Subject: Re: [PATCH v4 15/29] arm64: handle PKEY/POE faults

From: Dave Martin <[email-masked]>
Date: Tue, 6 Aug 2024 14:33:37 +0100

Hi,

On Thu, Aug 01, 2024 at 05:01:10PM +0100, Joey Gouly wrote:
> On Thu, Jul 25, 2024 at 04:57:09PM +0100, Dave Martin wrote:
> > On Fri, May 03, 2024 at 02:01:33PM +0100, Joey Gouly wrote:
> > > If a memory fault occurs that is due to an overlay/pkey fault, report that to
> > > userspace with a SEGV_PKUERR.
> > > 
> > > Signed-off-by: Joey Gouly <[email-masked]>
> > > Cc: Catalin Marinas <[email-masked]>
> > > Cc: Will Deacon <[email-masked]>
> > > ---
> > >  arch/arm64/include/asm/traps.h |  1 +
> > >  arch/arm64/kernel/traps.c      | 12 ++++++--
> > >  arch/arm64/mm/fault.c          | 56 ++++++++++++++++++++++++++++++++--
> > >  3 files changed, 64 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/arch/arm64/include/asm/traps.h b/arch/arm64/include/asm/traps.h
> > > index eefe766d6161..f6f6f2cb7f10 100644
> > > --- a/arch/arm64/include/asm/traps.h
> > > +++ b/arch/arm64/include/asm/traps.h
> > > @@ -25,6 +25,7 @@ try_emulate_armv8_deprecated(struct pt_regs *regs, u32 insn)
> > >  void force_signal_inject(int signal, int code, unsigned long address, unsigned long err);
> > >  void arm64_notify_segfault(unsigned long addr);
> > >  void arm64_force_sig_fault(int signo, int code, unsigned long far, const char *str);
> > > +void arm64_force_sig_fault_pkey(int signo, int code, unsigned long far, const char *str, int pkey);
> > >  void arm64_force_sig_mceerr(int code, unsigned long far, short lsb, const char *str);
> > >  void arm64_force_sig_ptrace_errno_trap(int errno, unsigned long far, const char *str);
> > >  
> > > diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> > > index 215e6d7f2df8..1bac6c84d3f5 100644
> > > --- a/arch/arm64/kernel/traps.c
> > > +++ b/arch/arm64/kernel/traps.c
> > > @@ -263,16 +263,24 @@ static void arm64_show_signal(int signo, const char *str)
> > >  	__show_regs(regs);
> > >  }
> > >  
> > > -void arm64_force_sig_fault(int signo, int code, unsigned long far,
> > > -			   const char *str)
> > > +void arm64_force_sig_fault_pkey(int signo, int code, unsigned long far,
> > > +			   const char *str, int pkey)
> > >  {
> > >  	arm64_show_signal(signo, str);
> > >  	if (signo == SIGKILL)
> > >  		force_sig(SIGKILL);
> > > +	else if (code == SEGV_PKUERR)
> > > +		force_sig_pkuerr((void __user *)far, pkey);
> > 
> > Is signo definitely SIGSEGV here?  It looks to me like we can get in
> > here for SIGBUS, SIGTRAP etc.
> > 
> > si_codes are not unique between different signo here, so I'm wondering
> > whether this should this be:
> > 
> > 	else if (signo == SIGSEGV && code == SEGV_PKUERR)
> > 
> > ...?
> > 
> > 
> > >  	else
> > >  		force_sig_fault(signo, code, (void __user *)far);
> > >  }
> > >  
> > > +void arm64_force_sig_fault(int signo, int code, unsigned long far,
> > > +			   const char *str)
> > > +{
> > > +	arm64_force_sig_fault_pkey(signo, code, far, str, 0);
> > 
> > Is there a reason not to follow the same convention as elsewhere, where
> > -1 is passed for "no pkey"?
> > 
> > If we think this should never be called with signo == SIGSEGV &&
> > code == SEGV_PKUERR and no valid pkey but if it's messy to prove, then
> > maybe a WARN_ON_ONCE() would be worth it here?
> > 
> 
> Anshuman suggested to separate them out, which I did like below, I think that
> addresses your comments too?
> 
> diff --git arch/arm64/kernel/traps.c arch/arm64/kernel/traps.c
> index 215e6d7f2df8..49bac9ae04c0 100644
> --- arch/arm64/kernel/traps.c
> +++ arch/arm64/kernel/traps.c
> @@ -273,6 +273,13 @@ void arm64_force_sig_fault(int signo, int code, unsigned long far,
>                 force_sig_fault(signo, code, (void __user *)far);
>  }
>  
> +void arm64_force_sig_fault_pkey(int signo, int code, unsigned long far,
> +                          const char *str, int pkey)
> +{
> +       arm64_show_signal(signo, str);
> +       force_sig_pkuerr((void __user *)far, pkey);
> +}
> +
>  void arm64_force_sig_mceerr(int code, unsigned long far, short lsb,
>                             const char *str)
>  {
> 
> diff --git arch/arm64/mm/fault.c arch/arm64/mm/fault.c
> index 451ba7cbd5ad..1ddd46b97f88 100644
> --- arch/arm64/mm/fault.c
> +++ arch/arm64/mm/fault.c

(Guessing where this is means to apply, since there is no hunk header
or context...)

> 
> -               arm64_force_sig_fault(SIGSEGV, si_code, far, inf->name);
> +               if (si_code == SEGV_PKUERR)
> +                       arm64_force_sig_fault_pkey(SIGSEGV, si_code, far, inf->name, pkey);

Maybe drop the the signo and si_code argument?  This would mean that
arm64_force_sig_fault_pkey() can't be called with a signo/si_code
combination that makes no sense.

I think pkey faults are always going to be SIGSEGV/SEGV_PKUERR, right?
Or are there other combinations that can apply for these faults?

> +               else
> +                       arm64_force_sig_fault(SIGSEGV, si_code, far, inf->name);

Otherwise yes, I think splitting things this way makes sense.

Cheers
---Dave