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.
To detect the support of this functionality, one can do err = sigaltstack(SS_DISABLE | SS_AUTODISARM); if (err && errno == EINVAL) unsupported(); Signed-off-by: Stas Sergeev <[email protected]> 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]> CC: Shuah Khan <[email protected]> --- include/linux/sched.h | 8 ++++++++ include/linux/signal.h | 4 +++- include/uapi/linux/signal.h | 4 +++- kernel/fork.c | 2 +- kernel/signal.c | 10 ++++++++-- 5 files changed, 23 insertions(+), 5 deletions(-) diff --git a/include/linux/sched.h b/include/linux/sched.h index a10494a..26201cd 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; @@ -2573,6 +2574,13 @@ static inline int sas_ss_flags(unsigned long sp) return on_sig_stack(sp) ? SS_ONSTACK : 0; } +static inline void sas_ss_reset(struct task_struct *p) +{ + p->sas_ss_sp = 0; + p->sas_ss_size = 0; + p->sas_ss_flags = SS_DISABLE; +} + static inline unsigned long sigsp(unsigned long sp, struct ksignal *ksig) { if (unlikely((ksig->ka.sa.sa_flags & SA_ONSTACK)) && ! sas_ss_flags(sp)) diff --git a/include/linux/signal.h b/include/linux/signal.h index 92557bb..3fbe814 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) \ + sas_ss_reset(t); \ } while (0); #ifdef CONFIG_PROC_FS diff --git a/include/uapi/linux/signal.h b/include/uapi/linux/signal.h index 7c73165..7388260 100644 --- a/include/uapi/linux/signal.h +++ b/include/uapi/linux/signal.h @@ -7,7 +7,9 @@ #define SS_ONSTACK 1 #define SS_DISABLE 2 +/* bit-flags */ +#define SS_AUTODISARM (1 << 4) /* disable sas during sighandling */ /* mask for all SS_xxx flags */ -#define SS_FLAG_BITS 0 +#define SS_FLAG_BITS SS_AUTODISARM #endif /* _UAPI_LINUX_SIGNAL_H */ diff --git a/kernel/fork.c b/kernel/fork.c index 2e391c7..68c8716 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -1483,7 +1483,7 @@ 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) - p->sas_ss_sp = p->sas_ss_size = 0; + sas_ss_reset(p); /* * Syscall tracing and stepping should be turned off in the diff --git a/kernel/signal.c b/kernel/signal.c index 9a24bc3..e1096ad 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -3133,6 +3133,7 @@ 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_flags; } error = 0; @@ -3163,9 +3164,14 @@ 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) + sas_ss_reset(t); + return 0; } #ifdef CONFIG_COMPAT -- 2.7.2
