[PATCH 2/2] exec: move core_pattern pipe helper into the crashing namespace

[Will Drewry <wad@chromium.org>]

Presently, a core_pattern pipe endpoint will be run in the init
namespace.  It will receive the virtual pid (task_tgid_vnr->%p) of the
core dumping process but will have no access to that processes /proc
without walking the init namespace /proc looking through all the global
pids until it finds the one it thinks matches.

One option for implementing this I've already posed:
  https://patchwork.kernel.org/patch/185912/
However, it's unclear if it is desirable for the core_pattern to run
outside the namespace.  In particular, it can easily access the mounts
via /proc/[pid_nr]/root, but if there is a net namespace, it will not
have access.  (Originally introduced in 2007 in commit
b488893a390edfe027bae7a46e9af8083e740668 )

Instead, this change implements the more complex option two.  It
migrates the ____call_usermodehelper() thread into the same namespaces
as the dumping process.  It does not assign a pid in that namespace so
the collector will appear to be pid 0.  (Using alloc_pid and change_pid
are options though. I avoided it because kernel_thread's returned pid
will then mismatch the actual threads pid.)

Signed-off-by: Will Drewry <wad@chromium.org>
---
 fs/exec.c |   56 +++++++++++++++++++++++++++++++++++++++++++++++++++++---
 1 files changed, 53 insertions(+), 3 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 828dd24..3b82607 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -51,6 +51,7 @@
 #include <linux/audit.h>
 #include <linux/tracehook.h>
 #include <linux/kmod.h>
+#include <linux/nsproxy.h>
 #include <linux/fsnotify.h>
 #include <linux/fs_struct.h>
 #include <linux/pipe_fs_i.h>
@@ -65,6 +66,12 @@ char core_pattern[CORENAME_MAX_SIZE] = "core";
 unsigned int core_pipe_limit;
 int suid_dumpable = 0;
 
+struct coredump_pipe_params {
+	struct coredump_params *params;
+	struct nsproxy *nsproxy;
+	struct fs_struct *fs;
+};
+
 /* The maximal length of core_pattern is also specified in sysctl.c */
 
 static LIST_HEAD(formats);
@@ -1819,8 +1826,34 @@ static int umh_pipe_setup(struct subprocess_info *info)
 {
 	struct file *rp, *wp;
 	struct fdtable *fdt;
-	struct coredump_params *cp = (struct coredump_params *)info->data;
-	struct files_struct *cf = current->files;
+	struct coredump_pipe_params *pipe_params =
+		(struct coredump_pipe_params *)info->data;
+	struct coredump_params *cp = pipe_params->params;
+	struct task_struct *tsk = current;
+	struct files_struct *cf = tsk->files;
+	struct fs_struct *cfs = tsk->fs;
+
+	/* Migrate this thread into the crashing namespaces, but
+	 * don't change its pid struct to avoid breaking any other
+	 * dependencies.  This will mean the process is pid=0 if it
+	 * was migrated into a pid namespace.
+	 */
+	if (pipe_params->nsproxy && pipe_params->fs) {
+		int kill;
+		switch_task_namespaces(tsk, pipe_params->nsproxy);
+		pipe_params->nsproxy = NULL;
+
+		task_lock(tsk);
+		tsk->fs = pipe_params->fs;
+		task_unlock(tsk);
+		pipe_params->fs = NULL;
+		/* Clean up the previous fs struct */
+		write_lock(&cfs->lock);
+		kill = !--cfs->users;
+		write_unlock(&cfs->lock);
+		if (kill)
+			free_fs_struct(cfs);
+	}
 
 	wp = create_write_pipe(0);
 	if (IS_ERR(wp))
@@ -1911,6 +1944,7 @@ void do_coredump(long signr, int exit_code, struct pt_regs *regs)
  	if (ispipe) {
 		int dump_count;
 		char **helper_argv;
+		struct coredump_pipe_params pipe_params = { .params = &cprm };
 
 		if (cprm.limit == 1) {
 			/*
@@ -1950,10 +1984,26 @@ void do_coredump(long signr, int exit_code, struct pt_regs *regs)
 			goto fail_dropcount;
 		}
 
+		/* Run the core_collector in the crashing namespaces */
+		if (copy_namespaces_unattached(0, current,
+			&pipe_params.nsproxy, &pipe_params.fs)) {
+			printk(KERN_WARNING "%s failed to copy namespaces\n",
+			       __func__);
+			argv_free(helper_argv);
+			goto fail_dropcount;
+		}
+
 		retval = call_usermodehelper_fns(helper_argv[0], helper_argv,
 					NULL, UMH_WAIT_EXEC, umh_pipe_setup,
-					NULL, &cprm);
+					NULL, &pipe_params);
 		argv_free(helper_argv);
+		/* nsproxy and fs will survive if call_usermodehelper_fns hits
+		 * ENOMEM prior to creating a new thread.
+		 */
+		if (pipe_params.nsproxy)
+			put_nsproxy(pipe_params.nsproxy);
+		if (pipe_params.fs)  /* not in use by anything else */
+			free_fs_struct(pipe_params.fs);
 		if (retval) {
  			printk(KERN_INFO "Core dump to %s pipe failed\n",
 			       corename);
-- 
1.7.0.4