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

From: Masami Hiramatsu <[email-masked]>
Date: Thu, 1 Aug 2019 23:08:18 +0900

On Thu, 1 Aug 2019 13:16:22 +0100
Will Deacon <[email-masked]> wrote:

> 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 <[email-masked]>
> > Fixes: commit 7419333fa15e ("arm64: kprobe: Always clear pstate.D in breakpoint exception handler")
> > Reviewed-by: James Morse <[email-masked]>
> > Tested-by: James Morse <[email-masked]>
> > Signed-off-by: Masami Hiramatsu <[email-masked]>
> > ---
> >   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

Oops, daifflags.h is included via kvm_host.h...
OK, kprobes.c must include daifflags.h directly in this case.

I'll update this patch too.

Thank you,

> Will

-- 
Masami Hiramatsu <[email-masked]>

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