[PATCH 4.9 89/93] signal: protect SIGNAL_UNKILLABLE from unintentional clearing.

[Greg Kroah-Hartman]

4.9-stable review patch.  If anyone has any objections, please let me know.

--

From: Jamie Iles 


[ Upstream commit 2d39b3cd34e6d323720d4c61bd714f5ae202c022 ]

Since commit 00cd5c37afd5 ("ptrace: permit ptracing of /sbin/init") we
can now trace init processes.  init is initially protected with
SIGNAL_UNKILLABLE which will prevent fatal signals such as SIGSTOP, but
there are a number of paths during tracing where SIGNAL_UNKILLABLE can
be implicitly cleared.

This can result in init becoming stoppable/killable after tracing.  For
example, running:

  while true; do kill -STOP 1; done &
  strace -p 1

and then stopping strace and the kill loop will result in init being
left in state TASK_STOPPED.  Sending SIGCONT to init will resume it, but
init will now respond to future SIGSTOP signals rather than ignoring
them.

Make sure that when setting SIGNAL_STOP_CONTINUED/SIGNAL_STOP_STOPPED
that we don't clear SIGNAL_UNKILLABLE.

Link: http://lkml.kernel.org/r/20170104122017.25047-1-jamie.i...@oracle.com
Signed-off-by: Jamie Iles 
Acked-by: Oleg Nesterov 
Cc: Alexander Viro 
Cc: Ingo Molnar 
Cc: Peter Zijlstra 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds 
Signed-off-by: Sasha Levin 
Signed-off-by: Greg Kroah-Hartman 
---
 include/linux/sched.h |   10 ++
 kernel/signal.c   |4 ++--
 2 files changed, 12 insertions(+), 2 deletions(-)

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -830,6 +830,16 @@ struct signal_struct {
 
 #define SIGNAL_UNKILLABLE  0x0040 /* for init: ignore fatal signals */
 
+#define SIGNAL_STOP_MASK (SIGNAL_CLD_MASK | SIGNAL_STOP_STOPPED | \
+ SIGNAL_STOP_CONTINUED)
+
+static inline void signal_set_stop_flags(struct signal_struct *sig,
+unsigned int flags)
+{
+   WARN_ON(sig->flags & (SIGNAL_GROUP_EXIT|SIGNAL_GROUP_COREDUMP));
+   sig->flags = (sig->flags & ~SIGNAL_STOP_MASK) | flags;
+}
+
 /* If true, all threads except ->group_exit_task have pending SIGKILL */
 static inline int signal_group_exit(const struct signal_struct *sig)
 {
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -346,7 +346,7 @@ static bool task_participate_group_stop(
 * fresh group stop.  Read comment in do_signal_stop() for details.
 */
if (!sig->group_stop_count && !(sig->flags & SIGNAL_STOP_STOPPED)) {
-   sig->flags = SIGNAL_STOP_STOPPED;
+   signal_set_stop_flags(sig, SIGNAL_STOP_STOPPED);
return true;
}
return false;
@@ -845,7 +845,7 @@ static bool prepare_signal(int sig, stru
 * will take ->siglock, notice SIGNAL_CLD_MASK, and
 * notify its parent. See get_signal_to_deliver().
 */
-   signal->flags = why | SIGNAL_STOP_CONTINUED;
+   signal_set_stop_flags(signal, why | 
SIGNAL_STOP_CONTINUED);
signal->group_stop_count = 0;
signal->group_exit_code = 0;
}

[PATCH 4.9 89/93] signal: protect SIGNAL_UNKILLABLE from unintentional clearing.

[Greg Kroah-Hartman]

4.9-stable review patch.  If anyone has any objections, please let me know.

--

From: Jamie Iles 


[ Upstream commit 2d39b3cd34e6d323720d4c61bd714f5ae202c022 ]

Since commit 00cd5c37afd5 ("ptrace: permit ptracing of /sbin/init") we
can now trace init processes.  init is initially protected with
SIGNAL_UNKILLABLE which will prevent fatal signals such as SIGSTOP, but
there are a number of paths during tracing where SIGNAL_UNKILLABLE can
be implicitly cleared.

This can result in init becoming stoppable/killable after tracing.  For
example, running:

  while true; do kill -STOP 1; done &
  strace -p 1

and then stopping strace and the kill loop will result in init being
left in state TASK_STOPPED.  Sending SIGCONT to init will resume it, but
init will now respond to future SIGSTOP signals rather than ignoring
them.

Make sure that when setting SIGNAL_STOP_CONTINUED/SIGNAL_STOP_STOPPED
that we don't clear SIGNAL_UNKILLABLE.

Link: http://lkml.kernel.org/r/20170104122017.25047-1-jamie.i...@oracle.com
Signed-off-by: Jamie Iles 
Acked-by: Oleg Nesterov 
Cc: Alexander Viro 
Cc: Ingo Molnar 
Cc: Peter Zijlstra 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds 
Signed-off-by: Sasha Levin 
Signed-off-by: Greg Kroah-Hartman 
---
 include/linux/sched.h |   10 ++
 kernel/signal.c   |4 ++--
 2 files changed, 12 insertions(+), 2 deletions(-)

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -830,6 +830,16 @@ struct signal_struct {
 
 #define SIGNAL_UNKILLABLE  0x0040 /* for init: ignore fatal signals */
 
+#define SIGNAL_STOP_MASK (SIGNAL_CLD_MASK | SIGNAL_STOP_STOPPED | \
+ SIGNAL_STOP_CONTINUED)
+
+static inline void signal_set_stop_flags(struct signal_struct *sig,
+unsigned int flags)
+{
+   WARN_ON(sig->flags & (SIGNAL_GROUP_EXIT|SIGNAL_GROUP_COREDUMP));
+   sig->flags = (sig->flags & ~SIGNAL_STOP_MASK) | flags;
+}
+
 /* If true, all threads except ->group_exit_task have pending SIGKILL */
 static inline int signal_group_exit(const struct signal_struct *sig)
 {
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -346,7 +346,7 @@ static bool task_participate_group_stop(
 * fresh group stop.  Read comment in do_signal_stop() for details.
 */
if (!sig->group_stop_count && !(sig->flags & SIGNAL_STOP_STOPPED)) {
-   sig->flags = SIGNAL_STOP_STOPPED;
+   signal_set_stop_flags(sig, SIGNAL_STOP_STOPPED);
return true;
}
return false;
@@ -845,7 +845,7 @@ static bool prepare_signal(int sig, stru
 * will take ->siglock, notice SIGNAL_CLD_MASK, and
 * notify its parent. See get_signal_to_deliver().
 */
-   signal->flags = why | SIGNAL_STOP_CONTINUED;
+   signal_set_stop_flags(signal, why | 
SIGNAL_STOP_CONTINUED);
signal->group_stop_count = 0;
signal->group_exit_code = 0;
}