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SubjectRe: [PATCH v2] x86: signal: move save_altstack_ex out of generic headers
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@zeniv.linux.org.uk>
> ---
> 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@gmail.com>

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