Re: [PATCH 1/2] sigaltstack: implement SS_AUTODISARM flag

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 <redacted>
CC: Peter Zijlstra <redacted>
CC: Richard Weinberger <richard-/L3Ra7n9ekc@public.gmane.org>
CC: Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
CC: Oleg Nesterov <redacted>
CC: Tejun Heo <redacted>
CC: Heinrich Schuchardt <redacted>
CC: Jason Low <redacted>
CC: Andrea Arcangeli <redacted>
CC: Frederic Weisbecker <redacted>
CC: Konstantin Khlebnikov <redacted>
CC: Josh Triplett <redacted>
CC: "Eric W. Biederman" <ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
CC: Aleksa Sarai <cyphar-gVpy/LI/lHzQT0dZR+AlfA@public.gmane.org>
CC: "Amanieu d'Antras" <redacted>
CC: Paul Moore <redacted>
CC: Sasha Levin <redacted>
CC: Palmer Dabbelt <redacted>
CC: Vladimir Davydov <redacted>
CC: linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
CC: linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
CC: Andy Lutomirski <redacted>

Signed-off-by: Stas Sergeev <redacted>
---
 include/linux/sched.h       |  8 ++++++++
 include/linux/signal.h      |  4 +++-
 include/uapi/linux/signal.h |  3 +++
 kernel/fork.c               |  2 +-
 kernel/signal.c             | 23 ++++++++++++-----------
 5 files changed, 27 insertions(+), 13 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 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

diff --git a/kernel/signal.c b/kernel/signal.c
index 0508544..fab6bea 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;