported only once for each function it appears in
arch/arm64/kernel/probes/kprobes.c: In function ‘kprobes_restore_local_irqflag’:
arch/arm64/kernel/probes/kprobes.c:190:19: error: ‘DAIF_MASK’ undeclared (first use in this function); did you mean ‘BIT_MASK’?
  regs->pstate &= ~DAIF_MASK;
                   ^~~~~~~~~
                   BIT_MASK
make[2]: *** [scripts/Makefile.build:274: arch/arm64/kernel/probes/kprobes.o] Error 1

Will