From: Al Viro <viro@xxxxxxxxxxxxxxxxxx> Subject: x86: signal: 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(). Link: https://lore.kernel.org/lkml/20200404170604.GN23230@xxxxxxxxxxxxxxxxxx/ Link: https://github.com/ClangBuiltLinux/linux/issues/876 Link: https://lkml.kernel.org/r/20210422230846.1756380-1-ndesaulniers@xxxxxxxxxx Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx> Tested-by: Nathan Chancellor <natechancellor@xxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- include/linux/compat.h | 2 -- include/linux/signal.h | 2 -- kernel/signal.c | 14 ++++---------- 3 files changed, 4 insertions(+), 14 deletions(-) --- a/include/linux/compat.h~x86-signal-dont-do-sas_ss_reset-until-we-are-certain-that-sigframe-wont-be-abandoned +++ a/include/linux/compat.h @@ -532,8 +532,6 @@ int __compat_save_altstack(compat_stack_ &__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); /* --- a/include/linux/signal.h~x86-signal-dont-do-sas_ss_reset-until-we-are-certain-that-sigframe-wont-be-abandoned +++ a/include/linux/signal.h @@ -462,8 +462,6 @@ int __save_altstack(stack_t __user *, un 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 --- a/kernel/signal.c~x86-signal-dont-do-sas_ss_reset-until-we-are-certain-that-sigframe-wont-be-abandoned +++ a/kernel/signal.c @@ -2829,6 +2829,8 @@ static void signal_delivered(struct ksig 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); } @@ -4147,11 +4149,7 @@ int __save_altstack(stack_t __user *uss, 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 @@ -4206,11 +4204,7 @@ int __compat_save_altstack(compat_stack_ &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 _
