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


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