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 <stsp@xxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxxxxx>
CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CC: Richard Weinberger <richard@xxxxxx>
CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
CC: Oleg Nesterov <oleg@xxxxxxxxxx>
CC: Tejun Heo <tj@xxxxxxxxxx>
CC: Heinrich Schuchardt <xypron.glpk@xxxxxx>
CC: Jason Low <jason.low2@xxxxxx>
CC: Andrea Arcangeli <aarcange@xxxxxxxxxx>
CC: Frederic Weisbecker <fweisbec@xxxxxxxxx>
CC: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
CC: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
CC: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
CC: Aleksa Sarai <cyphar@xxxxxxxxxx>
CC: "Amanieu d'Antras" <amanieu@xxxxxxxxx>
CC: Paul Moore <pmoore@xxxxxxxxxx>
CC: Sasha Levin <sasha.levin@xxxxxxxxxx>
CC: Palmer Dabbelt <palmer@xxxxxxxxxxx>
CC: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
CC: linux-kernel@xxxxxxxxxxxxxxx
CC: linux-api@xxxxxxxxxxxxxxx
CC: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
CC: Shuah Khan <shuahkh@xxxxxxxxxxxxxxx>
---
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
--
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