Re: [PATCH v12 05/12] namei: obey trailing magic-link DAC permissions

[Jann Horn]

On Wed, Sep 4, 2019 at 10:21 PM Aleksa Sarai <cyphar@xxxxxxxxxx> wrote:
> The ability for userspace to "re-open" file descriptors through
> /proc/self/fd has been a very useful tool for all sorts of usecases
> (container runtimes are one common example). However, the current
> interface for doing this has resulted in some pretty subtle security
> holes. Userspace can re-open a file descriptor with more permissions
> than the original, which can result in cases such as /proc/$pid/exe
> being re-opened O_RDWR at a later date even though (by definition)
> /proc/$pid/exe cannot be opened for writing. When combined with O_PATH
> the results can get even more confusing.
[...]
> Instead we have to restrict it in such a way that it doesn't break
> (good) users but does block potential attackers. The solution applied in
> this patch is to restrict *re-opening* (not resolution through)
> magic-links by requiring that mode of the link be obeyed. Normal
> symlinks have modes of a+rwx but magic-links have other modes. These
> magic-link modes were historically ignored during path resolution, but
> they've now been re-purposed for more useful ends.

Thanks for dealing with this issue!

[...]
> diff --git a/fs/namei.c b/fs/namei.c
> index 209c51a5226c..54d57dad0f91 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -872,7 +872,7 @@ void nd_jump_link(struct path *path)
>
>         nd->path = *path;
>         nd->inode = nd->path.dentry->d_inode;
> -       nd->flags |= LOOKUP_JUMPED;
> +       nd->flags |= LOOKUP_JUMPED | LOOKUP_MAGICLINK_JUMPED;
>  }
[...]
> +static int trailing_magiclink(struct nameidata *nd, int acc_mode,
> +                             fmode_t *opath_mask)
> +{
> +       struct inode *inode = nd->link_inode;
> +       fmode_t upgrade_mask = 0;
> +
> +       /* Was the trailing_symlink() a magic-link? */
> +       if (!(nd->flags & LOOKUP_MAGICLINK_JUMPED))
> +               return 0;
> +
> +       /*
> +        * Figure out the upgrade-mask of the link_inode. Since these aren't
> +        * strictly POSIX semantics we don't do an acl_permission_check() here,
> +        * so we only care that at least one bit is set for each upgrade-mode.
> +        */
> +       if (inode->i_mode & S_IRUGO)
> +               upgrade_mask |= FMODE_PATH_READ;
> +       if (inode->i_mode & S_IWUGO)
> +               upgrade_mask |= FMODE_PATH_WRITE;
> +       /* Restrict the O_PATH upgrade-mask of the caller. */
> +       if (opath_mask)
> +               *opath_mask &= upgrade_mask;
> +       return may_open_magiclink(upgrade_mask, acc_mode);
>  }

This looks racy because entries in the file descriptor table can be
switched out as long as task->files->file_lock isn't held. Unless I'm
missing something, something like the following (untested) would
bypass this restriction:

int readonly_fd = ...; /* some read-only fd we want to reopen as writable */
int writable_fd = open("/dev/null", O_RDWR);
int flippy_fd = dup(writable_fd);
char flippy_fd_path[100];
sprintf(flippy_fd_path, "/proc/%d/fd/%d", getpid(), flippy_fd);
if (fork() == 0) {
  while (1) {
    int reopened_fd = open(flippy_fd_path, O_RDWR);
    if (reopened_fd == -1) continue;
    char reopened_fd_path[100];
    sprintf(reopened_fd_path, "/proc/self/fd/%d", reopened_fd);
    char reopened_fd_target[1000];
    int target_len = readlink(reopened_fd_path, reopened_fd_target,
sizeof(reopened_fd_target)-1);
    reopened_fd_target[target_len] = 0;
    if (strcmp(reopened_fd_target, "/dev/null"))
      printf("managed to reopen as writable\n");
    close(reopened_fd);
  }
} else {
  while (1) {
    dup2(readonly_fd, flippy_fd);
    dup2(writable_fd, flippy_fd);
  }
}

Perhaps you could change nd_jump_link() to "void nd_jump_link(struct
path *path, umode_t link_mode)", and let proc_pid_get_link() pass the
link_mode through from an out-argument of .proc_get_link()? Then
proc_fd_link() could grab the proper mode in a race-free manner. And
nd_jump_link() could stash the mode in the nameidata.

