[v15,3/9] namei: LOOKUP_NO_XDEV: block mountpoint crossing
diff mbox series

Message ID 20191105090553.6350-4-cyphar@cyphar.com
State New
Headers show
Series
  • open: introduce openat2(2) syscall
Related show

Commit Message

Aleksa Sarai Nov. 5, 2019, 9:05 a.m. UTC
/* Background. */
The need to contain path operations within a mountpoint has been a
long-standing usecase that userspace has historically implemented
manually with liberal usage of stat(). find, rsync, tar and
many other programs implement these semantics -- but it'd be much
simpler to have a fool-proof way of refusing to open a path if it
crosses a mountpoint.

This is part of a refresh of Al's AT_NO_JUMPS patchset[1] (which was a
variation on David Drysdale's O_BENEATH patchset[2], which in turn was
based on the Capsicum project[3]).

/* Userspace API. */
LOOKUP_NO_XDEV will be exposed to userspace through openat2(2).

/* Semantics. */
Unlike most other LOOKUP flags (most notably LOOKUP_FOLLOW),
LOOKUP_NO_XDEV applies to all components of the path.

With LOOKUP_NO_XDEV, any path component which crosses a mount-point
during path resolution (including "..") will yield an -EXDEV. Absolute
paths, absolute symlinks, and magic-links will only yield an -EXDEV if
the jump involved changing mount-points.

/* Testing. */
LOOKUP_NO_XDEV is tested as part of the openat2(2) selftests.

[1]: https://lore.kernel.org/lkml/20170429220414.GT29622@ZenIV.linux.org.uk/
[2]: https://lore.kernel.org/lkml/1415094884-18349-1-git-send-email-drysdale@google.com/
[3]: https://lore.kernel.org/lkml/1404124096-21445-1-git-send-email-drysdale@google.com/

Cc: Christian Brauner <christian.brauner@ubuntu.com>
Suggested-by: David Drysdale <drysdale@google.com>
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Suggested-by: Andy Lutomirski <luto@kernel.org>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 fs/namei.c            | 34 ++++++++++++++++++++++++++++++----
 include/linux/namei.h |  1 +
 2 files changed, 31 insertions(+), 4 deletions(-)

Comments

Al Viro Nov. 13, 2019, 1:36 a.m. UTC | #1
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,
Aleksa Sarai Nov. 14, 2019, 4:49 a.m. UTC | #2
On 2019-11-13, Al Viro <viro@zeniv.linux.org.uk> wrote:
> 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...

This could be done, but the reason for stashing it away in
last_magiclink is because of the future magic-link re-opening patches
which can't be implemented like that without putting the open_flags
inside nameidata (which was decided to be too ugly a while ago).

My point being that I could implement it this way for this series, but
I'd have to implement something like last_magiclink when I end up
re-posting the magic-link stuff in a few weeks.

Looking at all the nd_jump_link() users, the other option is to just
disallow magic-link crossings entirely for LOOKUP_NO_XDEV. The only
thing allowing them permits is to resolve file descriptors that are
pointing to the same procfs mount -- and it's unclear to me how useful
that really is (apparmorfs and nsfs will always give -EXDEV because
aafs_mnt and nsfs_mnt are internal kernel vfsmounts).
Al Viro Nov. 14, 2019, 5:43 a.m. UTC | #3
On Thu, Nov 14, 2019 at 03:49:45PM +1100, Aleksa Sarai wrote:
> On 2019-11-13, Al Viro <viro@zeniv.linux.org.uk> wrote:
> > 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...
> 
> This could be done, but the reason for stashing it away in
> last_magiclink is because of the future magic-link re-opening patches
> which can't be implemented like that without putting the open_flags
> inside nameidata (which was decided to be too ugly a while ago).
> 
> My point being that I could implement it this way for this series, but
> I'd have to implement something like last_magiclink when I end up
> re-posting the magic-link stuff in a few weeks.
> 
> Looking at all the nd_jump_link() users, the other option is to just
> disallow magic-link crossings entirely for LOOKUP_NO_XDEV. The only
> thing allowing them permits is to resolve file descriptors that are
> pointing to the same procfs mount -- and it's unclear to me how useful
> that really is (apparmorfs and nsfs will always give -EXDEV because
> aafs_mnt and nsfs_mnt are internal kernel vfsmounts).

I would rather keep the entire if (nd->flags & LOOKUP_MAGICLINK_JUMPED)
out of the get_link().  If you want to generate some error if
nd_jump_link() has been called, just do it right there.  The fewer
pieces of state need to be carried around, the better...

And as for opening them...  Why would you need full open_flags in there?
Details, please...
Aleksa Sarai Nov. 14, 2019, 1:33 p.m. UTC | #4
On 2019-11-14, Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Thu, Nov 14, 2019 at 03:49:45PM +1100, Aleksa Sarai wrote:
> > On 2019-11-13, Al Viro <viro@zeniv.linux.org.uk> wrote:
> > > 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...
> > 
> > This could be done, but the reason for stashing it away in
> > last_magiclink is because of the future magic-link re-opening patches
> > which can't be implemented like that without putting the open_flags
> > inside nameidata (which was decided to be too ugly a while ago).
> > 
> > My point being that I could implement it this way for this series, but
> > I'd have to implement something like last_magiclink when I end up
> > re-posting the magic-link stuff in a few weeks.
> > 
> > Looking at all the nd_jump_link() users, the other option is to just
> > disallow magic-link crossings entirely for LOOKUP_NO_XDEV. The only
> > thing allowing them permits is to resolve file descriptors that are
> > pointing to the same procfs mount -- and it's unclear to me how useful
> > that really is (apparmorfs and nsfs will always give -EXDEV because
> > aafs_mnt and nsfs_mnt are internal kernel vfsmounts).
> 
> I would rather keep the entire if (nd->flags & LOOKUP_MAGICLINK_JUMPED)
> out of the get_link().  If you want to generate some error if
> nd_jump_link() has been called, just do it right there.  The fewer
> pieces of state need to be carried around, the better...

