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mount: fix a race between restoring namespaces and file mappings (v2)

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Currently we wait when a namespace will be restored to get its root.
We need to open a namespace root to open a file to restore a memory mapping.

A process restores mappings and only then forks children. So we can have
a situation, when we need to open a file from a namespace, which will be
"restored" by one of our children.

The root task restores all mount namespaces and opens a file descriptor
for each of them. In this patch we open root for each mntns in the root
task.

If we neeed to get root of a namespace which isn't populated, we can get
it from the root task. After the CR_STATE_FORKING stage, the root task
closes all namespace descriptors ane we know that all namespaces are
populated at this moment.

v2: don't close root_fd for root ns, because it was not opened
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
This commit is contained in:
Andrew Vagin 2015-12-09 18:58:00 +03:00 committed by Pavel Emelyanov
parent a9be7621b7
commit af55c059fb
2 changed files with 34 additions and 6 deletions
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1
include/namespaces.h
View File

@ -38,6 +38,7 @@ struct ns_id {
struct mount_info *mntinfo_list;
struct mount_info *mntinfo_tree;
int ns_fd;
int root_fd;
} mnt;
struct {

39
mount.c
View File

@ -2972,6 +2972,8 @@ void fini_restore_mntns(void)
if (nsid->nd != &mnt_ns_desc)
continue;
close(nsid->mnt.ns_fd);
if (nsid->type != NS_ROOT)
close(nsid->mnt.root_fd);
}
}
@ -3179,6 +3181,8 @@ int prepare_mnt_ns(void)
nsid->mnt.ns_fd = open_proc(PROC_SELF, "ns/mnt");
if (nsid->mnt.ns_fd < 0)
goto err;
/* we set ns_populated so we don't need to open root_fd */
futex_set(&nsid->ns_populated, 1);
continue;
}
@ -3199,6 +3203,11 @@ int prepare_mnt_ns(void)
if (nsid->mnt.ns_fd < 0)
goto err;
/* root_fd is used to restore file mappings */
nsid->mnt.root_fd = open_proc(PROC_SELF, "root");
if (nsid->mnt.root_fd < 0)
goto err;
/* And return back to regain the access to the roots yard */
if (setns(rst, CLONE_NEWNS)) {
pr_perror("Can't restore mntns back");
@ -3289,15 +3298,33 @@ set_root:
int mntns_get_root_fd(struct ns_id *mntns) {
/*
* We need to find a task from the target namespace and open its root.
* For that we need to wait when one of tasks enters into required
* namespaces.
* All namespaces are restored from the root task and during the
* CR_STATE_FORKING stage the root task has two file descriptors for
* each mntns. One is associated with a namespace and another one is a
* root of this mntns.
*
* The root task is born in the root mount namespace.
* When a non-root task is forked, it enters into a proper mount
* namespace, restores private mappings and forks children. Some of
* these mappings can be associated with files from other namespaces.
*
* After the CR_STATE_FORKING stage the root task has to close all
* mntns file descriptors to restore its descriptors and at this moment
* we know that all tasks live in their mount namespaces.
*
* If we find that a mount namespace isn't populated, we can get its
* root from the root task.
*/
if (mntns->type != NS_ROOT)
futex_wait_while_eq(&mntns->ns_populated, 0);
if (!futex_get(&mntns->ns_populated)) {
int fd;
fd = open_proc(root_item->pid.virt, "fd/%d", mntns->mnt.root_fd);
if (fd < 0)
return -1;
return mntns_set_root_fd(mntns->ns_pid, fd);
}
return __mntns_get_root_fd(mntns->ns_pid);
}