[PATCH] add new prctl for a per process wide close on exec

Stefani Seibold
2013-10-22T19:27:50Z

This small patch adds a runtime prctl config option for a per process wide
"close on exec" without breaking existing code.

With this feature a developer can decide if the application will pass all non
"close on exec" file descriptors to a new process or not.

The mode of the process wide "close on exec" can be set with PR_SET_CLOEXEC and
PR_GET_CLOEXEC returns the current mode. Mode is one of the following:

- PR_CLOEXEC_DEFAULT closes only the fd's marked as "close on exec" in
the child process, this is the linux default behaviour.

- PR_CLOEXEC_ONCE closes all fd's expect 0, 1 and 2 which are regular
handled as in PR_CLOEXEC_DEFAULT and reset the mode of the child to
PR_CLOEXEC_DEFAULT.

- PR_CLOEXEC_INHERIT is like PR_CLOEXEC_ONCE, but the mode will stay in the
child

STDIO file descriptors will be passed to the child process depending on the
..._CLOEXEC flag. So the new modes should be compatible to regular code.

This patch will increase security since no developers can review all libraries
which there are using. Also in a team of developers it is not always possible
to have a full survey over the code which is produced. Or the output of a code
generators and so one. This patch allows a kind of preventive measures.

It can also prevent resource occupation. Imagine a long running process (a
daemon) is execute from the application after open some file desciptors. For
example libpcsclite.so will not open the socket with SOCK_CLOEXEC. Or a device
driver which alows only a single open. In both cases the resource cannot
reopened after a close. Sigh!

What do you think?

ChangeLog:

2013-10-21 First release to the mailing list
2013-10-22 Fix fork for non main threads

The patch is against 3.12.0-rc6

Greetings,
Stefani

Signed-off-by: Stefani Seibold <stefani@seibold.net>
---
fs/exec.c | 2 ++
fs/file.c | 12 +++++++++++-
include/linux/sched.h | 6 ++++++
include/uapi/linux/prctl.h | 18 ++++++++++++++++++
kernel/fork.c | 3 +++
kernel/sys.c | 32 ++++++++++++++++++++++++++++++++
6 files changed, 72 insertions(+), 1 deletion(-)

diff --git a/fs/exec.c b/fs/exec.c
index 8875dd1..c8af524 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1546,6 +1546,8 @@ static int do_execve_common(const char *filename,
/* execve succeeded */
current->fs->in_exec = 0;
current->in_execve = 0;
+ if (!current->cloexec_inherit)
+ current->cloexec = 0;
acct_update_integrals(current);
free_bprm(bprm);
if (displaced)
diff --git a/fs/file.c b/fs/file.c
index 4a78f98..7372252 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -622,7 +622,17 @@ void do_close_on_exec(struct files_struct *files)
fdt = files_fdtable(files);
if (fd >= fdt->max_fds)
break;
- set = fdt->close_on_exec[i];
+ if (!current->cloexec)
+ set = fdt->close_on_exec[i];
+ else {
+ set = fdt->open_fds[i];
+
+ /* special handling for stdio */
+ if (!i) {
+ set &= ~7;
+ set |= fdt->close_on_exec[i] & 7;
+ }
+ }
if (!set)
continue;
fdt->close_on_exec[i] = 0;
diff --git a/include/linux/sched.h b/include/linux/sched.h
index e27baee..469e6f8 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1112,6 +1112,12 @@ struct task_struct {
unsigned sched_reset_on_fork:1;
unsigned sched_contributes_to_load:1;

+ /* close non stdio on exec */
+ unsigned cloexec:1;
+
+ /* inherit cloexec flag on exec */
+ unsigned cloexec_inherit:1;
+
pid_t pid;
pid_t tgid;

diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 289760f..e1c2d66 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -149,4 +149,22 @@

#define PR_GET_TID_ADDRESS 40

+/*
+ * PR_CLOEXEC allows to configure the inheritance of the non stdio file
+ * handles to a child process:
+ *
+ * - PR_CLOEXEC_DEFAULT: close only the fd's marked with as close on exec
+ * in the child process
+ * - PR_CLOEXEC_ONCE: close all fd's expect 0, 1 and 2 which are regular
+ * handled as in PR_CLOEXEC_DEFAULT and reset the mode of the child to
+ * PR_CLOEXEC_DEFAULT
+ * - PR_CLOEXEC_INHERIT: like PR_CLOEXEC_ONCE, but the mode will stay in the
+ * child process
+ */
+#define PR_SET_CLOEXEC 41
+#define PR_GET_CLOEXEC 42
+# define PR_CLOEXEC_DEFAULT 1
+# define PR_CLOEXEC_ONCE 2
+# define PR_CLOEXEC_INHERIT 3
+
#endif /* _LINUX_PRCTL_H */
diff --git a/kernel/fork.c b/kernel/fork.c
index 086fe73..1aacf2e 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1190,6 +1190,9 @@ static struct task_struct *copy_process(unsigned long clone_flags,
if (!p)
goto fork_out;

+ p->cloexec = current->group_leader->cloexec;
+ p->cloexec_inherit = current->group_leader->cloexec_inherit;
+
ftrace_graph_init_task(p);
get_seccomp_filter(p);

diff --git a/kernel/sys.c b/kernel/sys.c
index c18ecca..7e53e14 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1999,6 +1999,38 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
if (arg2 || arg3 || arg4 || arg5)
return -EINVAL;
return current->no_new_privs ? 1 : 0;
+ case PR_SET_CLOEXEC:
+ if (arg3 || arg4 || arg5)
+ return -EINVAL;
+ switch(arg2) {
+ case PR_CLOEXEC_DEFAULT:
+ current->group_leader->cloexec = 0;
+ current->group_leader->cloexec_inherit = 0;
+ break;
+ case PR_CLOEXEC_ONCE:
+ current->group_leader->cloexec = 1;
+ current->group_leader->cloexec_inherit = 0;
+ break;
+ case PR_CLOEXEC_INHERIT:
+ current->group_leader->cloexec = 1;
+ current->group_leader->cloexec_inherit = 1;
+ break;
+ default:
+ return -EINVAL;
+ }
+ break;
+ case PR_GET_CLOEXEC:
+ if (arg2 || arg3 || arg4 || arg5)
+ return -EINVAL;
+ if (!current->group_leader->cloexec)
+ error = PR_CLOEXEC_DEFAULT;
+ else {
+ if (!current->group_leader->cloexec_inherit)
+ error = PR_CLOEXEC_ONCE;
+ else
+ error = PR_CLOEXEC_INHERIT;
+ }
+ break;
default:
error = -EINVAL;
break;
--
1.8.4