Sure, I can make nd_jump_link() give -ELOOP and drop the current need
for LOOKUP_MAGICLINK_JUMPED -- if necessary we can re-add it for the
magic-link reopening patches.

> And as for opening them...  Why would you need full open_flags in there?
> Details, please...

I was referring to [1] which has been dropped from this series. I
misspoke -- you don't need the full open_flags, you just need acc_mode
in nameidata -- but from memory you (understandably) weren't in favour
of that either because it further muddled the open semantics with namei.

So the solution I went with was to stash away the i_mode of the
magiclink in nd->last_magiclink.mode (though to avoid a race which Jann
found, you actually need to recalculate it when you call nd_jump_link()
but that's a different topic) and then check it in trailing_magiclink().

However, I've since figured out that we need to restrict things like
bind-mounts and truncate() because they can be used to get around the
restrictions. I dropped that patch from this series so that I could work
on implementing the restrictions for the other relevant VFS syscalls
separately from openat2 (upgrade_mask will be re-added to open_how with
those patches).

My point was that AFAICS we will either have to have nd->acc_mode (or
something similar) or have nd->last_magiclink in order to implement the
magic-link reopening hardening.

[1]: https://lore.kernel.org/lkml/20190930183316.10190-2-cyphar@cyphar.com/

Patch
diff mbox series

diff --git a/fs/namei.c b/fs/namei.c
index 1f0d871199e5..b73ee1601bd4 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -504,6 +504,9 @@  struct nameidata {
 	struct filename	*name;
 	struct nameidata *saved;
 	struct inode	*link_inode;
+	struct {
+		bool same_mnt;
+	} last_magiclink;
 	unsigned	root_seq;
 	int		dfd;
 } __randomize_layout;
@@ -837,6 +840,11 @@  static inline void path_to_nameidata(const struct path *path,
 
 static int nd_jump_root(struct nameidata *nd)
 {
+	if (unlikely(nd->flags & LOOKUP_NO_XDEV)) {
+		/* Absolute path arguments to path_init() are allowed. */
+		if (nd->path.mnt != NULL && nd->path.mnt != nd->root.mnt)
+			return -EXDEV;
+	}
 	if (nd->flags & LOOKUP_RCU) {
 		struct dentry *d;
 		nd->path = nd->root;
@@ -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);
+			}
 		}
 		if (IS_ERR_OR_NULL(res))
 			return res;
@@ -1271,12 +1285,16 @@  static int follow_managed(struct path *path, struct nameidata *nd)
 		break;
 	}
 
-	if (need_mntput && path->mnt == mnt)
-		mntput(path->mnt);
+	if (need_mntput) {
+		if (path->mnt == mnt)
+			mntput(path->mnt);
+		if (unlikely(nd->flags & LOOKUP_NO_XDEV))
+			ret = -EXDEV;
+		else
+			nd->flags |= LOOKUP_JUMPED;
+	}
 	if (ret == -EISDIR || !ret)
 		ret = 1;
-	if (need_mntput)
-		nd->flags |= LOOKUP_JUMPED;
 	if (unlikely(ret < 0))
 		path_put_conditional(path, nd);
 	return ret;
@@ -1333,6 +1351,8 @@  static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
 		mounted = __lookup_mnt(path->mnt, path->dentry);
 		if (!mounted)
 			break;
+		if (unlikely(nd->flags & LOOKUP_NO_XDEV))
+			return false;
 		path->mnt = &mounted->mnt;
 		path->dentry = mounted->mnt.mnt_root;
 		nd->flags |= LOOKUP_JUMPED;
@@ -1379,6 +1399,8 @@  static int follow_dotdot_rcu(struct nameidata *nd)
 				return -ECHILD;
 			if (&mparent->mnt == nd->path.mnt)
 				break;
+			if (unlikely(nd->flags & LOOKUP_NO_XDEV))
+				return -EXDEV;
 			/* we know that mountpoint was pinned */
 			nd->path.dentry = mountpoint;
 			nd->path.mnt = &mparent->mnt;
@@ -1393,6 +1415,8 @@  static int follow_dotdot_rcu(struct nameidata *nd)
 			return -ECHILD;
 		if (!mounted)
 			break;
+		if (unlikely(nd->flags & LOOKUP_NO_XDEV))
+			return -EXDEV;
 		nd->path.mnt = &mounted->mnt;
 		nd->path.dentry = mounted->mnt.mnt_root;
 		inode = nd->path.dentry->d_inode;
@@ -1491,6 +1515,8 @@  static int follow_dotdot(struct nameidata *nd)
 		}
 		if (!follow_up(&nd->path))
 			break;
+		if (unlikely(nd->flags & LOOKUP_NO_XDEV))
+			return -EXDEV;
 	}
 	follow_mount(&nd->path);
 	nd->inode = nd->path.dentry->d_inode;
diff --git a/include/linux/namei.h b/include/linux/namei.h
index a8b3f93338da..6105c8a59fc8 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -43,6 +43,7 @@  enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND};
 /* Scoping flags for lookup. */
 #define LOOKUP_NO_SYMLINKS	0x020000 /* No symlink crossing. */
 #define LOOKUP_NO_MAGICLINKS	0x040000 /* No /proc/$pid/fd/ "symlink" crossing. */
+#define LOOKUP_NO_XDEV		0x080000 /* No mountpoint crossing. */
 
 extern int path_pts(struct path *path);