[PATCH] arm64: kprobe: Always clear pstate.D in breakpoint exception handler

[Pratyush Anand]

Hi Sandeepa,

On 24/08/2016:03:09:16 PM, Sandeepa Prabhu wrote:
> On Mon, Aug 22, 2016 at 12:16 PM, Pratyush Anand <panand at redhat.com> wrote:
> 
> > Whenever we are hitting a kprobe from a none-kprobe debug exception
> > handler, we hit an infinite occurrences of "Unexpected kernel single-step
> > exception at EL1"
> >
> > PSTATE.D is debug exception mask bit. It is set whenever we enter into an
> > exception mode. When it is set then Watchpoint, Breakpoint, and Software
> > Step exceptions are masked. However, software Breakpoint Instruction
> > exceptions can never be masked. Therefore, if we ever execute a BRK
> > instruction, irrespective of D-bit setting, we will be receiving a
> > corresponding  breakpoint exception.
> > For example:
> > - We are executing kprobe pre/post handler, and kprobe has been inserted in
> >   one of the instruction of a function called by handler. So, it executes
> >   BRK instruction and we land into the case of KPROBE_REENTER. (This case
> > is
> >   already handled by current code)
> > - We are executing uprobe handler or any other BRK handler such as in
> >   WARN_ON (BRK BUG_BRK_IMM), and we trace that path using kprobe.So, we
> >   enter into kprobe breakpoint handler,from another BRK handler.(This case
> >   is not being handled currently)
> > In all such cases kprobe breakpoint exception will be raised when we were
> > already in debug exception mode.
> > SPSR's D bit (bit 9) shows the value of PSTATE.D immediately before the
> > exception was taken.So, in above example cases we would find it set in
> > kprobe breakpoint handler.
> > Single step exception will always be followed by a kprobe breakpoint
> > exception.However, it will only be raised gracefully if we clear D bit
> > while returning from breakpoint exception. If D bit is set then, it results
> > into undefined exception and when it's handler enables dbg then single step
> > exception is generated, however it will never be handled(because address
> > does not match and therefore treated as unexpected).
> > This patch clears D-flag unconditionally in setup_singlestep, so that we
> > can always get single step exception correctly after returning from
> > breakpoint exception.
> > Additionally, it also removes D-flag set statement for KPROBE_REENTER
> > return path, because debug exception for KPROBE_REENTER will always take
> > place in a debug exception state. So, D-flag will already be set in this
> > case.
> >
> > Signed-off-by: Pratyush Anand <panand at redhat.com>
> > ---
> >  arch/arm64/kernel/probes/kprobes.c | 29 +++++++++++++----------------
> >  1 file changed, 13 insertions(+), 16 deletions(-)
> >
> > diff --git a/arch/arm64/kernel/probes/kprobes.c
> > b/arch/arm64/kernel/probes/kprobes.c
> > index e51f4ad97525..6e13361b9c01 100644
> > --- a/arch/arm64/kernel/probes/kprobes.c
> > +++ b/arch/arm64/kernel/probes/kprobes.c
> > @@ -166,13 +166,18 @@ static void __kprobes set_current_kprobe(struct
> > kprobe *p)
> >  }
> >
> >  /*
> > - * The D-flag (Debug mask) is set (masked) upon debug exception entry.
> > - * Kprobes needs to clear (unmask) D-flag -ONLY- in case of recursive
> > - * probe i.e. when probe hit from kprobe handler context upon
> > - * executing the pre/post handlers. In this case we return with
> > - * D-flag clear so that single-stepping can be carried-out.
> > - *
> > - * Leave D-flag set in all other cases.
> > + * 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)
> > @@ -245,10 +250,7 @@ static void __kprobes setup_singlestep(struct kprobe
> > *p,
> >
> >                 set_ss_context(kcb, slot);      /* mark pending ss */
> >
> > -               if (kcb->kprobe_status == KPROBE_REENTER)
> > -                       spsr_set_debug_flag(regs, 0);
> > -               else
> > -                       WARN_ON(regs->pstate & PSR_D_BIT);
> > +               spsr_set_debug_flag(regs, 0);
> >
> >                 /* IRQs and single stepping do not mix well. */
> >                 kprobes_save_local_irqflag(kcb, regs);
> > @@ -333,8 +335,6 @@ int __kprobes kprobe_fault_handler(struct pt_regs
> > *regs, unsigned int fsr)
> >                         BUG();
> >
> >                 kernel_disable_single_step();
> > -               if (kcb->kprobe_status == KPROBE_REENTER)
> > -                       spsr_set_debug_flag(regs, 1);
> >
> >                 if (kcb->kprobe_status == KPROBE_REENTER)
> >                         restore_previous_kprobe(kcb);
> > @@ -457,9 +457,6 @@ kprobe_single_step_handler(struct pt_regs *regs,
> > unsigned int esr)
> >                 kprobes_restore_local_irqflag(kcb, regs);
> >                 kernel_disable_single_step();
> >
> > -               if (kcb->kprobe_status == KPROBE_REENTER)
> > -                       spsr_set_debug_flag(regs, 1);
> > -
> >                 post_kprobe_handler(kcb, regs);
> >         }
> >
> > Thanks for the fix, feel free to add my ACK as well.
> Acked-by: Sandeepa Prabhu <sandeepa.s.prabhu at gmail.com>

Thanks.

> 
> btw, has it been tested on guest kernel?

I have not tested it on guest kernel. I will setup a kvm guest to see if all
works fine there too.

~Pratyush