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Peter Anvin" , x86@kernel.org, Sebastian Andrzej Siewior , Andy Lutomirski , Sami Tolvanen , Marco Elver , Brian Gerst , Arnd Bergmann , Andrew Morton , "Eric W. Biederman" , linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] x86: signal: move save_altstack_ex out of generic headers Message-ID: <20200404225036.GA12416@ubuntu-m2-xlarge-x86> References: <20200324220830.110002-1-ndesaulniers@google.com> <20200403231611.81444-1-ndesaulniers@google.com> <20200404160100.GB26298@redhat.com> <20200404170604.GN23230@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200404170604.GN23230@ZenIV.linux.org.uk> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > --- > 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