This patch implements the SS_AUTODISARM flag that can be ORed with SS_ONSTACK when forming ss_flags. When this flag is set, sigaltstack will be disabled when entering the signal handler; more precisely, after saving sas to uc_stack. When leaving the signal handler, the sigaltstack is restored by uc_stack. When this flag is used, it is safe to switch from sighandler with swapcontext(). Without this flag, the subsequent signal will corrupt the state of the switched-away sighandler.
CC: Ingo Molnar <[email protected]> CC: Peter Zijlstra <[email protected]> CC: Richard Weinberger <[email protected]> CC: Andrew Morton <[email protected]> CC: Oleg Nesterov <[email protected]> CC: Tejun Heo <[email protected]> CC: Heinrich Schuchardt <[email protected]> CC: Jason Low <[email protected]> CC: Andrea Arcangeli <[email protected]> CC: Frederic Weisbecker <[email protected]> CC: Konstantin Khlebnikov <[email protected]> CC: Josh Triplett <[email protected]> CC: "Eric W. Biederman" <[email protected]> CC: Aleksa Sarai <[email protected]> CC: "Amanieu d'Antras" <[email protected]> CC: Paul Moore <[email protected]> CC: Sasha Levin <[email protected]> CC: Palmer Dabbelt <[email protected]> CC: Vladimir Davydov <[email protected]> CC: [email protected] CC: [email protected] CC: Andy Lutomirski <[email protected]> Signed-off-by: Stas Sergeev <[email protected]> --- include/linux/sched.h | 1 + include/linux/signal.h | 4 +++- include/uapi/linux/signal.h | 3 +++ kernel/fork.c | 4 +++- kernel/signal.c | 23 ++++++++++++----------- 5 files changed, 22 insertions(+), 13 deletions(-) diff --git a/include/linux/sched.h b/include/linux/sched.h index a10494a..f561d34 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1587,6 +1587,7 @@ struct task_struct { unsigned long sas_ss_sp; size_t sas_ss_size; + unsigned sas_ss_flags; struct callback_head *task_works; diff --git a/include/linux/signal.h b/include/linux/signal.h index 92557bb..be3ebe0 100644 --- a/include/linux/signal.h +++ b/include/linux/signal.h @@ -432,8 +432,10 @@ int __save_altstack(stack_t __user *, unsigned long); stack_t __user *__uss = uss; \ struct task_struct *t = current; \ put_user_ex((void __user *)t->sas_ss_sp, &__uss->ss_sp); \ - put_user_ex(sas_ss_flags(sp), &__uss->ss_flags); \ + put_user_ex(t->sas_ss_flags, &__uss->ss_flags); \ put_user_ex(t->sas_ss_size, &__uss->ss_size); \ + if (t->sas_ss_flags & SS_AUTODISARM) \ + t->sas_ss_size = 0; \ } while (0); #ifdef CONFIG_PROC_FS diff --git a/include/uapi/linux/signal.h b/include/uapi/linux/signal.h index e1bd50c2..4691bc5 100644 --- a/include/uapi/linux/signal.h +++ b/include/uapi/linux/signal.h @@ -6,5 +6,8 @@ #define SS_ONSTACK 1 #define SS_DISABLE 2 +#define SS_VALMASK 0xf +/* bit-flags */ +#define SS_AUTODISARM (1 << 4) /* disable sas during sighandling */ #endif /* _UAPI_LINUX_SIGNAL_H */ diff --git a/kernel/fork.c b/kernel/fork.c index 2e391c7..ce840a9 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -1482,8 +1482,10 @@ static struct task_struct *copy_process(unsigned long clone_flags, /* * sigaltstack should be cleared when sharing the same VM */ - if ((clone_flags & (CLONE_VM|CLONE_VFORK)) == CLONE_VM) + if ((clone_flags & (CLONE_VM|CLONE_VFORK)) == CLONE_VM) { p->sas_ss_sp = p->sas_ss_size = 0; + p->sas_ss_flags = SS_DISABLE; + } /* * Syscall tracing and stepping should be turned off in the diff --git a/kernel/signal.c b/kernel/signal.c index 0508544..25199d4 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -3100,13 +3100,14 @@ do_sigaltstack (const stack_t __user *uss, stack_t __user *uoss, unsigned long s if (uss) { void __user *ss_sp; size_t ss_size; - int ss_flags; + unsigned ss_flags; + unsigned ss_xflags; error = -EFAULT; if (!access_ok(VERIFY_READ, uss, sizeof(*uss))) goto out; error = __get_user(ss_sp, &uss->ss_sp) | - __get_user(ss_flags, &uss->ss_flags) | + __get_user(ss_xflags, &uss->ss_flags) | __get_user(ss_size, &uss->ss_size); if (error) goto out; @@ -3115,14 +3116,8 @@ do_sigaltstack (const stack_t __user *uss, stack_t __user *uoss, unsigned long s if (on_sig_stack(sp)) goto out; + ss_flags = ss_xflags & SS_VALMASK; error = -EINVAL; - /* - * Note - this code used to test ss_flags incorrectly: - * old code may have been written using ss_flags==0 - * to mean ss_flags==SS_ONSTACK (as this was the only - * way that worked) - this fix preserves that older - * mechanism. - */ if (ss_flags != SS_DISABLE && ss_flags != SS_ONSTACK && ss_flags != 0) goto out; @@ -3137,6 +3132,7 @@ do_sigaltstack (const stack_t __user *uss, stack_t __user *uoss, unsigned long s current->sas_ss_sp = (unsigned long) ss_sp; current->sas_ss_size = ss_size; + current->sas_ss_flags = ss_xflags; } error = 0; @@ -3167,9 +3163,14 @@ int restore_altstack(const stack_t __user *uss) int __save_altstack(stack_t __user *uss, unsigned long sp) { struct task_struct *t = current; - return __put_user((void __user *)t->sas_ss_sp, &uss->ss_sp) | - __put_user(sas_ss_flags(sp), &uss->ss_flags) | + 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) + t->sas_ss_size = 0; + return 0; } #ifdef CONFIG_COMPAT -- 2.7.2
