signals/sigaltstack: Implement SS_AUTODISARM flag

This patch implements the SS_AUTODISARM flag that can be OR-ed 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 <stsp@list.ru>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Amanieu d'Antras <amanieu@gmail.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Jason Low <jason.low2@hp.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Moore <pmoore@redhat.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Richard Weinberger <richard@nod.at>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vladimir Davydov <vdavydov@parallels.com>
Cc: linux-api@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/1460665206-13646-4-git-send-email-stsp@list.ru
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 52c4847b05e2882a72d04c3c75fc4d55c2b4a6b9..2950c5cd30054eb9a6d702b7da5a4b98382a0cfd 100644 (file)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1596,6 +1596,7 @@ struct task_struct {
 
        unsigned long sas_ss_sp;
        size_t sas_ss_size;
+       unsigned sas_ss_flags;
 
        struct callback_head *task_works;
 
@@ -2592,6 +2593,13 @@ 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 92557bbce7e7b9a3d9c6c6ed4c4d4f54ff7ff9e5..3fbe81444d31e85db8b1bb0bb517a7d5ae4b5860 100644 (file)
--- 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 7c73165d11cee8bcec0f219cd475881c22f85ee7..738826048af22f56828069a28b9c6f58ad3f2e22 100644 (file)
--- 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 d277e83ed3e06d702875e4823a827c8a4c48b729..3e8451527cbe2ac87eed8bc3c9d64aeade9d10ef 100644 (file)
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1494,7 +1494,7 @@ 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 b1c6eb4df2a86ce5987d1d06010392683bd34930..bf97ea5775ae30959fde4ada06391ad2ee015fec 100644 (file)
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -3137,6 +3137,7 @@ 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;
@@ -3167,9 +3168,14 @@ 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