A sketch of how I imagine that:
===============================
diff --git a/fs/namei.c b/fs/namei.c
index 6b936038319b..14c6790203c7 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -506,6 +506,7 @@ struct nameidata {
        struct inode    *link_inode;
        unsigned        root_seq;
        int             dfd;
+       umode_t         last_link_mode;
 } __randomize_layout;

 static void set_nameidata(struct nameidata *p, int dfd, struct filename *name)
@@ -890,7 +891,7 @@ 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)
+void nd_jump_link(struct path *path, umode_t link_mode)
 {
        struct nameidata *nd = current->nameidata;
        path_put(&nd->path);
@@ -898,6 +899,7 @@ void nd_jump_link(struct path *path)
        nd->path = *path;
        nd->inode = nd->path.dentry->d_inode;
        nd->flags |= LOOKUP_JUMPED | LOOKUP_MAGICLINK_JUMPED;
+       nd->last_link_mode = link_mode;
 }

 static inline void put_link(struct nameidata *nd)
@@ -3654,9 +3656,9 @@ static int trailing_magiclink(struct nameidata
*nd, int acc_mode,
         * strictly POSIX semantics we don't do an acl_permission_check() here,
         * so we only care that at least one bit is set for each upgrade-mode.
         */
-       if (inode->i_mode & S_IRUGO)
+       if (nd->last_link_mode & S_IRUGO)
                upgrade_mask |= FMODE_PATH_READ;
-       if (inode->i_mode & S_IWUGO)
+       if (nd->last_link_mode & S_IWUGO)
                upgrade_mask |= FMODE_PATH_WRITE;
        /* Restrict the O_PATH upgrade-mask of the caller. */
        if (opath_mask)
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 297242174402..af0218447571 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1614,6 +1614,7 @@ static const char *proc_pid_get_link(struct
dentry *dentry,
 {
        struct path path;
        int error = -EACCES;
+       umode_t link_mode;

        if (!dentry)
                return ERR_PTR(-ECHILD);
@@ -1622,11 +1623,11 @@ static const char *proc_pid_get_link(struct
dentry *dentry,
        if (!proc_fd_access_allowed(inode))
                goto out;

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

-       nd_jump_link(&path);
+       nd_jump_link(&path, link_mode);
        return NULL;
 out:
        return ERR_PTR(error);
diff --git a/fs/proc/fd.c b/fs/proc/fd.c
index 9b7d8becb002..9c1d247177b1 100644
--- a/fs/proc/fd.c
+++ b/fs/proc/fd.c
@@ -163,7 +163,8 @@ static const struct dentry_operations
tid_fd_dentry_operations = {
        .d_delete       = pid_delete_dentry,
 };

-static int proc_fd_link(struct dentry *dentry, struct path *path)
+static int proc_fd_link(struct dentry *dentry, struct path *path,
+                       umode_t *link_mode)
 {
        struct files_struct *files = NULL;
        struct task_struct *task;
@@ -184,6 +185,7 @@ static int proc_fd_link(struct dentry *dentry,
struct path *path)
                if (fd_file) {
                        *path = fd_file->f_path;
                        path_get(&fd_file->f_path);
+                       *link_mode = /* something based on fd_file->f_mode */;
                        ret = 0;
                }
                spin_unlock(&files->file_lock);
diff --git a/fs/proc/internal.h b/fs/proc/internal.h
index cd0c8d5ce9a1..a090fff984ed 100644
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -74,7 +74,7 @@ extern struct kmem_cache *proc_dir_entry_cache;
 void pde_free(struct proc_dir_entry *pde);

 union proc_op {
-       int (*proc_get_link)(struct dentry *, struct path *);
+       int (*proc_get_link)(struct dentry *, struct path *, umode_t *);
        int (*proc_show)(struct seq_file *m,
                struct pid_namespace *ns, struct pid *pid,
                struct task_struct *task);
===============================