Re: [PATCH v15 3/9] namei: LOOKUP_NO_XDEV: block mountpoint crossing

Al Viro
2019-11-13T01:37:08Z

On Tue, Nov 05, 2019 at 08:05:47PM +1100, Aleksa Sarai wrote:

> @@ -862,6 +870,8 @@ static int nd_jump_root(struct nameidata *nd)
> void nd_jump_link(struct path *path)
> {
> struct nameidata *nd = current->nameidata;
> +
> + nd->last_magiclink.same_mnt = (nd->path.mnt == path->mnt);
> path_put(&nd->path);
>
> nd->path = *path;
> @@ -1082,6 +1092,10 @@ const char *get_link(struct nameidata *nd)
> if (nd->flags & LOOKUP_MAGICLINK_JUMPED) {
> if (unlikely(nd->flags & LOOKUP_NO_MAGICLINKS))
> return ERR_PTR(-ELOOP);
> + if (unlikely(nd->flags & LOOKUP_NO_XDEV)) {
> + if (!nd->last_magiclink.same_mnt)
> + return ERR_PTR(-EXDEV);
> + }
> }

Ugh... Wouldn't it be better to take that logics (some equivalent thereof)
into nd_jump_link()? Or just have nd_jump_link() return an error...

I mean, look at the callers of nd_jump_link().
static const char *policy_get_link(struct dentry *dentry,
struct inode *inode,
struct delayed_call *done)
{
struct aa_ns *ns;
struct path path;

if (!dentry)
return ERR_PTR(-ECHILD);
ns = aa_get_current_ns();
path.mnt = mntget(aafs_mnt);
path.dentry = dget(ns_dir(ns));
nd_jump_link(&path);
aa_put_ns(ns);

return NULL;
}
- very close to the end of ->get_link() instance.

static const char *proc_pid_get_link(struct dentry *dentry,
struct inode *inode,
struct delayed_call *done)
{
struct path path;
int error = -EACCES;

if (!dentry)
return ERR_PTR(-ECHILD);

/* Are we allowed to snoop on the tasks file descriptors? */
if (!proc_fd_access_allowed(inode))
goto out;

error = PROC_I(inode)->op.proc_get_link(dentry, &path);
if (error)
goto out;

nd_jump_link(&path);
return NULL;
out:
return ERR_PTR(error);
}
Ditto.

static const char *proc_ns_get_link(struct dentry *dentry,
struct inode *inode,
struct delayed_call *done)
{
const struct proc_ns_operations *ns_ops = PROC_I(inode)->ns_ops;
struct task_struct *task;
struct path ns_path;
void *error = ERR_PTR(-EACCES);

if (!dentry)
return ERR_PTR(-ECHILD);

task = get_proc_task(inode);
if (!task)
return error;

if (ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS)) {
error = ns_get_path(&ns_path, task, ns_ops);
if (!error)
nd_jump_link(&ns_path);
}
put_task_struct(task);
return error;
}

The same. And that's it - there's no more of them. So how about
this in the beginning of the series, then having your magiclink
error handling done in nd_jump_link()?

diff --git a/fs/namei.c b/fs/namei.c
index 671c3c1a3425..8ec924813c30 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -859,7 +859,7 @@ static int nd_jump_root(struct nameidata *nd)
* Helper to directly jump to a known parsed path from ->get_link,
* caller must have taken a reference to path beforehand.
*/
-void nd_jump_link(struct path *path)
+const char *nd_jump_link(struct path *path)
{
struct nameidata *nd = current->nameidata;
path_put(&nd->path);
@@ -867,6 +867,7 @@ void nd_jump_link(struct path *path)
nd->path = *path;
nd->inode = nd->path.dentry->d_inode;
nd->flags |= LOOKUP_JUMPED;
+ return NULL;
}

static inline void put_link(struct nameidata *nd)
diff --git a/fs/proc/base.c b/fs/proc/base.c
index ebea9501afb8..ac4e57a3dfa5 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1626,8 +1626,7 @@ static const char *proc_pid_get_link(struct dentry *dentry,
if (error)
goto out;

- nd_jump_link(&path);
- return NULL;
+ return nd_jump_link(&path);
out:
return ERR_PTR(error);
}
diff --git a/fs/proc/namespaces.c b/fs/proc/namespaces.c
index dd2b35f78b09..dde0c501b2f3 100644
--- a/fs/proc/namespaces.c
+++ b/fs/proc/namespaces.c
@@ -54,7 +54,7 @@ static const char *proc_ns_get_link(struct dentry *dentry,
if (ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS)) {
error = ns_get_path(&ns_path, task, ns_ops);
if (!error)
- nd_jump_link(&ns_path);
+ error = nd_jump_link(&ns_path);
}
put_task_struct(task);
return error;
diff --git a/include/linux/namei.h b/include/linux/namei.h
index 397a08ade6a2..f3e8438e5631 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -68,7 +68,7 @@ extern int follow_up(struct path *);
extern struct dentry *lock_rename(struct dentry *, struct dentry *);
extern void unlock_rename(struct dentry *, struct dentry *);

-extern void nd_jump_link(struct path *path);
+extern const char *nd_jump_link(struct path *path);

static inline void nd_terminate_link(void *name, size_t len, size_t maxlen)
{
diff --git a/security/apparmor/apparmorfs.c b/security/apparmor/apparmorfs.c
index 45d13b6462aa..98aef94b4777 100644
--- a/security/apparmor/apparmorfs.c
+++ b/security/apparmor/apparmorfs.c
@@ -2453,18 +2453,16 @@ static const char *policy_get_link(struct dentry *dentry,
struct inode *inode,
struct delayed_call *done)
{
- struct aa_ns *ns;
- struct path path;
-
- if (!dentry)
- return ERR_PTR(-ECHILD);
- ns = aa_get_current_ns();
- path.mnt = mntget(aafs_mnt);
- path.dentry = dget(ns_dir(ns));
- nd_jump_link(&path);
- aa_put_ns(ns);
-
- return NULL;
+ const char *err = ERR_PTR(-ECHILD);
+
+ if (dentry) {
+ struct aa_ns *ns = aa_get_current_ns();
+ struct path path = {.mnt = mntget(aafs_mnt),
+ .dentry = ns_dir(ns)};
+ err = nd_jump_link(&path);
+ aa_put_ns(ns);
+ }
+ return err;
}

static int policy_readlink(struct dentry *dentry, char __user *buffer,