Re: [PATCH v2] x86: signal: move save_altstack_ex out of generic headers

[Nathan Chancellor]

On Sat, Apr 04, 2020 at 06:06:04PM +0100, Al Viro wrote:
> On Sat, Apr 04, 2020 at 06:01:00PM +0200, Oleg Nesterov wrote:
> > On 04/03, Nick Desaulniers wrote:
> > >
> > > --- a/arch/x86/kernel/signal.c
> > > +++ b/arch/x86/kernel/signal.c
> > > @@ -416,6 +416,7 @@ static int __setup_rt_frame(int sig, struct ksignal *ksig,
> > >  	return 0;
> > >  Efault:
> > >  	user_access_end();
> > > +	reset_altstack();
> > >  	return -EFAULT;
> > >  }
> > >  #else /* !CONFIG_X86_32 */
> > > @@ -507,6 +508,7 @@ static int __setup_rt_frame(int sig, struct ksignal *ksig,
> > >  
> > >  Efault:
> > >  	user_access_end();
> > > +	reset_altstack();
> > >  	return -EFAULT;
> > >  }
> > 
> > I must have missed something, but this looks just wrong.
> > 
> > reset_altstack() should be called when __setup_rt_frame() (and
> > unsafe_save_altstack() in particular) succeeds, not when it fails.
> > 
> > Nevermind, Al has already suggested to use signal_delivered()...
> 
> FWIW, I propose to do is the patch below (against the current mainline);
> objections?
> 
> Don't do sas_ss_reset() until we are certain that sigframe won't be abandoned
> 
> Currently we handle SS_AUTODISARM as soon as we have stored the
> altstack settings into sigframe - that's the point when we have
> set the things up for eventual sigreturn to restore the old settings.
> And if we manage to set the sigframe up (we are not done with that
> yet), everything's fine.  However, in case of failure we end up
> with sigframe-to-be abandoned and SIGSEGV force-delivered.  And
> in that case we end up with inconsistent rules - late failures
> have altstack reset, early ones do not.
> 
> It's trivial to get consistent behaviour - just handle SS_AUTODISARM
> once we have set the sigframe up and are committed to entering
> the handler, i.e. in signal_delivered().
> 
> Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> ---
> diff --git a/include/linux/compat.h b/include/linux/compat.h
> index 0480ba4db592..f614967374f5 100644
> --- a/include/linux/compat.h
> +++ b/include/linux/compat.h
> @@ -461,8 +461,6 @@ int __compat_save_altstack(compat_stack_t __user *, unsigned long);
>  			&__uss->ss_sp, label); \
>  	unsafe_put_user(t->sas_ss_flags, &__uss->ss_flags, label); \
>  	unsafe_put_user(t->sas_ss_size, &__uss->ss_size, label); \
> -	if (t->sas_ss_flags & SS_AUTODISARM) \
> -		sas_ss_reset(t); \
>  } while (0);
>  
>  /*
> diff --git a/include/linux/signal.h b/include/linux/signal.h
> index 05bacd2ab135..28fe9cc134f7 100644
> --- a/include/linux/signal.h
> +++ b/include/linux/signal.h
> @@ -450,8 +450,6 @@ int __save_altstack(stack_t __user *, unsigned long);
>  	unsafe_put_user((void __user *)t->sas_ss_sp, &__uss->ss_sp, label); \
>  	unsafe_put_user(t->sas_ss_flags, &__uss->ss_flags, label); \
>  	unsafe_put_user(t->sas_ss_size, &__uss->ss_size, label); \
> -	if (t->sas_ss_flags & SS_AUTODISARM) \
> -		sas_ss_reset(t); \
>  } while (0);
>  
>  #ifdef CONFIG_PROC_FS
> diff --git a/kernel/signal.c b/kernel/signal.c
> index e58a6c619824..4cfe0b9af588 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -2769,6 +2769,8 @@ static void signal_delivered(struct ksignal *ksig, int stepping)
>  	if (!(ksig->ka.sa.sa_flags & SA_NODEFER))
>  		sigaddset(&blocked, ksig->sig);
>  	set_current_blocked(&blocked);
> +	if (current->sas_ss_flags & SS_AUTODISARM)
> +		sas_ss_reset(current);
>  	tracehook_signal_handler(stepping);
>  }
>  
> @@ -4070,11 +4072,7 @@ int __save_altstack(stack_t __user *uss, unsigned long sp)
>  	int err = __put_user((void __user *)t->sas_ss_sp, &uss->ss_sp) |
>  		__put_user(t->sas_ss_flags, &uss->ss_flags) |
>  		__put_user(t->sas_ss_size, &uss->ss_size);
> -	if (err)
> -		return err;
> -	if (t->sas_ss_flags & SS_AUTODISARM)
> -		sas_ss_reset(t);
> -	return 0;
> +	return err;
>  }
>  
>  #ifdef CONFIG_COMPAT
> @@ -4129,11 +4127,7 @@ int __compat_save_altstack(compat_stack_t __user *uss, unsigned long sp)
>  			 &uss->ss_sp) |
>  		__put_user(t->sas_ss_flags, &uss->ss_flags) |
>  		__put_user(t->sas_ss_size, &uss->ss_size);
> -	if (err)
> -		return err;
> -	if (t->sas_ss_flags & SS_AUTODISARM)
> -		sas_ss_reset(t);
> -	return 0;
> +	return err;
>  }
>  #endif
>  
> 

On next-20200404, this makes the three objtool warnings about the call
to memset() with UACCESS disabled disappear and does not add any new
ones.

Tested-by: Nathan Chancellor <natechancellor@xxxxxxxxx>